The Radiant Heat Experiment (on a seriously low budget)

The house rebuilding project is going well. We’ve finished all the framing, and the higher ceilings and more open floorplan are hinting at a level of awesomeness that surprises even me. A picture from just this morning:


Here is the new living room and the kitchen around the corner in the back. Old ceiling height was at the bottom of that steel beam. 2 more giant window openings still to come behind those plywood squares.

While I’ve destroyed and rebuilt quite a few houses for other people, this is the first one I have been fortunate enough to create from nearly scratch for my own family, so I am treating it as a bit of a science experiment. I want to build neat energy-saving features into it, but they need to be cost-effective and homegrown whenever possible.

Any old rich guy can hire the top architect and boutique builder to make him the latest LEED-Platinum superhouse to show off in Dwell magazine… at $1000 per square foot. But with energy cheap and skilled labor and high-end home materials expensive, it takes more thought and experimentation to save energy AND money at the same time. And one of these experimental projects is to build my own radiant under-floor heating system.

If you have never heard about this, you’ll want to tune in. The dominant heating method in the US right now is the forced air furnace – a big box in your basement that blows air (and dust) through a huge network of bulky air tubes so it can reach all parts of your house through floor vents. It works, but it is not elegant: they make noise, waste a surprising amount of interior space with ducts and chases, and are a hassle to install or upgrade.

When my small construction company was building some houses from the ground up a few years ago, the architect highly recommended that we use hydronic (radiant) heat instead of forced air. “It is a world of difference”, he said, “To have that silent warmth radiating at you through the floor instead of just blowing around some hot air.”

Unfortunately, when I got quotes from some plumbers for this type of heating system, the cost was astronomical: $35,000 or more, when a full conventional heating system was only $10,000 installed. Since these homes were being built to sell, on a tight budget to compete with other houses in the forced-air price range, I reluctantly decided to skip the luxury option. Plus, the passive solar design in our architecture would ensure that the furnace was used only lightly anyway.

Now, the picture has changed. I have learned to do my own plumbing, and new technologies have come down in price that make radiant heat much more affordable. After quite a few long nights of research and online training videos, I have bought all the necessary parts and we are about to put this sucker in.

What is Hydronic Radiant Heating?

Have you ever walked past a large brick building long after the sun goes down, and felt warmth all over your body even without touching the wall? How about feeling the heat from a hot bed of campfire coals even when sitting some distance away? This is radiant heat in action: a warm surface shines out infrared light (also known as heat), which directly warms your skin. With a hot campfire on a still mountain night, you can feel completely warm even when the air temperature around you is below freezing.

This same concept applies nicely to warming a house with hydronic radiant heat: warm water circulates in tubes under your floor, causing the floor to warm up and shine heat at you from all directions. There are no ducts and no blowing dust, and the system operates silently. And because the system is warming your skin directly at the same time it warms the air of your house, you feel warmer at a lower temperature setting, which allows you to keep the house cooler, saving energy. But the best part may be that you have constantly warm feet, wherever you go in your house.

So how do we build one of these systems? In a nutshell, you need something to heat the water (sometimes called a boiler), a network of tubing under your floor, and a pump to circulate that water through all that tubing:


While the concept is simple, my summary leaves out a lot of detail. When you look at the typical “boiler room” in a luxury house, there are all sorts of valves and sensors, and miles of meticulously soldered copper from the $35,000 plumber.  I mean, shit, does this look like an easy do-it-yourself project to you?


Me neither. This is why I have always gone with forced air furnaces in the past.

On top of that, hydronic heating is an art and science unto itself, with things like ΔT, GPM, BTU/hr, and R-value calculations involved. If you can get through all that, you’re faced with boilers that start at $2000, a complicated selection of parts that nobody except the experts really understands (you won’t be getting advice at Home Depot on building one of these systems), and all sorts of other hurdles.

However, after digging through all this rubble, I found a few simplifications that bring the cost and complexity of radiant heat way down, to make it a DIY-compatible project for the average handy Mustachian. The tricks I’m applying for my system:

  1. Using drinking-water-safe components allows an “open loop” system which requires fewer valves and allows item 2:
  2. Using a single tankless water heater for both hot water and house heating cuts out the $2000-$4000 boiler cost. I chose this extremely efficient Rheem Tankless unit that runs only about $1200.
  3. A single variable speed circulator pump eliminates most of the loss and loop size calculations by sensing the water temperature and adjusting its speed automatically (this also saves energy).
  4. Using a pre-made manifold from Rifeng allows easy multi-zone control and adjustment, without the need to ever mess with the tubing after you install it.
  5. And of course, everything is done in PEX, to eliminate the cost, slow installation, and boiler-room heat loss of copper pipes.
Disclaimer: Like all of my experiments, and indeed my whole lifestyle, there is some unproven stuff in here. I am using myself as a guinea pig, and there may be some trial and error, and even risk involved. Enjoy and learn, but don’t dive in as a beginner just to follow me (another beginner) blindly!

Boiling it all down, the system I ended up with is relatively simple, and I drew it out for you in this picture:


My proposed radiant heat system (click for larger)

So far, this is a work in progress. I have already run this by a system designer and received his nod of approval, and completed some of the installation, so I am sure we can get this to work. But there are surely improvements to be made.

The great thing about this blog is that there are many people reading right now (including professional plumbers) who have already done this, so if you have any suggestions on how to improve or simplify it further, it would be much appreciated and I will update the article as new information is received. I will also publish a second post when everything is done, to show a few of the steps in progress and the finished pictures.

