Is A Two Stage Furnace Worth It?

 April 30, 2020

By  Robert Bradford

(Updated April 30, 2020)

A common question when someone needs to replace their furnace is whether to choose single stage or two stage.

Most people pay very little attention to their furnace. The first time we even hear the option of two stage heating is from a contractor’s salesman. Often these salesman wax poetic about how much better a two stage furnace is over a single stage furnace.

But before you choose, I’d like to give you my real world experience with these furnaces. You’ll find it quite different from what salesmen are telling you.

What Is A Two Stage Furnace?

Two stage heating describes a furnace with two levels of heating output. First stage (low fire) will run between 70-80% of the rated capacity of the furnace. When a furnace initially turns on it will start in this low fire mode.

The furnace will go to second stage (high fire) based on one of two conditions. 1) The time the furnace has been operating or 2) if the thermostat senses it needs to raise the temperature more than two degrees.

For example, when a furnace rated at 80,000 BTU’s first turns on it will operate at 60,000 BTU’s (75% of 80,000). After running for 10 minutes the furnace will determine it is not keeping up with the need and switch to high fire to meet demand.

The difference between single stage and two stage furnace is the single stage furnace has only one heating output. If it's rated at 80,000 BTU's, it will only operate at that capacity.

Beware Of The Two Stage Furnace Hype

Certain heating contractors will rave about the claimed benefits of the two stage furnace. "It will save you money in utilities, be quieter and keep your home more comfortable" they'll claim.

At first, the two stage furnace might look attractive. After all, who doesn’t want those things. But does a two stage furnace really offer those benefits over a single stage furnace?

Is A Two Stage Furnace More Efficient?

A two stage furnace is NOT more efficient than a single stage furnace. It will not save you any money on your utility bills compared to a single stage furnace. In fact, until 2019, it has most likely cost MORE to operate.

A 2006 study completed by ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers) said this: “…while the 2006 ASHRAE test procedure shows only a 0.4% decrease in fuel consumption. The electricity consumption of two-stage furnaces as opposed to single-stage furnaces increases by 11%…”.

This increase in electricity consumption was mostly from the blower motor due to increased running time. It is only recently that has changed.

In 2014 the US Department of Energy established a national efficiency standard for furnace fans. The DOE’s FER rule (Fan Efficiency Rule) took effect in 2019. This meant manufactures could only make furnaces with more efficient ECM (electronically commutated motor) motors. The good news is these new standards required a 46% watt reduction over the older style motors.

These motors were not new. You could buy a two stage furnace with an ECM motor for over 15 years. By far though, most two stage furnaces were sold with the older less efficient motors.

But more importantly, remember this changed in 2019. And manufacturers and contractors have been pushing the “save you money” claim for over 20 years. So that means for 20 years consumers that bought a two stage furnace believing they’ll save money have been hornswoggled.

Saving money comes down to the efficiency of the furnace. And efficiency is determined by heat exchanger design. Single stage and two stage furnaces of the same efficiency style use the exact same heat exchanger.

In other words, a 95% efficient furnace is 95% efficient whether its running in low or high fire. No savings.

Is A Two Stage Furnace Quieter?

Yes. But so is any new furnace compared to the old clunkers we've had in our homes for the past 30 years.

In addition, most of the noise that we complain about comes from the amount of air that is circulating through the duct work. Whether your furnace is two stage or a single stage, the required amount of air needed for the furnace to operate properly will be the same.

Does A Two Stage Furnace Keep You More Comfortable?

Who doesn’t want a more comfortable home? The two stage furnace claims it creates this by helping prevent temperature swings. How? By running at 70-80% of it’s rated capacity in first stage, the furnace will run longer thus reducing temperature swings.

The idea seems plausible but the field tested numbers do not support this claim. Here’s an example:

Let’s use an 80,000 BTU furnace as an example. If the airflow is set properly this furnace would raise the temperature of the air circulating through it about 50°. (Ex: 70° air coming into the furnace, 120° out to the supply ductwork)

At 75% capacity the furnace would be burning 60,000 BTU’s. Although heat output is changing, if the airflow is set properly the temperature of the air at 75% capacity would be only 7°-10° lower. It would be difficult for anyone to notice that small of a change.

