How to Replace Your Own Furnace

Egads! This is the furnace that came with an old Victorian house I'm currently helping some friends renovate.

Egads! This is the furnace that came with an old Victorian house I’m currently helping some friends renovate.

It seems that every house I’ve ever owned has needed a new furnace. I mean sure, the old furnace was still there clunking along and producing heat, but it was always some embarrassingly old thing installed by someone that obviously didn’t care about energy efficiency (or it predated the invention of efficient furnaces entirely).

But like all owners of these same houses before me, I let it slide and let projects that seemed more urgent on the surface suck up my time. I renovated kitchens and bathrooms or replaced roofs. It was financially easy to justify the procrastination as well: heating bills for a typical house are under $1000 per year in my area, but if you hire out the installation of a new furnace you’re looking at about five grand. Even more if you’re replacing the air conditioning system at the same time. Even if you could find one that ran on free magic unicorn dust you would have a six year payback and more realistically it will take decades.

So I let the slow leakage continue and always felt a small hole in my heart every time that machine kicked on, because for Mr. Money Mustache, energy efficiency is a moral issue even more than it is a financial one.

I figured the numbers would work out much better if I could actually do the replacement job myself, because a top-of-the-line gas furnace only costs about $1200 online these days.  But I didn’t know exactly how to do it and there never seemed to be a good time to learn*. Nobody I knew had ever replaced their own furnace, and the building materials stores don’t even sell them – everybody says you need to hire a pro for such a thing.

But finally, here in the year 2015 and at the embarrassingly late age of 41, I have finally studied up on the necessary tricks, successfully installed two beautiful high-efficiency gas furnaces alongside friends, and am here to tell you it is a perfectly reasonable do-it-yourself project after all**. So let’s get started.

Step 1: How the hell does a furnace work?

When you get right down to it, a gas furnace is just a box-shaped heater connected to some tubes. These days, they have added more internal complexity to make them more efficient, but all you really need to know as the installer is this: Cold Air in, Warm air out, Gas and Electricity in, Combustion air In and Out. It gets even easier if you write these same things on a picture of a box (aka furnace).

Figure 1: Furnaces are Simple

Figure 1: Furnaces are Simple

Step 2: What kind do I need and where do I buy it?

In general, you’ll want a high-efficiency (94% or higher) condensing furnace, with variable speed blower and roughly the same overall heating capacity as the one you’re replacing. It can be smaller in physical size (they have shrunk nicely over the years), but probably not much bigger since you have to fit it into the same space.

Actually finding a place that sells furnaces can be tricky. Like plumbing was a few decades ago, the heating and cooling industry is still an insider’s game, with low-profile stores that only sell to contractors, and contractors that insist their field is far too dangerous and exacting for any homeowner to master. If your personality type is at all similar to mine, the very words “consult a qualified installer” piss you off a little and make you want to learn the trade.

Typing “where to buy a gas furnace” into Google leads to a mixed bag you can sift through, but I ended up finding the best results for my situation at a place called Alpine Home Air. Specifically, for both recent installs, my friends just went for the top-of-the-line Goodman 96% unit.

For a bit more background reading on the field, Consumer Reports has a free furnace buying guide.

Step 3: OK, Got The Furnace. What Other Parts Do I Need?

Remembering that diagram above, you’re hooking up air, gas, intake, exhaust, and electricity. Everything will be available at your local building materials shop, with the possible exception of a condensate pump.

If you’re installing the furnace from scratch or replacing a Crazy Spaghetti Octopus monster and want to re-do the ducting in your basement completely, you might also pick up:

  • a return air box : this is just a big sheet metal box that you set your furnace on. It serves as a big air scoop where you can connect all your return air ducts, and it also has a convenient slot to hold the air filter.
  • a supply air plenum to handle the heated air on its way out. You’ll cut holes in this to connect supply ducts to the rest of your house.

Step 4: Let’s Hook This Sucker Up

Read the Manual:

Somebody actually cared when they wrote this instruction manual.

Somebody actually cared when they wrote this instruction manual.

I was pleasantly surprised to find that the Goodman furnace we used came with a fully detailed installation manual – none of this consumer-oriented “run screaming and consult a professional” attitude. Every hookup and specification, right down to how far to keep your vent pipes away from exterior windows, was described clearly with pictures. I spent a leisurely hour at home reading it from cover to cover the night before beginning the installation, which allowed me to have the big picture in mind on the big day.

Household Air:

custom_supply_boxCool air (also called ‘return air’) gets sucked in through the bottom of the furnace, heated, and blown out the top (‘supply’). If you are replacing an existing furnace, you just need to carefully extract the old furnace from the big metal boxes, then seal the new unit to the same boxes. You can reshape or extend them as necessary.

If you have central air conditioning, there will be a separate metal box stuck in with everything else. Just leave it in place, be careful not to break the tubes and wires, and it will continue to work with your new setup.

Tools and tips: You cut the metal with tin snips or a grinder with metal cutoff wheel. Fold pieces nicely with a metal brake. Screw things together with sheet metal screws. Create airtight and heat-resistant joints with silver foil tape (not duct tape). Brush on duct sealant to all potential air joints to create a better seal. And above all, instantly master sheet metal duct work with a few YouTube videos on the subject.