Reader corrections so far:

  • Add the expansion tank before the pump, not after it as currently shown
  • Watch out for Legionella bacteria growth in an open-loop system like this. While rare, the bacteria is dangerous. Exactly the same risk exists if you have a tank-type water heater and keep the water under about 120F. Solution: make sure my tankless unit is hot (legionella dies above 122F, so perhaps 130 or higher), to exterminate bacteria. Also, drain or flush the heating loops during the offseason so the water does not sit stagnant for months.
  • OR, create two-loop system with a heat exchanger in between the two loops, so the heating water never touches the hot tap water. This requires oxygen-barrier PEX and an air eliminator. You can also buy tankless heaters with two independent loops: one for heating, one for potable water.
  • Add a check valve on the 3/4″ return line so cold water cannot sneak back into the manifold instead of going to the Rheem (I guess this could happen when the pump is off)
  • Actually, add a check valve on every loop, just to make sure there are no flow surprises and water goes the direction you want it to. Otherwise, cold water can be drawn through loops unexpectedly.
  • Many tankless heaters (including the Rheem I recommend above) are not warrantied for use in heating systems. This is fine for me, as I find warranties are generally useless anyway. But it is good to note.
  • Further criticism about this experiment showed up here on the forum of the useful site called heatinghelp.com. While the thread almost convinced me that I am an idiot, the thing is that a similar discussion forms somewhere on the internet about every single article that ever appears on this blog. Many plumbers spend their days cursing this site just because I recommend doing some of your own plumbing with PEX, for example. The problem is that my fellow tradesmen tend to use anecdotes rather than statistics to make these safety decisions. The experiment will go on undeterred, but I will make a point of doing some tests with my friend who works at the city’s drinking water treatment/analysis lab.

What About Cooling?

Every house should be designed to suit its own climate. Here in Colorado, we have intense sun nearly every day but much cooler nights due to our elevation 5000 feet above sea level. So the house has loads of South-facing windows to capture heat and more loads of thermal mass to smooth out day and night temperature swings.

In the summer, this picture flips around: the Earth tilts so the sun is nearly overhead (and the large overhangs I built onto the house shade the windows from the rest of it). You keep the windows closed during the 90F days and the interior stays cool. On summer nights, the temperatures drop below 60F, so you run a large fan blowing out the day’s heat to cool everything down and begin the cycle anew.

I find this strategy (along with not being a Giant A/C Wussy) allows us to live happily without turning on our A/C in Colorado. But there’s always a backup: most modern houses without ducts use a ductless mini-split air conditioning system for cooling. These can be more efficient than central A/C systems, because you only cool the rooms where heat is building up. I will add a system like this if necessary, but we will be sure to test out a summer without air conditioning first, since the place is likely to be even more comfortable than our current house, even without cooling.

As the final cheat sheet, here is my shopping cart from PexSupply, my preferred supplier of plumbing stuff. There are a few extra things in my shopping cart for building out the bathrooms, but in general this is a complete system for a 1500 square foot house: about $1100, with free shipping and no sales tax. Add in the water heater and you have a complete heat and hot water system that costs less than a single low-efficiency furnace.


An efficiency upgrade to this system: I also purchased two boxes of aluminum heat reflector plates from Amazon which should improve heat transfer and efficiency slightly. Cost was $2.45 per 4-foot plate ($245 for each box of 100).

Update: After building the system with these, I feel it was highly worthwhile as it makes installation faster and cleaner, and improving heat transfer is a worthwhile goal with wood floors – while they work well, you do need all the heat you can get.


Update: One Year Later

This system is now up and running and you can read the results in the update article here:

The Radiant Heat Experiment – Did it Work?


  • Stan February 18, 2014, 8:02 am

    This is a good stand-alone system if you can live without AC. In Iowa where it usually gets to and above 100 degrees for a week or so there is no way of getting by without cooling and dehumidifying the house.
    We do have many heating systems like this I’ve seen. People are happy with the heating they receive from this system.
    Going with a natural gas tankless system is a good idea. I’ve seen too many people use an ordinary hot water heater and they just wear out from use because they aren’t designed to be run 24/7 to heat a house.

  • TMan February 18, 2014, 10:07 am

    As a counterpoint, there are other ways to achieve this.

    My house (in Austin TX) is on a slab foundation, so putting in the piping for this type of system as a remodel is out of the question. Also, we don’t have access to gas heating for water or air. So during a large master bath remodel, we put in an electric system.

    Basically, you run a dedicated circuit to a system which uses resistance in an electrical wire to make heat under the floor. You can put it in under tile or wood flooring (not sure about carpet). The company we bought from, Warmly Yours, made us a custom system specifically to a floor diagram we provided. Only added about $1200 installed (parts, labor, electrical) to the cost of the remodel where we were ripping up the floor anyway.

    We love it and would do it again in a heartbeat!

  • Free to Pursue February 18, 2014, 10:32 am

    Nice article MMM. I am pleased to hear more people are using radiant heat (of all types). We are still in the minority with our home features. We almost didn’t pass the home inspections because the inspector was not familiar with the primary heating solution we were using:

    When my husband and I built our home in 2001, we decided to do the following:
    – Installed in-ceiling radiant heating panels throughout the house and in the garage – heats objects in the room as opposed to the air. Each room has a thermostat. Very efficient and comfortable and NO noise, NO space needed for a furnace…and about 1/2 the cost of a forced air furnace (and we installed it ourselves).
    – Installed a wood-burning stove, with a fan above to move the hot air around the very open-concept vaulted-ceiling home (a bonus for heat propagation).
    – Included suspended wood floors in the basement, making floors more comfortable than the concrete alternative.
    – Added a sun room to our home’s south-facing wall for heat capture in the winter. The sun room double doors are open as I type and can guarantee it is heating the house right now, as I am nice and cozy 20ft away!
    – Installed in-floor electric radiant heat in the kitchen and bathrooms to heat tile and slate to ensure our feet were toasty warm in those important areas.
    – Designed dormers on each side of our home to act as skylights because of the vaulted ceilings, but without the hassle (leaks, mold, concerns over snow accumulation). We do not have to have lights on in the house at all during daylight hours. Sweet!

    If these features can work efficiently and effectively in a place that was deemed colder than Mars this past winter, (Winnipeg, Canada) I’m pretty sure they can work anywhere!

    Good luck in completing the project. Sounds like quite the adventure, but we would expect nothing less ;).

  • MM Wannabe February 18, 2014, 10:44 am

    MMM, your place is an inspiration for my family’s next home. Your experiments will surely lend some practical insight for me. We had been thinking about building new, but we love older homes. I renovated my current 100 year old 1300 sf home to accomodate a family of 4. I’m excited about the opportunity to do it again and make it even more badass than the first thanks to your inspiration. Anyway, about your place. That fire place looks to have some ideally placed thermal mass. Its right in the middle of your large living space. Heck, its the middle of winter and the sun is shining right on it. Well thought out sir! Looking at that steel reinforced wall has me wonding how you attached your framing to it? I though for sure you were going to need collar ties and a center load bearing beam for the roof. Also, those engineered I-beam joists look like they are going to allow you plenty of insulation thickness for your ceiling. I can’t wait to see more of this project.