Combustion Air and Exhaust:

pvcHere we are just running two pieces of 3″ PVC pipe (you can even use 2″ for shorter runs) from the furnace to an inconspicuous place on the outside of your house. It’s a fun design puzzle, deciding how to route the pipe and figuring out which fittings to use to accomplish it. Your goal is a classy looking job. You cut it with a miter saw or sawzall, and glue it with purple PVC primer and PVC glue. Again, watch a few videos if you need to learn how to handle this stuff.

As a huge bonus, these same plastic piping skills will allow you to run drain pipes, which lets you build your own bathroom from scratch (future article?)

Gas Supply:


My friend Mike uses a pipe wrench to twist on a length of black pipe.

Although people tend to be afraid of working on gas piping (after all, you can blow up your entire house if you get just the perfect gas leak and ignite it), it is easier than ever and quite rewarding to do yourself.

An existing furnace will already have a gas line, complete with shutoff. So in most cases, you can just connect your new furnace with a standard flexible gas connector.

But if you need to change the routing, you can turn off  your gas supply at the outside meter, use a big pipe wrench to unthread the existing black gas pipe, and buy new lengths and fittings at the store to create your new layout. They will even custom-cut and thread the pipe for you, or you can do it yourself if you own a pipe threading tool. Once you have the right pieces, wrench everything together with plenty of pipe thread sealant (aka “pipe dope”) to create gas-tight joints.

Three pieces of CSST come together at one traditional T-joint, where we split off a branch to feed the existing water heater.

Three pieces of CSST come together at one traditional T-joint, where we split off a branch to feed the existing water heater.

These days, I usually bypass the black gas pipe entirely and use the newer flexible Corrugated Stainless Steel Tubing (CSST for short) instead. This saves time and allows you to thread the tubing right through joists and around corners, keeping it out of sight – especially useful if you’re installing in basement that may eventually be finished. The fittings come with detailed instructions and you can of course watch videos on CSST installation as well.

When everything looks perfect, you turn on the gas and check thoroughly for leaks with a soap solution, your nose, and a gas leak detector.


The furnace uses electricity to run its control electronics, igniter, and blower. But you just need to connect it to one normal (15 amp) household circuit. Black wire is hot, white is neutral, and green/copper is ground. You make the connection right inside the junction box built into the furnace, although you’ll want to mount a switch somewhere in the circuit so you can power down the furnace.

Another option (if your inspector allows it) is to wire on a cord and simply plug the furnace into a nearby electrical outlet. This is simpler, and allows you to plug the furnace into a backup power source (generator or large battery) to restore heating in the event of a power failure.

Condensate (aka dripping water):

When an efficient furnace runs, it condenses some water out of the hot combustion gases. This drips slowly out of the furnace through a little plastic spout, and you need to connect that to a flexible plastic tube that takes the water somewhere safe. If you have a floor drain in the basement, pipe the water there. If you need to lift the water higher, you dump it into a condensate pump, and have the output go to a nearby plumbing drain.

Thermostat wiring:

You are down to the really easy stuff now! You can follow the instructions for the furnace and thermostat, but in my case I just connected the red, green, yellow and white wires on both sides.  For a longer explanation of what the wires do, here’s a guide.

If you’re looking to upgrade your thermostat at the same time, I am a fan of the Ecobee Lite Wi-Fi enabled thermostat. It is reasonably priced but also has a beautiful physical form, and a really nice, robust user interface (and phone app). The default operation is ridiculously simple, but you can also dig in and adjust lots of detailed settings if you are an engineer like me.

And You’re Done!

It’ll take some work and you will learn a few things, but at the end of the project you’ll have a beautiful new furnace that provides a sizable return on your investment of time and money.

Here’s a picture of one of the finished installations at a friend’s house. From bottom to top, notice the custom return plenum, furnace, existing A/C box, PVC combustion air piping, and my homemade supply plenum that funnels the air to the old ductwork.



Successful DIY mentality


Two local friends exchange witty banter even as they build custom ductwork for a current project house. This was a bigger job as the original 1910 house had no real ducting at all – we had to open up walls and run pipes to every room in the house. Your furnace upgrade will be easier.

When I first started do-it-yourself home renovation, at least part of the motivation was a desire to save some serious money. But in recent years the need to conserve money has faded away completely and yet I find myself more enthusiastic about building and fixing stuff than ever. This is because learning new skills, solving puzzles and creating finished products you can be proud of is not just something you do for money – it’s the purpose of life itself.

So when confronted with a choice between fixing something yourself and hiring it out, you do well if you push your comfort zone just little a bit further each time. Just remember the mantra: “This is possible, and plenty of people with fewer advantages than me have accomplished the same thing many times in the past”

Then you get to work, read the instructions, tinker, make mistakes, learn, and succeed. And continue to build on that success, forever.

Further Reading: You can find many more of my DIY-Themed Articles with the shortcut https://www.mrmoneymustache.com/tag/diy/

* When I bought my current house I ditched the hot air furnace entirely and built an under-floor radiant heat system instead. Now into its second winter, we are still loving it.

**Do you need a permit and Inspection? 
In general, yes – replacing a furnace is something your city wants you to get a permit for. But it’s not a big deal – going through this process is a nice low-cost safety check to make sure you get the details right. And having an inspected, approved permit on file will make it easier to sell your house further down the road.

  • KY November 25, 2015, 7:27 am

    Has anyone dealt with a “Homestead Permit” with the city of Austin, Texas? Generically, not just for furnace replacements? I just found a “Homestead Permit” which appears to allow a homeowner to apply for a permit for various modifications. From my reading, it definitely allows for a furnace replacement (as long as gas plumbing is not affected).

  • Dan November 25, 2015, 7:27 am

    How do you go about calculating your energy savings by replacing a furnace?

    I think you could calculate the amount of gas your furnace uses taking the total for the year, then subtracting the amount in July x 12. In July my furnace is not used so it is only the gas for the hot water tank.
    Yearly Furnace Gas = Total yearly gas – (12 x Gas in July)

    Then if I assume a 60% efficient unit, versus a 94% newer furnace, would the savings rate be:
    Gas Units Saved = Yearly Furnace Gas x (1 – 60%/94%)

    Then my yearly savings is Gas Units Saved x Unit Rate for Gas

    (I think I answered my own question by asking it. Please let me know if there are any errors or suggestions)

  • Atroc November 25, 2015, 8:21 am

    Funny thing: I am from Germany and I never even heard of such a heating system.
    All heating systems here (that I know of) are water circulation based. The water gets heated (gas, oil, solar, what ever) and than circulates through the radiators.

    But, I think this warm air system makes perfect sense when you also can use the same air system for air conditioning. A think one can normal not find in German houses. Only in Offices and Malls.

  • Mr Zombie November 25, 2015, 10:28 am

    Awesome article. Reading articles like these over the last couple of months has moved my mindset from “I don’t have time” to “I’m going to make time to learn some new skills”.

    Not quite the same, but the doors all need replacing in our house. Ripping one of the old frames out has certainly been an kick up the arse to get started. No excuses now.

    I’ve been practising chipping out mortises with a mallet and chisel. Even something as simple as this, it’s nice to actually work with your hands and make something.

    I fear my time of working on excel in a cubicle is getting shorter and shorter.


  • Chris November 25, 2015, 4:38 pm

    Good job Pete, it looks like a professional install. I do Hvac work in southern Minnesota. The one thing that may be overlooked is checking the manifold gas pressure,and making sure it is inline with manufactures specs. We use U tube manometers which are very accurate with multistage furnaces.

  • Doug November 26, 2015, 9:08 am

    I see a lot of you commenters here are concerned about not having a licensed gas fitter doing this kind of job. If you have that concern, why not install the furnace yourself and have the gas fitter hook up the gas or propane supply? That would still be cheaper than having the whole job done by a contractor.

    from what I’ve read here over the years you not only believe in living efficiently to save money, but also for environmental concerns. In other words you have a conservation ethic. Did you make provisions to put a heat exchanger in the ductwork (a car radiator would work fine) for a future solar heat installation? In such a configuration, the gas heat would be the backup for the heating system.

    • JC December 1, 2015, 11:17 pm

      I do this with 160SF of thermal collectors on the roof of my midwest 70’s wood-frame construction house – but it is supplemental heat only. By no means is the gas furnace a “back-up” – in winter months, it is primary. Unless your house is very well insulated and air-tight, you are not going to replace an 60-80,000btu furnace with a few hundred gallons of 140*F water on the sunny days.

  • mralistair November 26, 2015, 3:30 pm

    For the love of moustaches, also install a Carbon Monoxide detector near the furnace and allong the length of the flue pipes if they go anywhere near occupied spaces. that stuff will kill you.

    nobody does ducted air heating in the UK, it looks pretty damned inefficient and take up a lot of room, which we rarely have.

    Didn’t you insall underfloor heating? does that work on a separate system

  • Steve Ferguson November 27, 2015, 4:15 am

    Nice write up. I would have REALLY been impressed if you had measured the external static pressure and/or total static pressure both to see how the whole system (furnace + duct work) was actually performing. Normal HVAC guys won’t do that (even though they should). Doing this could be likened to taking ones blood pressure when going to the doctor. The heart (furnace) must work a lot harder if the arteries or veins (both return and supply ducts) are clogged with plaque (ventsare often undersized.)

  • diymark November 27, 2015, 10:49 am

    Thanks for demystifying another closed world of residential construction. Nice looking install. For what it is worth there was an interesting article in this months Journal of Light construction (http://www.jlconline.com/how-to/hvac/drain-pain-furnace-condensate-proves-tricky-to-manage_o) that talks about the high acid of condensing furnace discharges, which can have catastrophic consequences for your plumbing (and the city) if not handled correct. Unbeknownst by me the condensing water is highly corrosive due to the combustion chemistry. It can eat your pipes, such as a cast iron basement floor drain or even the pipes in the street. The article enlightened me that a typical high efficiency furnace can produce 2,000 gallons of acidic fluid per year. The solution is a discharge neutralizer or at least the awareness that this might be a problem going forward. I’m not trying to scare anyone off from these superior high efficiency units. But it is another aspect to consider when installing.

  • CaBuilder November 27, 2015, 5:38 pm

    We ordered a top of the line goodman 40,000 BTU furnace from Alpine. When the furnace arrived, I read through the manual and found that the all the airflow specs were 2x as high as the spec sheet on Alpine’s site. That means the ducts would have to be able to carry twice as much air as designed for.
    I know this has to be a production error.
    I called Alpine and talked with Brian he said specs change all the time. Not by 200%!!
    I called Goodman, the techs said something is wrong and they would have the Product Specialist Don Mauk call. I am still waiting.
    After weeks of phone calls, we are sending the furnace back. They are going to try to charge us $260-$400 for their mistake.

    The problems only seem to be with 40,000 BTU 96% models.