    • Mr. Money Mustache February 18, 2014, 10:34 pm

      Thanks MMW. All the surrounding framing is screwed into the steel beams at 2′ intervals with somewhat beefy 1/4″ x 4″ lag screws. AND glued with construction adhesive, because I think my structural engineer is a bit over-cautious. And that was before he made me pour a whole new steel-reinforced footing in the basement and strap it to one of two new shear walls :-)

  • Andy Connors February 18, 2014, 11:40 am

    hi MMM – you mentioned possibly installing a mini-split heat pump later on to offer some air cooling. Some of the MSHP’s can actually provide enough heating for a whole house your size at high efficiency down to minus 13 degress (as long as the air molecules are moving the air still has heat to be extracted!). You could down size your radiant plan and install a MSHP yourself and have both heating and cooling….

  • Erich February 18, 2014, 12:26 pm

    I have seen a lot of conversations about the potential for stagnant water in an open system. It seems like you could add in a check valve and run all the new (cold municipal water) water through the radiant tubing prior to heating and it would prevent stagnation and any potential for water quality issues. Have you looked into that for your system?

  • Mr. Grump February 18, 2014, 2:22 pm

    I don’t mean to brag or anything but with a lot of help one time I put in a hot water heater. I’d like to see you try THAT!! Any body can rip off a roof and replace it or design and install a radiant heat system in a few hours. Why don’t you share with us a challenging DIY project instead of these cupcakes!!! Gotcha! Holy cow the pictures look awesome and I couldn’t even imagine the hard work and studying you have had to put in. Keep up the good work, for the aspiring handyman in all of us these articles are awesome!

    • Mr. Money Mustache February 18, 2014, 5:49 pm

      Haha.. thanks Mr. Grump. You are right that I should really do a USEFUL DIY article that is applicable to people in more typical situations.

      What do you think of this oldie? http://www.mrmoneymustache.com/2012/04/27/nicer-legs-and-some-new-carpentry-skills-for-only-15-bucks/

      I was thinking of getting another one of these great tankless heaters and installing it in another house (maybe my current house), for a simple article “how to install a tankless water heater”.

      My old heater has been a bit fussy lately anyway, and I figure the new tankless would increase my sale value by at least its cost, plus save the new owners a shitload of energy when I sell this place, since this is a 4BR/4BA house that will probably be occupied by the typical insane-shower-and-laundry family after we leave :-)

      • Mr. Grump February 19, 2014, 12:31 am

        Your response time is fast and it’s good to see an article on something more impressive than radiant heat and roofs. A wooden box, that’s what I am talking about! Something challenging! I kid of course, there is only a few articles you have that I disagree with but building boxes and jumping I can easily give a thumbs up!!!

  • Emily February 18, 2014, 2:33 pm

    Hey Mr. MM. Big fan here. You’ve been instrumental in changing my financial life. Wanted to share a peripheral success story with you. I was a Managing Editor of a small B2B mag in the manufacturing industry in 2008 when the housing downturn happened. In 2009 when the shit really hit the fan and the government wanted to pay people $8k to buy a house I was in the fortunate position to do so. Although I lived in Chicagoland I found a home across the border in NW Indiana where the taxes are 1/3 of Illinois and commute downtown is a breeze. The home was a 1960s 1650 sft Frank Lloyd Wright style brick ranch sitting on a beautiful 1/2 acres wooded lot in a fancy suburb with excellent schools and dirt cheap taxes 1 mile from I-94. It had been a hoarders’ house but after only about a year and $10,000 of work I turned my $150,000 investment into a $200,000 prize (I had it appraised so I could refinance into a 4% mortgage). Being 32 and debt free (except my mortgage, which I’m paying extra on) with a decent start to retirement savings and a great work-from-home situation, I couldn’t be happier. The kicker is, the house has radiant floor heating, and came with a 2005 boiler. There are only a couple weeks in the summer I could really use the central A/C, but a window unit suffices. Now I can focus on my real passion, working with dog rescues and foster dogs. There was a shift that happened in my mind when I realized I didn’t need stuff to be satisfied. It’s was like a spiritual experience – it hit me one day while reading your blog. I wish you, Mrs. MM and the youngster many years of happiness in your new radiant floor heated home! It really is lovely! Best, Emily

  • Gabe February 18, 2014, 4:05 pm

    I love radiant heat. I also love tank less water heaters. And I absolutely love PEX. Great write up. I really wanted to install radiant heat in my house, but I currently lack the time as I’m working my full time job and building my ‘stache. I also wanted to do it all with a single boiler, but costs and time eventually kept me from the project.

    A little about me…

    In high school I did water treatment equipment installation and maintenance for my Dads company. Then I went to college and earned my mechanical engineering degree. Since then my work has been hot water, steam, heat transfer, pumps, high purity water, and process controls with an 18 month stint as an energy auditor. I say this only because I’m very familiar with both the theory and practice of pumping, boilers, water and water chemistry, and heat transfer. That said I’m not specifically an expert in radiant floor heating, so in practice some of these items may not be problems or concerns.
    That said I think there are a couple of potential pitfalls that you should watch for, or be prepared to address if they arise. I’m going to do a little theory work (which you might already know) first and follow it up with a few recommendations (things I would certainly do) and a few ideas (things you might do or might do if the original design gives you trouble).

    First a quick word about heat transfer. To solve a heat transfer problem you have to calculate both how much energy is being supplied by the hot side, and how much energy can be accepted by the cold side. The lower heat transfer number is the limiting condition. You can’t provide more heat than the cold side can accept. So, one needs to insure not only will your boiler and PEX system deliver enough heat, but will your floor move that heat into the floor and then into the room.

    There are several different heat transfer points in a radiant floor system. There is the burning gas to the heat exchanger, the heat exchanger to the water, the water to the PEX, the PEX to the underfloor space below the floor, the underfloor space to the subfloor, subfloor to the hardwood and finally the hardwood to the room. Each one of these steps has a certain drop in temperature “delta T”, that it takes to move BTUs per hour across that material. For this system the boiler controls the water temperature, so we can pick the temperature of the water. The other temperatures will be determined by the materials, installation, and physics.

    The heater supplies water at 120°F (you won’t want to run hotter than that for a radiant set up) assuming that your underfloor space temperature is 90°F (30° across the PEX, 10-15°F drop across the floor and a floor temp of 75-80) and your PEX loop dumps all the way down to the underfloor temp you’ve got a delta T of 30°F in your water loop. (If your underfloor temp is 90, your return water temp will actually be higher but this a concepts post, not detailed numbers)

    The boiler you’ve selected has a minimum heat input of 11,000 BTU/h. With the 30°F delta T, it takes about 45 gallons per hour(11000/(30*8.34)), or 0.75 GPM, to put that heat into the space. This assumes that the loop/zone will accept that heat. Remember, the zone has to accept all 11,000 BTUs/h from the loop, else it is the limiting condition. For a 500 square foot area, that 11,000 BTU/h is 22 BTU/ft^2. Depending on the thermal resistance of the floor this may be more than can be moved at the assumed delta T of 10-15 degrees. It’s also possible that the PEX will not be able to move enough heat at 30° delta T for such a small area. This can lead to heat building up in the hot water loop.

    Ah! You say (perhaps), but I have thermostats! They will cycle the loop on and off as needed! And change pump speed! While that’s true, you also have a significant process delay – the time it takes for a change in PEX temperature to cause a change in the room temperature. This is likely tens of minutes long due to the slow conduction of heat through PEX and wood. Because of this delay your underfloor temperatures may spike because you’re continuing to supply heat to an already warm under floor area, and consequently, the return temperature to your boiler will spike, and as we’re already talking about the minimum heat output condition, the boiler can’t turn down any more, so your hot water loop get even hotter. This can continue until your boiler trips out on an error. All the time, due to process delay, the room thermostat is saying “we need more heat!” Slowing the pump speed also will not fix this, because the boiler is at its minimum, putting 11,000 BTU/h into the water. Slowing the water will lower the return water temperature, but it will also increase supply temperature (adding the same energy to less flow) causing the boiler to trip out sooner.

    There are solutions. Running a closed heating loop with a thermal reservoir (a tank) can allow you to use a lower heating hot water temperature and cycle your boiler as needed to heat the tank. You can even use your single boiler; just use an indirect hot water tank. I would look at using the heat exchanger coil on the hot side. Then you can control the tank temperature and choose any hot water loop temp, say 100 or 90 or whatever is needed to keep your floors just slightly warm. This allows you to run the pumps more, giving a much more even floor temperature through the day and a more stable system.

    Add a valve to each loop, either at the supply header or the return header. This will allow you to tune the flow in the loops.
    Add a system check valve (others have suggested both of these) I would put the check in the return line where it ties back into the cold water supply.
    Place your expansion tank on the cold water side of the boiler. This way it doesn’t get hotter than it must. (not sure if other have suggested this)

    Use a hot water storage tank to smooth out the temperature in your loops. I’ll not go into a lot of detail now, (because I’m already a giant wall of text!) but I can explain my reasoning for that if desired in another comment. Good news is this can be done after the fact, if you have a little room for a short 15-25 gallon insulated tank. Also, have you considered the Rinnai 175C? It is designed to supply both domestic hot water and heating hot water, in separate loops, for just think kind of thing. http://www.rinnai.us/boiler/product/q175c I think you would still need a insulated heating hot water tank to smooth things out.

    Let us know how it goes! I’m super excited for you! (and jealous)

  • Dragline February 18, 2014, 5:11 pm

    Very interesting/exciting project. It’s funny, though, it reminds me of the old-fashioned heating system we had in the house I grew up in, which was built in 1917. It had a boiler and those old-fashioned iron radiators in almost every room. One of my childhood tasks was taking the “key” and letting the air out of all the radiators so that they would be all water and no air and heat properly without gurgling.

    So it made me wonder if you were going to have some valves to let the air out of the system. Or maybe built it with one of those old iron antique radiators in one room just for show and an access point.

    I also wonder if you couldn’t run cold water through the system as a cooling mechanism when it was hot. But that probably would be more trouble than its worth and wouldn’t work that well.

  • markincolo February 18, 2014, 6:38 pm

    Cool, or Hot, one, put a close spaced set of tee’s for a solar thermal loop, sweet them in, put a ball valve on each one and cap it for now, ya got them in. this will save you trouble latter, and when you do get some panels on the roof your set. Getting some panels or evacuated tube collectors will save you from buying gas you don’t have to. Really, I did it, and I live a dang close to 10,000ft.
    I would like to see your numbers on this, if you want?

  • Janet February 18, 2014, 9:20 pm

  • Brock Hammill February 18, 2014, 9:50 pm

    I built and designed my house and included radiant heat in the concrete slab. After one winter here in Montana I gave up on it and got a woodstove. Read about what happened.

    It works but not as well as I would have liked.

    • Mr. Money Mustache February 18, 2014, 10:11 pm

      A useful story Brock, thanks! Sounds like your main issue was with the time lag with a heavy slab to heat up. Under the wood floor, I expect to see shorter lags.. but like you, I also have a fun woodstove for instant heat blasts. Best of both worlds :-)

      • Paularado February 19, 2014, 12:07 pm

        Yes, that is true. We have radiant heat in our basement in 4″ concrete. You set the temperature and you leave it…for months. It takes a whole day to heat up the floor or cool it down. That’s because it’s so efficient. We lived in our basement for 6.5 years (not a typo!) while DIY building our home. We probably only changed the temperature 5 or 6 times the ENTIRE TIME!

        We have our main floor pipes in gypcrete, which I commented on below. It works much quicker because it’s only 1.5 inches and very lightweight, airy, concrete with flooring over the top. This works much better in my opinion.

  • Snor February 18, 2014, 11:00 pm

    A small tip: take pictures of all the tubes of your heating system before they disappear beneath your floor. This is a small and probably unnescessary precaution, but it might save you some hassle in the future and it’ll cost you only a couple of minutes.