  • Ardhanari November 27, 2015, 9:23 pm

    I rarely post on MMM although I have been a reader and admirer for a while. Frankly I wouldn’t dare take on something like this myself– I have yet to get comfortable with even much smaller home improvement jobs. However, I wanted to make a suggestion (which was also mentioned by MMM in a post a few months ago about chasing electrical demons): consider a geothermal system if your furnace is old. The capital cost is high (ranging from $20,000 to $50,000 for a typical house after rebates), but there are numerous advantages:
    – the geothermal system can easily be set up to do heating, cooling, and hot water heating;
    – you can live completely fossil-fuel free (after switching from an old oil-based furnace, we have no gas or heating oil coming in, and the electricity needed for kitchen appliances, washer-dryer, and geothermal is sourced from a wind-based provider in our state);
    – the operating cost of the geothermal system is very low in the summer (because 10 feet below the ground is invariably cooler than we want the house to be, so the system is very efficient in summer);
    – the main electricity cost increase is in winter, since the geothermal system will only heat the house to around the ground temperature of 55 deg F, and any additional heating is done by expensive electricity, but this is still significantly cheaper (in our case, 30% cheaper, and we expect savings to increase substantially as we improve insulation on our 50-year old home with single-pane doors/windows and leaky attic) than buying heating oil.

    While the high capital cost means that we may not see full return of investment for 8-9 years, the satisfaction of being completely fossil-fuel free and not having to worry about scheduling fillings of our oil tank (which, by the way, was a huge potential liability since it was underground) was worth it to us.

  • Nelson November 29, 2015, 12:22 pm

    Wow… I made it! I’ve finally read every single MMM article on the blog. What a journey! It’s taken me about 3 months of reading about 3 or 4 articles every day, on average, but I’m finally here! I suppose now I get to have the prestige of the title of full Senior Mustache, like MMM suggested in the comments of one of the articles in 2013 :)

    • Mr. Money Mustache November 29, 2015, 2:14 pm

      Congratulations Nelson! If I recall correctly, you’re supposed to add a mustache to your avatar picture. You don’t even have any picture yet, so you have some work to do. Gravatar.com

  • dave November 30, 2015, 9:09 am

    Lets hope there isn’t a sudden increase in home fires because of this article.

  • Mike November 30, 2015, 10:57 am

    Just wanted to say that I really enjoyed this article. Your DIY articles are my favorite. I’ve done a ton of DIY on my fixer upper house and will soon be tackling a couple bathroom remodels one of which will include the DIY concrete shower pan.

    A new furnace may be in the near future. we have an efficient one, but its from the early 90’s and may not last much longer, but we will keep using it until a major repair pops up.

  • Padobs November 30, 2015, 11:24 pm

    Great post!! I actually installed my own furnace many years ago being “gasp” a woman! Mine was damaged when the basement flooded and as a single parent I did not have the money to pay for a replacement. I found the exact furnace I originally had which was pretty modern for its time. Borrowed a truck and an appliance dolly to pick it up to and managed to maneuver it to the basement and remove the old one. After that, it was just hook up where the other one was, and cut a hole in the side for cool air return. Gas pipe, pvc, and everything was in the same place so I just hooked it up & checked for leaks. I did call a local hvac company for a “tune up” before firing it up the first time so total installation cost was about $90.00. That furnace still heats my house today. Never fear trying to do things yourself. A little common sense and patience will take you far. How much better and easier mustachism is now with all the knowledge available on the Internet. Thanks to all who share their wisdom and knowledge with the rest of us!!

  • Marty December 1, 2015, 7:09 am

    I am interested in installing two high efficiency natural gas furnaces in a duplex I own. These would replace the aging steam boiler that currently heats the building. In my own house I hired out the install and the duct work for the furnace three years ago to the tune of $7,000. I considered doing the install myself, but had trouble finding resources on running ductwork (where to put returns and registers, how many, etc) and sizing the furnace. Can anyone reccomend some books on the subject? Last month the motherboard went (after only 3 years!) and I had to pay $500 to have a replacement installed, it would have cost me $1000 if it wasn’t under warranty. I now realize I my should have done this repair myself, although I’m not sure how hard it would be to come by parts. Does anyone know how hard it is for an individual to come by replacement parts?

    • Scoop December 2, 2015, 9:03 pm

      When mine went it cost $107 off of eBay, way cheaper than even your “warranty” repair. Google and eBay are your friends, pull the part number off of almost anything from an appliance, furnace, car or anything else and pump it into a search. It’s very rare to not find a match and a source, then it becomes a best price vs timing exercise. A search for my furnace controller a “White-Rodgers 50A55-843” on eBay yields tons of the things that anyone can just order. Swapping it is a snap, set the DIP switches the same and move the wires over one by one to the same positions, just watch out for any revision differences/updates. Had to get creative a few times with temporary band-aid solutions while parts were on order. There are a bunch of sensors that can cause the thing to not start, troubleshooting up front is required to determine the root cause of the failure. Can be simple stuff liked plugged up venting or condensate drains and no parts are required.

      • Marty December 8, 2015, 8:03 am

        Thanks for the tip. I’ve pretty much lost all confidence in the value of warranties. The best “warranty” is taking the time to learn how to fix it yourself.

  • Metzler December 1, 2015, 12:27 pm

    Story time. My father-in-law has knowledge of this type of work, and he offered to help me take out the oil furnace and install a gas one, along with a water heater (we already had a gas line in the house). I learned a lot doing it with him, and the work was not too hard. The best part is now I understand how the whole think works. The house has 4 zones of hydronic (water) heat. We installed plenty of cut-off valves so that we can easily drain single section instead of the entire piping system if anything needs to be replaced. I doubt a plumber would go the extra mile to lay it out like we did, unless you specifically ask. The previously piping we took down certainly didn’t have any of it.