  • Tom February 19, 2014, 12:36 am

    MMM, you’re a genius! I’m a big fan of the tiny house movement (under 200 sq. ft. crowd) and I’ve asked fairly often why people are almost universally using a $1200 wall hung, marine grade propane heater, in addition to a tankless water heater, when they can use a tankless water heater to provide hot water and radiant floor heating.
    I thought it was particularly feasible due to the small square footage of tiny homes and the pex required, as well as the smaller heating load.

    I didn’t think a tankless heater could keep up with heat load in a larger home, but I’m thrilled to see you’ve applied MMM theory and DIY to finding a solution. Please keep us updated on the performance.

    • Roch Naleway February 21, 2014, 4:22 pm

      Tankless water heaters can be set up in series. It’s very easy to do, so capacity should never be an issue for the gas tankless solutions. The only problem to run into is whether you have a sufficiently large enough gas line coming into the home.

  • MattD February 19, 2014, 8:31 am

    What amazes me is the parallel worlds which seem to exist in heating technology on different continents – and the reaction of people to the other system (you have hot water in pipes going all around your house ? – surely that causes all sorts of problems! )

    Admittedly climate varies a lot, but why did forced air take off in North America, while wet heating systems are the norm in Europe where many countries have access to natural gas which is pretty good at heating air….and air-con is not usually an issue (southern Med in high summer being the exception).

    I don’t quite get what you are describing as ‘open loop’ – so apologies in advance if what I say below is rubbish….
    – in the UK you can get a gas fired device (Combi boiler) which will produce instant hot water for washing etc, and also happily drive a separate circuit for running hot water around your heating system – the heating is controlled using normal thermostats and valves etc and it can be driving radiators or underfloor pipes.

    The temperature you want your domestic hot water at (55-65C) is unlikely to be the same as the temperature you want the floor pipes to be running at for optimal efficiency (probably around 30C – a few degrees above target room temp – just enough heat flow to compensate for heat loss.).

    And condensing boilers are most efficient for heating when they are ticking away at a low level constantly, not modulating from full on to off.

    Wet heating pipework is very DIY-able – gas appliances are best left to those who know what they are doing.

  • Julien B. Ouellet February 19, 2014, 8:32 am

    Since you may already have the walls down, you might also want to look a a drain water heat recovery. It’s an easy way of slightly reducing your energy consumption. It might not be worth it if you buy it, but you can make one rather easily with a little ingenuity. In my opinion, the best place to recover waste heat is the shower.

    Take a look here for some info : http://delafleur.com/blog/?m=201012

    • Marcus de la fleur March 7, 2014, 8:31 am

      Julien, I wasn’t sure if the DWHR would be worth it, but recently did the first measuring on the unit. At the peak, the cold water feeding in at the bottom was at 48.2 F while the water leaving the DWHR at the top was at 79.9 F. It picked up a good chunk of thermal energy from the shower drain water. I will post about the official test results in the next few weeks.

  • George February 19, 2014, 10:51 am

    My apologies if this has already been brought up, but the first thing that strikes me with your use of PEX rather than copper is the heat transfer. You alluded to the ‘heat losses’ you deal with using copper, however, you get heat loss simply because copper more efficiently transfers heat into your floors. With that said, you may not get the heat into the floor you want using PEX. A happy medium may be using PEX between the water heater and manifold and any transfer spaces where the heat transfer isn’t wanted (i.e. heat loss) but using copper on the long runs under the floor. Just my two cents. I wish you luck with the project!

  • Clayton February 19, 2014, 11:20 am

    Great ideas, back when I lived in northern Utah this would have been great, especially on our tiles. Another thought to add is that instead of having the water for the facets come out a separate system could you have them come out of your loops, this would decrease the average time you have to have the warm/hot water running before it becomes warm or hot.

    The only thing that might get in the way of this is if that section of the house’s thermostat is off or some such.

    Thanks for all your thoughts and insights.

  • Paularado February 19, 2014, 11:57 am

    We love our radiant heat. We built our home about 10 years ago. Our PEX is in gypcrete, which adds a whole lot more radiant mass and efficiency. Our neighbors all have staple up and they have trouble sometimes keeping their homes warm (we’re at 9000 ft in the mountains, by the way). It’s probably too late for gypcrete, but I highly recommend it. I also highly recommend Advanced Hydronics in Denver.

    Your house looks great by the way. I hope that the experiment works out because radiant heat is glorious!

  • Lyn Wilkinson February 19, 2014, 12:07 pm

    Whoa dude, we’re like twins sons from different mothers. I just finished a tankless HW HVAC system back in January. Like you, I’m a builder who prefers to experiment with this type of stuff on my own home before passing judgement and peddling the tech/ideas to others. I too used a Rheem DV unit (the larger 199 BTUH unit) and sourced lead-free materials from PEX Supply/local vendors and did the install work. My system also provides the domestic HW via a tempering valve. Works great thus far! Unlike yours, mine is mated to a Rheem/Ruud fan coil unit specifically designed for this application. A local sheetmetal shop provided the FCU and did an excellent job on the ductwork and zoning controls. I decided to go with forced air since I already had floor registers cut in tile floors, duct installed in all the impossible to reach-in-a-retrofit spots, and I really need AC as well. I took extra measures to keep from oversizing the FCU and to keep the system quiet and efficient. It’s set up as a two-zone system, with returns on both levels of the house. I tore the old gas-fired forced air system out last year, shortly after we moved from VA to our current home in CA. It was a basically a dust pump installed in a nasty crawl space. The blower sat on top of a moldy mildewed wood plenum box, that, upon tearing the system out, I found a dried up rat carcass in the bottom of. The indoor air quality of our house last winter was a real bummer, especially considering what we had to fork out to buy the house. Neither home inspectors nor realtors spend a week in a house in the dead of a cool, damp December so these types of issues rarely get disclosed during the house buying process. I encapsulated all crawl space area, built a proper mechanical/utility room to house the installation and went to great pains to either run ductwork in conditioned spaces and/or ensure ductwork in un-conditioned space is well insulated. I thought long and hard about going with radiant heat and utilizing a couple of mini-splits for the AC. My wife and I are used to cold winter nights from living back east, so letting the house cool down to 58-60 degrees at night and then quickly spiking the main level up to 64-65 degrees first thing in the am seems to be working well for us. I love radiant heat, but I think I’ll be ok with this particular forced air heat. I also got rid of 2 holes in the roof (the vents for the old gas fired AHU and the gas-fired water heater) and the new Fan coil unit has a built-in pump and controller which automatically circulates the water loop periodically. I also installed an air separator and drain valves on every low spot so I can easily isolate and drain the system down during the non-heating seasons. If you would like to see pics of my installation, please forward an email address I can attach them to. I don’t see a way to do it here. I think you would get a kick out of it, after reading a couple of your blogs it’s a little scary how we think alike!

  • Tim Fulton February 19, 2014, 2:52 pm

    Great write up MMM. I love that you’re going with hydronic heat. As I go through my house renovating I am adding it as well. In fact, right now I have an office torn apart and I just stapled down my aluminum heat spreaders the other night:


    I am also in the middle of adding a fairly large solar hot water install that should provide 20% of my heat (worst case). The best part the solar panels were free!

  • drea February 19, 2014, 8:20 pm

    Did I just get lucky and find a deal on a Rheem tankless water heater or is this not the kind of model you’d want for a project like this?


    • Roch Naleway February 21, 2014, 4:38 pm

      You definitely do not want to use an electric tankless water heater for this type of project. They will not have the heating capacity needed to power a radiant floor heating system. Electric tankless water heater are made for “point of use” applications…..a sink, a shower, etc….They are not made to provide water for a whole house or a radiant floor heating system.

  • hopper February 19, 2014, 8:57 pm

    I import and sell mini split AC and at a recent trade show there was Heat Pump Hot water set ups that would probably be a lot more efficient at heating the water than the rheem tankless unit.

    • Roch Naleway February 21, 2014, 4:40 pm

      The operating costs for a electric heat pump water heater and a gas tankless water heater are about the same.

      Heat pump water heaters are a great option though when an all electric standard water heaters can be replaced. The operating costs for a heat pump water heater are between 50-65% less when compared to standard electric water heaters.

  • matt February 20, 2014, 11:03 am

    does the manifold allow you to shut off each loop individually?
    If not, I would add a shut off for each loop in case you aren’t going to be using a room for a while, or need to work on a leak without losing heat in the rest of the house.

  • Jess February 20, 2014, 12:03 pm

    So have you considered how you plan to get flow down each line. I work in a process industry and proper line sizing and length of those lines is very important to insure that each line will receive water equally. i have talked with a Process engineer i work with and you will for sure want to do a first out/ last in setup. so the full loop you show on your drawing should return to the manifold on the far right side. Have you put thought into the hydraulics of the system?

    • Mr. Money Mustache February 20, 2014, 3:10 pm

      Good question Jess – the Rifeng manifold allows you to monitor and adjust the flow rate on each loop individually. So I bet it will be pretty easy to balance everything out. Will let you know.

  • Jesse February 20, 2014, 12:35 pm

    Sounds like an interesting project. However I would be concerned about the service life of the tankless water heater. For a residential model it is designed to provide hot water on demand, which might be a few hours per day in a typical household. But the radiant heat system will demand hot water almost continuously during the winter. My experience has been that standard tankless water heaters only last a few months when they are required to run 24-7. You might want to look into the specifics of the model you are considering or maybe look at commercial models.

    • Mr. Money Mustache February 20, 2014, 3:08 pm

      This is a great point. Mine is a pretty heavy-duty commercial unit, but the warranty expressly is void if it is used for radiant heat, probably for that reason.

      In my own system, based on BTU calculations I expect the heater to run less than 2 hours a day in winter, and only a few minutes per day outside the heating system (a few 5-minute showers per week plus a little bit for the kitchen sink/dishwasher). And I have a general life policy that I don’t care at all about voiding warranties.. but I will still add this to the “reader corrections” section.

      • MacGruber November 17, 2014, 4:55 pm

        Also look at the conditions it is designed to work under. The warranty being voided is not a capacity issue as much as a water issue.

  • Alasdair February 21, 2014, 1:06 am

    Check out cansolair.com. Looks very promising, though could be hard to regulate the heat that flows into your house. Don’t think it’d be hard to build your own.

    Seems like something that could be used to blow hot air under floor space, too.

  • teacherman February 21, 2014, 6:09 am

    Thanks for the promo code on GPconservation.com, MMM–just purchased the tankless water heater I’ve been mulling over for a while!!

    • Roch Naleway February 21, 2014, 4:41 pm

      Thank you for the order!

  • TommyVee February 21, 2014, 10:30 am

    Sounds like you are deep into and enjoying your radiant heat experiment.

    However, I think in Colorado for reasonably energy-efficient or passive houses, the combination of mini-split heat pumps and PV is way simpler and way more economic. Many passive houses have such reduced heating/cooling demand that only 2 (upstairs/downstairs) mini-splits are used to heat/cool even in Vermont (http://www.greenbuildingadvisor.com/blogs/dept/musings/just-two-minisplits-heat-and-cool-whole-house). Plus lots of passive houses use heat recovery ventilation units with some resistance heat elements included for backup.
    A few electric space heaters or electric baseboards are easy to throw in and that allows tuning zone temps using your own kWh from PV on the roof.

    Although I work writing software to model energy use in buildings, many design decisions are still very hard to quantify, with complicated and difficult to predict consequences beyond energy consumption. Personally, I hate all the envelope perforations and outside air requirements for gas heating, whether for hot water boilers or forced air. But our house in Boulder is a passive remodel and we stuck with gas forced air since it was already in place.
    If the house has a very tight envelope, temperatures will be so nearly uniform inside that I think the difference between in-floor radiant and mini-split heated air would be nearly imperceptible.

  • Jessica w February 21, 2014, 1:01 pm

    For information on radiant heat flooring this blogger is building a whole home from scratch in Alaska with ICF and shows step by step pictures. http://ana-white.com/2011/09/momplex/heated-slab-installation

  • Jeff February 21, 2014, 3:17 pm

    Heating with hot water is relatively common in the UK. I installed all my own system, except for the gas boiler, which the law say I cannot do myself.

    1 If you run fresh water through the boiler, what stops it, the pump or anything else in the system corroding? I prefer the use of an indirect system, where the main circuits are sealed and filled with corrosion inhibitor (useful if the system contains aluminium, copper & ferrous materials).