    On the other hand, getting the town to approve it was a nightmare. First, this work is only allowed to be done by a licensed plumber, so I was not allowed to submit the permit myself. I called a bunch of local plumbers, and no one was interested in looking it over and signing the permit.

    Finally, I got a plumber who was an acquaintance that lived 5 towns over to submit the permit. There was some minor work to be done, but since he was far away, and didn’t really know the local town ordinances, so he had no interest in doing the work. I also think he was scared about the liability.

    I let a year pass, and started calling local plumbers again, this time saying that the other plumber started the job, but he flaked out on me. That was more palatable to them, so a local guy did the minor work, and resubmitted the permit. I swear making it legal was much harder and stressful than doing the work.

  • Genevieve Hawkins December 1, 2015, 6:34 pm

    Awesome….do you know anything about adding a second story to an existing house? I’ve heard it’s insanely expensive, only for experts blah blah blah and that I should just convert the garage into living space if I need the extra room that bad. But I’m curious what any Mustachians would think of that impossible renovation….

    • Simon Kenton February 14, 2016, 7:43 pm

      A real estate agent familiar with men will tell you never to convert a garage to a room. You lose about 75% of male buyers. We want garages, we want them big, and the more DIY we are, the bigger. On resale you will lose everything you spent converting the garage, plus some extra loss.

      Second story additions (popups) are not hard assuming the first story walls are sound. But they take a longer time than most people anticipate, and the lower story is exposed to the elements for nearly all that time. It’s nice to have dry storage from the get-go; rent a warehouse or a friend’s empty garage. Trying to tarp the exposed lower area seldom works out very well. Trying to live in the exposed lower area for months is a marital strain made worse if there are kids. Other than that, it’s standard major renovation – no special skills for framing, wiring, plumbing, siding, ducting or radiant routing, drywall, roof trusses, sheathing, roofing, but you’ll be doing all of this. A plus is no foundation work. A minus is solving the heating problem. You’ll need a more capacious furnace, or use the height to increase the uptake of your solar collectors – but then if you want heat storage you’ll need to reinforce the floor/ceiling to take the new load.

      Put on a utility belt and help some friends with major renovations: popups or room additions or remodels. That will teach you what you need to know to do it, and give you a base for deciding yes or no and if yes how much professional help you want – plus it will give you some helpers when it is your turn.

  • Pooperman December 2, 2015, 7:12 pm

    My parents house has a furnace that is 90 years old. Seriously! It was originally coal, then it was changed to oil, and finally to gas. It still works, though it’s massive. It’s part of the steam heat system (also 90 years old). It might get replaced in another 90 years, who knows.

  • Brad December 5, 2015, 3:39 am

    I DID IT!!!!!
    As of today i have read from start to finish the entirety of this blog :D
    One article at a time, next button after next button press I’ve devoured every single word!!!
    Fantastic stuff and as i am only 21 at the moment i have found it early enough to make a difference, early retirement here i come. Woohoo!!!!

    • KY December 8, 2015, 2:03 pm

      Congrats! I wish I discovered mrmoneymustache in my 20’s or 30’s, I could have ruled the world!

  • Becky December 5, 2015, 10:04 pm

    I love reading your DIY articles, thanks for posting this! My skills still have a lot of room for improvement, but my mantra is that even tackling small repairs is better than nothing. Just last week my mom was out visiting and when I explained that the lock wouldn’t unlock from the outside on my driver side car door, she immediately said I should take the car in to the shop. I said I just hadn’t had time to research a solution. That night I spent some time googling, and ended up spending less than 5 minutes in the garage blowing a hair dryer into the lock. We had some unusually cold weather here in WA (it often doesn’t even get down to freezing) and it hadn’t even occurred to me that the lock could be frozen. It’s been working fine ever since, and my mom thought I was brilliant (the bar is set pretty low ;) ).

  • Jarrod December 6, 2015, 9:13 am

    Be careful in the sizing of the unit. Old equipment was 60% efficient… A 100k btuh unit only yielded a 60k output of heat. A 95% 70k unit yields the same output. Oversizing by 30% will cause short cycling and rot out your heat exchanger prematurely.
    As far as “licensed contractors”, why do I pay them to send out $10/hr hillbillyies that have worked for this slave diver for three whole weeks? Every trade is poisoned by these egomaniacs that hire the cheapest labor, treat them horribly and have insane turnover. If you hire a contractor make sure the license holder will be turning the wrenches.
    I have 17yrs of hvac experience and I have an engineering degree as well as licenses. I ALWAYS do my own work and I make a living correcting “the pros” who have fancy vans, logo shirts, and catchy adds on the radio. GET REFERENCES!!! It makes me money when you do!

  • Chad December 8, 2015, 9:27 am

    To clarify – you didn’t replace the furnace in your house. You’re still running your Radiant Heat, correct? I see your little * note, but I wanted to clarify.

    If you had furnace and A/C sharing the same ductwork, would you still go with radiant heating even though you cannot do away with the ductwork? That’s the dilemma I have. Someone before me installed a central A/C unit.


  • Brandon December 14, 2015, 9:44 pm

    Thank you for this awesome blog. My fiancée and I have been planning on having our natural gas furnace replaced next year. She purchased the house 2 years ago, and replacing the furnace has always been at the top of mind. I’m quite handy as my father is a General Contractor and owns his own business in a much larger city. My fiancée and I have been looking around at various companies for our furnace upgrade, but with my background and the guidance posted I feel I am more than capable of completing this myself.

    I have been also lucky to “rub elbows” with a member of our local gas and plumbing company, so I’m hoping he will be able to assist with any questions, or labour.

    I will make sure I visit City Hall to ensure all paperwork is covered, and also I’ll visit the homeowner insurance company to make sure everything is legitimate for them.

  • Brian December 16, 2015, 9:29 am

    Great article, I’ve been thinking about doing this myself. This helps.

    Have you read anything about the condensate water of high efficiency HVAC causing issues with the plumbing? The water is very acidic and it can corrode the pipes so it should be treated before dumping into your pipes:

    The nice thing about DIY projects is that they start out very tricky but eventually a lot of them become pretty similar.

  • KY January 1, 2016, 4:25 pm

    I’m thinking about tackling this task. Is there a online forum to ask questions?

  • Gordy January 12, 2016, 1:41 am

    My son showed me your site; and I’m glad he did, ’cause I need a new furnace, and your article has pushed me over the hump, where I am gonna do this myself. Thanks too for the link to Alpine. One more thing I need, is where to get that box that you show in the Household Air section, the one that makes an elbow from the return air plenum, and that the new furnace sits on. Is this a custom-built item?

  • 1WineDude February 14, 2016, 11:43 am

    This is AWESOME.

    Can *anyone* recommend the best way to find a contractor who is qualified and willing to do this type of work?

    I’ve been putting feelers out to the local/regional heating/cooling outfits about doing this kind of replacement, and NONE of them even want to touch it. DIYing this is a bit over my paygrade, but there is no way I am spending $9K on a replacement when $2k of equipment and maybe $2k of labor could get me to > 90% of the same solution that those guys want to sell me.


  • CapitalistRoader February 24, 2016, 5:57 pm

    Great site! Directed here from the New Yorker article.

    A couple of years ago I decided to replace the 35-year-old furnaces in my rental duplex. Considered DIY but Goodman—as noted above—would not warranty a DIY installation. Using Angie’s List I interviewed and got quotes from five HVAC contractors. The one that got the job suggested ways for me to reduce the cost including raising one of the furnace’s exhaust duct in the existing brick chimney (easy brick & mortar work) and extending the existing clay lining a couple of inches above the brick chase so the Denver building inspector could see it from the ground instead of having to crawl up on the roof. I think the two 4′ sections of clay lining cost me less than $50. Oh, and I installed 6″ cold air supplies on both sides. Bought all the sheet metal from HVAC Supply in Denver, who will sell to anyone.

    $4000 later I had two new 80% Goodmans installed. Since natural gas is so cheap and will probably stay that way for another 50 years, I figure it didn’t make sense to install high efficiency furnaces due to the added expense/complexity of running new PVC or stainless exhaust piping. As it turned out the gas/electricity bills are about 10% less in the winter than they were with the old (60%?) furnaces. Everything’s good.

    BTW one advantage of living in CO is evaporative cooling. No A/C required.

  • Bikeguy April 20, 2016, 9:44 am

    Another success story. I overheard a coworker say his furnace had failed and it would cost $4000 to replace. Sent him a link to this article, and he ordered his own and installed. He wishes he had waited for his mail order tools to arrive, as the metal bending tools from Harbor Freight were garbage.

  • porkchucker June 22, 2016, 12:24 pm

    Excellent, but I must confess I didn’t read the entire article. I just wanted to add two cents from my own experience. I do do not believe the high efficiency 90+ units are the way to go. An 80% is better and I’ll try to explain why. Yes, the 90+ efficiency units burn the gas more efficiently, but they also produce much more condensation. They rust and rot from the inside out. I’ve replace two of these for every 80 per-center that I’ve installed. So what in truth is greener? In my book its using less natural resources to manufacture these things. Installers push the 90% knowing that they’ll be back in 10 years. My 80 percent units are still in great shape, some at 20+ years. ……PS I’ve noticed reluctance from local stores to sell to DIY’s. I found some nice new Goodman 50k t0 75k BTU units on Ebay for around $600 delivered. Can’t beat that! Reliable, and perfect for any 1000 sq/ft house. IMHO.

  • Liebling August 17, 2016, 12:43 pm

    My husband is an HVAC technician here in Ontario, Canada. According to the gas code, a homeowner can work on their own equipment if it is in a fully detached home. The equipment must then be inspected and the gas turned on by a fully licensed technician. I absolutely agree with doing as much DIY as possible. My husband and I purchased our first fixer upper recently and our goal is to do as much as we can on our own. On the flip side, people need to acknowledge when something is beyond their scope of practice. I used to work at a big box home improvement store and met many DIY customers who were truly frightening. Some people wandered in looking for random furnace parts they had no name for… Another guy wanted spools of electrical wire to wire his basement for lights and outlets… Scary stuff. Knowledge and skillset should always be considered when tackling a project and not just total expense.