    2 If you then have an indirect hot water storage cylinder, it has a couple of advantages:
    (i) You could specify one that has a couple of loops through it, to facilitate solar heating.
    (ii) Add an electric heater, so there is a back up in case the main boiler needs attention.

    3 Your capital costs seem modest. I see no harm in spending a bit more to get max efficiency, a robust long life and upgrade possibilities (especially solar). The main cost will still be fuel.

    4 I made a very simple & cheap pressure tester when doing mine. Got a T- piece, put a schrader valve from a tyre in one end, a pressure gage on another & third end can be connected to the system with a quick release coupling. Then just pressurise with a bicycle pump, take a pressure reading & go have a coffee.

  • Brant February 21, 2014, 9:33 pm

    A couple of thoughts:

    1. Colorado is amazing not just for the thermal difference between day and night, but because of the difference between ground temperature and the opportunity to do solar. I know I’m going beyond the scope of your current project, but I’ve considered doing the same thing to my house in Boulder County over the last several years:
    a. Do what industry does on heating systems – circulate year round. Bring the cool of your basement to the rest of your house (esp. the 2nd floor) by circulating year round (if possible).
    b. Leave the possibility of combining your radiant project with solar – and improving the efficiency of both… pull away the heat that destroys the efficiency of solar panels – and use it to even out the temperature of your basement… or pre-heat the water for your tankless. Stick it to Xcel the best you can.

    2. Take another cue from industry and eliminate flushing of the system (save for once every 30 years)… use PEG on your closed loop. PEG (polyethylene glycol) is the primary component of antifreeze, is more viscous and has higher thermal capacity than water. Bugs are bad (esp. Legionella).

    3. If you need to cheap it further – remember that heat rises. The system that I have wanted to install would be for my basement wood floors only… (3′ above the floor of my sub-basement). I’m a big fan of reflectors too, so I think that’s money well spent.

    Good luck.

  • Joe February 21, 2014, 10:06 pm

    I know the thought of it hurts but why not spend another $1,000 and use two Rheem heaters. One for hot water one for radiant. I have thought of doing a system like this. I’ll be watching. Thanks.

  • DrewP February 22, 2014, 7:11 am

    Neat stuff! Why not buy two tank water heaters for the price of the tankless? One for the hydronic heater and one for your hot water. That way you could use distilled water with a closed loop system for the hydronic and avoid scale buildup. Plus you could add solar wand heat exchangers to the tanks which would really amp up the WOW factor!

  • Jeff February 22, 2014, 3:09 pm

    Probably best to go for an indirect system. Then the heating loop can be protected from corrosion with an inhibitor. Useful when you probably have a mix of materials, including ferrous, copper & aluminium in the system.

  • William Krick February 22, 2014, 8:03 pm

    I remember watching a TV show many years ago about energy efficient heating where the homeowners had tanks that I believe were made of metal, painted a dark color, and filled with paraffin. I think they were 4 feet wide, 6 feet tall, and maybe 1 foot thick. They were on wheels and in the morning, before they went to work, they rolled the tanks in front of their large south-facing windows. The sun would heat the tanks and melt the paraffin. Then when they got home from work, they would roll the tanks into other places in the house, including the bedrooms where the they would slowly radiate heat all night as the paraffin cooled.

  • Scott February 23, 2014, 11:25 am

    I really enjoyed this article, and all of the comments! I discovered PEX tubing about fifteen years ago, and we have retrofitted the potable water system to PEX for three houses. No leaks, ever. Then, when I was building a shop next to my home about ten years ago, I designed and installed in-floor hydronic radiant heat using the regular potable PEX and a tankless water heater. This was a closed loop system, and it worked great for years. Then about two years ago the tankless heater stopped working. Since it’s a “newfangled thing” I couldn’t find anybody to work on it. Since it’s not imperative that I heat my shop at all times, I just haven’t got it working again. So my conclusion is that the tankless water heater is the weak link in such a system.

    From your design schematic I think you about have it right! Will there be glitches? Sure! But nothing you can’t handle. I would imagine down the road you might opt for a second heater, and maybe split the two systems. Again, nothing you can’t handle. Please keep us up on how it’s working!

  • Jason Natael February 24, 2014, 6:53 am

    Quick question on this, just out of curiosity. Do you continue to use PEX in the pipes under the floor itself?

    It seems to me that while PEX is superior in almost all other situations, particularly in the boiler room as you mention, copper piping might actually be preferable in the under-floor section simply because it would conduct heat so much better. How much less effective is the PEX at allowing the heat to pass through from the water to the surrounding material? And is that difference enough to be problematic for this heating system?

    • Mr. Money Mustache February 24, 2014, 6:11 pm

      Yes! Using PEX under the floor makes the installation much simpler than copper (where you’d need to solder elbows every time you turn a corner). The aluminum heat deflector plates help with the heat transfer a bit. Insulating below the Pex helps even more. It all works surprisingly well, which is why this method of heating is surprisingly popular.

  • Lonni February 24, 2014, 5:01 pm

    I am only looking at your drawing and do not know what others have said, but make sure you add a high point vent for starting up the system and a low point drain for the inevitable drain down.

    You could also add a back flow preventer at you potable water connection to the system and put a chemical feed port to inject some chlorine if you are really worried about bacteria.

    Good Luck!

  • Joe February 25, 2014, 11:48 am

    With all those heating lines below the subfloor, how do you avoid dumping a majority of heat in the the unused basement space below, or worse, into the unconditioned crawlspace beneath the house, if that is the case?

    I see you will be insulating beneath the pipes, but I wonder if that would be sufficient.

    • Mr. Money Mustache February 25, 2014, 3:51 pm

      Great question Joe – I learned through reader feedback that you want at least 5x more R-value BELOW the pipes than you have above. With my wood floor being around R-1 to 1.5, I’d need 7.5 below.

      I was going to stuff some standard R-13 batts into each joist bay anyway (to insulate the house from its crawlspace), so that will cover both purposes.