  • Cheda Martis October 23, 2016, 3:38 pm

    I know this is an old thread that may not be monitored, but I’m hoping I might be a reply :-) . I bought myself an incredible old Victorian on the fall line in middle GA in 2015. November last year I discovered the one million year old gas furnace did not work (it had been converted to propane. We ordered a propane tank, but when they brought it out the guy tried to light the furnace and said it was kaput. I wasn’t there, my 72 yo mother was. Interestingly, he had a new furnace he could install! Uhuh. He also told her all the ductwork was bad, which made me know he was looking to scam because the ductwork is perfect). Anyway, I did a lot of debating but I really want an all-electric house. I plan on installing solar and working toward off the grid. I also think piping something into your house that can blow you up, poison you, light your house on fire, etc is a little mad if u don’t have to. For some reason it’s really difficult to find electric furnaces. I’m assuming this is because combustibles make more heat and more efficiently. Am I wrong? If this is the case, then elec for me! It was 80 degrees on Xmas last year. It only gets cold for about 2 months and maybe 3 days the whole year below freezing. I also don’t need it hot in the house, 65 is pretty good.
    Ok, all that down, can you tell me if swapping out the gas for electric is as straightforward? I’m assuming I don’t need gas venting and I will probably have to run a 210/220v line with separate breaker. But, otherwise is it going to be the same?
    Not only am I like u and prefer to do things myself if I can, for many reasons, not the least of which is I hate getting hosed, but add to that the fact that I can’t get anyone to come out to my tiny town to even do an estimate let alone the work. I have no choice here, really.
    So, if anyone’s still there, I would be really grateful for a little advice. And thanks for being there with what a lot of us know; it’s ok to take care of your stuff yourself.

    • Mr. Money Mustache October 25, 2016, 5:37 pm

      If you’re in such a warm climate and want to do electric, why not just throw in a few baseboard heaters and/or some radiant pads under tile floors as you upgrade?

      I agree that the days of the natural gas furnace are numbered – it’s a fossil fuel after all, and cheap renewable electricity is already here. But forced air is really a concession to the limitations of burning gas. When you give that up, you have much better options for heating.

  • Dan The Gas Man January 31, 2017, 10:11 am

    I would like to add this post. I live in Ontario, Canada. Please be aware that while any home owner may install anything in their own house as long as you get a permit and an inspection done, it is illegal to do any kind of certified trades work in your neighbor’s house or any other house without being a Certified Gas Technician as is the case for a furnace install. A G2 can work with up to 400,000 BTU’s in residential homes only and nothing in commercial buildings. Just saying. Be well.

  • LLB June 18, 2017, 7:58 pm

    Wow, legendary post MMM. Seems like this is the classic thing where everybody would say “oh my goodness, you can’t do that yourself”. If also seems to be a very high-profit industry. Here in Calgary, the furnace guys charge over $100 just to come, and then they bill for everything they do and it always seems very expensive.

    Way to think outside the box, very impressive!

  • Fernando October 18, 2017, 8:57 pm

    Thanks for this post, it was one of the first things to come up when i searched for DIY gas furnace install. So glad I did it. Alpine should pay you a commission for all the customers you’ve brought them! ACwholesalers.com is another good source, they offer free shipping over $100. I bought a 2 stage 96% efficient goodman for just under a grand. Was able too get it up and running in about 10 hours. The hardest part was cutting a 2″ hole through the wall, now that I have the right drill bits, it’ll be easier for the next one. I am lucky enough to have a father in law with a ton of tools, I used his pipe cutter and pipe threading tools- it was good exercise, very rewarding to cut your own as pipe lengths on the spot. I’m a hard core DIY guy, I taught myself guitar, rebuilt engines in cars, sheet rocked an entire home, replaced gas and electric water heaters, replaced a home oil tank, built my own shed from scratch, even helped my wife have our first girl at home recently :) I thank my father for giving me the guts to take a stab at anything and everything before opening my wallet. And it paid off. My wife and I have two homes paid off and we’re in our early 30s with no debt whatsoever (I dont make a ton of money and shes not working anymore to raise kid). I will be ordering another Goodman furnace for another tenant if this one proves itself. Good luck everyone. Just be smart and play it safe when tearing into things, that’s all. Do your research; with youtube and google, there’s no excuse anymore.

  • Bill Carson November 12, 2017, 4:39 pm

    Thanks for posting this article. The way you broke it down step by step made me feel confident to take on replacing my 58 year old furnace. I just finished installing my new furnace and it’s purring like a kitten. Not only was it gratifying it saved me a lot of money. Thanks again!

  • Courtney August 2, 2018, 12:17 pm

    Thank you for the very informative article! I’m 25 and bought my first home two months ago. What you have talked about with most of the home improvement industry being so secretive and not willing to provide any actual information to DIY-ers has been a nightmare. My house was built in 1910 and the main soil stack was cracked, and when I had a professional quote the job they said 12k for 3 days of work. So I did some research and through a bit of trial and two weeks and only 2k in materials have replumbed my whole house. It’s such a reward to have been able to pull that off. But my next step is replacing the super old gas furance with a high efficiency electric one. And finding any information has been super difficult. I very much appreciate your blog and your attitude towards helping DIY-ers invest in their own futures without ripping them off.

  • J April 23, 2019, 6:28 am

    I’m planning on DIY replacing a rusty 25 year old 80% furnace and 30 year old AC unit with a 96% Goodman unit. Looks like about $2700 for the furnace + AC. Will pay an HVAC guy to remove the R22 from the old AC unit. Then after I finish the install I’ll call him back to braze the copper lines, pump down, and check for leaks.

    Thank you for writing this post. It’s a bit disheartening to read comments like “in my country it is ILLEGAL to even THINK about working on ____ myself!” how did we get to this point where so many gladly relinquish their rights to bureaucrats? People who like the challenge of a DIY install — researching and understanding a new skill — and enjoy the satisfaction of a job well done are turned into outlaws.

    • Mr. Money Mustache April 24, 2019, 8:13 am

      I agree, J. Laws are always a balance – so while it’s good to have laws to protect us from truly dangerous stuff like killing each other drunk driving and speeding, we probably don’t to over-regulate low-risk stuff like connecting a few pipes and wires.

      Sure, you CAN cause an explosion or a fire with a gas leak or electrical fire. But you can do the same thing by tripping over your water heater accidentally while carrying the laundry basket through the utility room. Do we therefore also need to regulate laundry?

      No, because the risk is low enough that the cost to society of the new rules is greater than the savings from MAYBE preventing a few injuries or deaths across a country of 320 million. Every rule comes with both a benefit AND a cost, which I think is what makes me a political centrist in the olden days definition of it.

  • Monte Montgomery October 30, 2019, 7:24 am

    I was going to bring up the setting the gas valve pressure as well, you should definitely include that. Easy to do and there are videos to help using a manometer to set it. I was having minor furnace ignition problems and like you, I hate calling for outside contractors. Turns out my LP furnace gas pressure was set up too low by the pro installer. Pressure from the propane tank was too low and was never adjusted properly in the first place on my new house to achieve proper pressure at the furnace, which was not even close to spec. This was not the cause of my problem but it shows that even large furnace contractors are far from perfect. Anyway, learned how everything is supposed to work and be replaced need be through the internet and component instructions, and am now totally self reliant on furnace needs. I’m looking at this website because I do plan on replacing my furnace when the time comes. My furnace is 14 years old and running perfectly, so I’m not sure when that’s supposed to be, never serviced by a “pro” btw, just me. I love the DIY stuff on the internet, haven’t called for outside help on anything in decades. I was always a DIY guy and a self taught pro auto mechanic, the internet has helped immensely on so many things. There has never been a better time for the DIY’er.

  • Curtis November 17, 2019, 7:28 pm

    I’d like to see other angles so I can see how your cold air return box draws air into the furnace. Your furnace is like the one I’m installing, but previous ones I’ve used draw air into the side. Also, a link to that part would be great so it is easy to order.

  • Richard December 18, 2019, 8:29 am

    I like your ‘Scary’ photo at the top. The house we lived in 1967-84 had one of these. Coleman brand, little 3 1/2″ round ducts led to the room outlets. The room outlets were tall ugly things that jetted heat out at chest level while pulling cold air off the floor. Located near the center of the house, not by the windows. Efficient in that only the top 1/2 of the room was heated.

  • James L April 20, 2020, 2:52 am

    Some of the posts I read from licensed contractors could not be any more blatant on their disdain for losing business to DIYers. With an inspection and and 100 different error codes most systems are equipped with via an internal cpu, you are safer on your own than say a Home Depot sub contractor that lies about the inspection and 9 years later find out your son has been being poisoned with carbon monoxide and hasn’t had bronchitis multiple times per year for any other reason than a poor install. If you are confident enough, everyone should do their own work. Workers installing a unit in an upscale neighborhood don’t give a rats ass if you die or not. I’m currently in the midst of litigation with HD and it has been mentioned many times that HD will avoid inspection at all costs.

  • Jonathan August 19, 2020, 5:14 pm

    What a nice blog!
    Has anyone installed a mrcool heat pump? The universal model, with quick connect fittings. No hvac technician required.

    I will attempt to do this year, and wanted to see if anyone has any pointers.


  • Doug Kirchner February 14, 2021, 12:35 pm

    Great detailed outline. You are absolutely correct that this wouldn’t make it above a 6 on the diy scale. I just want to point out that there is cost in owning and operating a business and most contractors I know in the HVAC business are reputable and do a very good job. I am not a contractor but I work within the Hvac community and almost every company I speak with takes great pride in providing quality in their work. I will also point out that living in Wisconsin (it is -12 without the wind chill as I am writing this) that if something fails most don’t have time to wait to troubleshoot and locate parts to repair their equipment. They call a professional to come in and get the heat back on and expect it within a couple of hours.

    I would never contend that anyone not endeavor to know and work with all the systems in their home. It is becoming a lost art. I am one of those guys too! I would point out that there is a difference between brands. You chose Goodman which 10 years ago was just garbage. It was bought out by Diakin and the quality is much better, but there are other brands in the on-line universe that are suspect. Do you homework and try to stay with the more mainstream of brands of which there are many. Next understand that the prices charged by contractors is more than just $50/hr. They have insurance on vehicles buildings, workmanship (errors and omittions) , along with healthcare, etc. They have training costs CE requirements, and they have dreams and goals just like you and I and $45K a year isn’t a kings ransom. They are a service not a charity. So go ahead if you want and do this work yourself, read carefully and be prepared with the right tools to finish the job. Good on you! But lets at least appreciate that the professionals do have a role in the safety and comfort of your home. All the best.

  • jane-o February 15, 2021, 10:58 am

    Why not a heat pump space heater to reduce greenhouse gases? As electric grid becomes more and more de-carbonized, this would seem a better environmental choice, right?

    • Mr. Money Mustache February 15, 2021, 3:28 pm

      I agree! Heat pumps have advanced in efficiency and dropped in price even since I wrote this article, so I’m currently getting ready to do a DIY whole-building heat pump replacement for yet another old gas furnace. If it goes well, I’ll write another article about it. And link to the new article from this older one.


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