  • Jamison February 26, 2014, 10:11 am

    Non-plumbing comment: did you just build up off the lower level wall framing’s top plate when you pushed the roof up? If so, you have a nasty, unstable hinge in your wall when the wind blows. Or are those black horizontal and vertical bands pieces of steel you put in to help with this problem?

    I’m not a residential framing expert but I know my way around structures generally and would hate to see your work compromised! figured you wouldn’t take offense at the question.

    Cheers, and his luck with the rest of the remodel!

    • Mr. Money Mustache February 26, 2014, 9:04 pm

      Yes! About 2400 pounds of 6″x4″ steel behemoths, welded together and bolted into everything in sight from foundation to roof sheathing, along with some shear walls and hold-down straps. Looking back, it probably would have been more efficient to just bulldoze the house and build it from scratch, but that is always the case with fixing up old houses. It is a labor of love.

  • JT February 26, 2014, 10:15 am

    Excited to read your follow up post on this one. I designed a system for hydroponic radiant heating for my house a few months ago (won’t be able to build it until fall, though). I was talked into doing closed loop system because of legionella concerns, though. Damned heat exchanger added a degree of
    DIY difficulty I didn’t want. I may reconsider your design, and just make the water hotter.

    I also live in a neighborhood with no natural gas available, which really limits my options for affordable hot water.

  • MJ February 26, 2014, 11:41 am

    Hi MMM,

    What do you think about running some of the radiant heat loops in the shower/underneath the shower pan/tub and the walls? Is there a good reason not to do this? If your shower area has an outside wall or two, maybe this would make showering a bit more pleasant. Sure does sound wussy when I put it on paper….

    • Mr. Money Mustache February 26, 2014, 9:00 pm

      I don’t know if heated walls would make that much of a difference in shower comfort, but the bathroom floor (and by extension the concrete showerpan) will sure have the radiant heat tubing underneath. Great for bare feet after showers, and also great for rapidly drying the shower floor so mold can’t develop and you can clean it even less frequently :-)

  • Kevn Lambson February 27, 2014, 2:41 pm

    I’m in southwest Colorado, living in a straw bale house with 128 sq feet of solar hot water panels that provide 90 degree glycol/water to my propane boiler before it goes into the floor. I created the system myself, like you, with help and advice from various sources. It works wonderfully!

    Heat from the panels and boiler go directly into the floor without going through a heat exchanger. If you opt for one, be prepared to kiss efficiency good-bye. The heating demands for your home will be far greater than heating demands for potable water, so I’d manage efficiency there first by eliminating a heat exchanger, and use the tankless heater for showers and such.

  • Josh February 28, 2014, 5:28 pm

    Good stuff and stoked to see you’re going with radiant. It’s awesome to wake up, get out of bed and your feet are on warm floors. I don’t know if anyone has mentioned this device, but when I get a chance to do it again, this is what I’ll be using the next time around – http://myheatworks.com/

    All the best,

  • Heather D. February 28, 2014, 7:43 pm

    I like the radiant heating and passive solar plan! i just have a suggestion on the cooling- I would consider a well built whole house fan. We have an Airscape fan that we use in the evenings/mornings here in UT. It sucks all the lovely cool, dry nighttime air inside and pushes the hot attic air out. Keeps us so cool at night, that the kids need blankets in summer. It may extend the life of our roof as well, by getting the built-up daytime heat out. You can buy cheaper fans at the home centers, but they sound like an airplane takeoff and I think that would affect how long you’d want to run them. Ours is so quiet, I have to get close to tell if it turned on! They’re very good on electricity usage, as well. I won’t have another house in the west without one.

  • Zalo March 1, 2014, 4:31 pm

    …or I can just put on a sweater & some long johns! :D

  • Scott Lee March 2, 2014, 3:57 pm

    My friend just told me about your site, I am looking forward to it. I zoned right in on your heating system. So, here are a few of my comments for what they are worth. I am a lifelong do it yourself builder, contractor, solar installer and live in NH.
    1. Hydronics are very popular in the east.
    2. I use a similar boiler to yours, from Triangle Tube. Has an internal circulator which is a great bonus.
    3. put in solar hot water panels. They are the best. your boiler will not turn on in the summer in CO. I recommend evacuated tube panels.
    4. Purchase a 2 coil indirect water tank, one coil for the solar ( bottom) and the other for the boiler. This will eliminate the open loop debate.
    5. I have manufactured manifolds for my radiant. The flow is still not perfect and seems to have a cold leg sometimes. a cold leg means no flow, no flow means dead water. In an open loop system this is bad.
    6. in the grand scheme an indirect heater to protect from the open loop is small change.
    7. Love the radiant, love the gas boilers. Love the self made plumber. Tell all them curmudgeons calling BS to pound sand.

  • ray March 3, 2014, 11:43 am

    You need a check valve in every loop. This drove me batty in our radiant heat system. Without check valves, what happens is that the pump will sometimes start pulling cold water (backwards) from some loops. (Or pushing, depending on which side you have your pump on. Ours was on the cold-water side, before the heater.)

    Take a look at your disgram. You are assuming that all 6 zones will have an equal flow. They won’t. What’ll happen is something like Z1 will get a lot of flow, so returns a lot of water to the cool side of the manifold. You *think* that this cold water will flow to the outlet. You think that because that’s the way you want it to work, and you assume the water will flow the way you drew the arrows.

    But the water will flow where it wants to. Some of that cold water from Z1 will flow to Z2. The Z2 loop will now have backward flow, and will be cold.

    It gets worse if/when you discover that one pump isn’t enough and you add another pump.

    Spend the extra $7 for check valves and you’ll save yourself a lot of hair-pulling-out.

    It is quite probable that you’ll need a separate thermostat for each loop. The water flow in each loop is highly dependent on the resistance of the loop compared to the reistance of every other loop. It’s all too easy for 90% of the flow go through one or 2 loops while the others get very little flow.

    • MacGruber November 17, 2014, 4:47 pm

      His zones actually all will have the same flow. Normally when you have different lengths of circuits you will run into flow issues because of the whole ‘path of least resistance’ from shortest to longest length.

      He wont have this problem because by luck or good advise he has a self balancing valve. The key now for him is to balance it correctly.

      Had he of chosen a standard off the shelf manifold or built his own manifold the system would operate horribly as you rightly note. That said if you dont balance it right it will work like crap :)


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