Field tested numbers also do not agree with effects on run-time. Remember the study from ASHREA in 2006? They showed an 11% higher electrical consumption from a two stage furnace. Most of that from the blower motor. That means the furnace operated about 10% longer than a single stage furnace.

So if, on average, your single stage furnace ran for 10 minutes, the two stage furnace would run for 11 minutes. This would have little to no effect on temperature swings.

Single Stage Versus Two Stage Furnace

A single stage furnace has two major advantages over a two stage furnace. They are less prone to breakdown and cost less to repair.

The parts also tend to be more universal. A heating contractor working on your furnace in twenty years will be more likely to find needed parts. This also helps to get your furnace up and running quickly in the event you do need a repair.

Are there any two stage furnace benefits? After digging past all the marketing it’s easy to see that a two stage furnace does not have any benefits over a single stage furnace.

Now you’re asking “If the two stage furnace doesn't save me money or make my home more comfortable why should I spend the extra money?” The answer is you shouldn’t.

Keep this in mind when deciding between a single stage or two stage furnace: A new single stage furnace installed by a good heating contractor will keep you comfortable just as well as a two stage furnace.

Why The Push For Two Stage?

Contractors often push two stage furnaces because they are lazy​.

Rather than taking the time to do a proper calculation to determine the correct size furnace for your home, they install an oversized furnace.

They figure the low fire mode on the two stage furnace will make up for their poor workmanship. It doesn’t and creates a host of other problems.

Manufacturers on the other hand push features that have benefit for themselves. They’ve tried for years to improve profits by having higher equipment turnover.

They need furnaces to fail after 10-20 years not 20-30. They have succeeded by adding expensive and repair prone features all while telling you its for your benefit.

Two stage furnaces are designed to fail earlier. They won't fall apart sooner than a single stage furnace. But repairs on a two stage furnace in the 10-20 year mark will cost considerably more.

Replace the furnace or pay for high mark up parts. Either way manufacturers and contractors win.

You can avoid the manufacturers trap though. Don’t focus on equipment options. Focus on choosing the right contractor. Nothing is more important.

You will be happy with any single stage furnace as long as it's installed by a good contractor. And a good contractor won’t recommend features that you don’t need.

If you want more information on how to find good contractors click here.

Robert Bradford

I'm Robert Bradford. I've been in the heating and air conditioning industry for 30 years. Ov​​​​​er 40 if you count the years I helped my father as a kid. On this site, I share everything I've learned about finding the best HVAC contractors and equipment for your home. I'm happy to say that over the last few years, The Comfort Academy has grown into a trusted site with thousands of informed site visitors each year.

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  1. Thanks a lot. Your information was very helpful. After 30 years of the same oil furnace it was time to switch to a natural gas furnace. The decision was between a single or double stage furnace. Reading your article it was a know brainer. Thanks

  2. Well thank you for this article. We have a 30 year old furnace that has really given us no problems. However, we want air conditioning and so if we are doing one we decided to do both. We have been given three bids one salesman in particular was pushing a class 2 high priced unit about 2k more than the others. He also mentioned dry air and allergies trying to upsell a new humidifier and electronic air cleaner. This article has just made our decision so much easier.

    I am a home renovation contractor and until doing our research I was completely lost on this issue. Now we have go choose between Lennox and Trane or go with the cheaper model made by Coleman???? Single stage of course!

    thanks andy

  3. Thank you for this. I’m just putting a new system in and this was helpful I’ve wondered why many sales people and places online claim a two stage would be more fuel efficient — which seems illogical given the parameters of a fairly “closed system”.

  4. Thank you , great article. I was wondering what size furnace I would need, mine is a 35 year old ICS 90,000 btu @ 80 % efficiency in a 2000 sq ft 1981 house with newer windows in Toronto, Ontario. It really doesn’t use a lot of gas but I believe it is near it’s end of life. I have had quotes for 80,000 and 60,000 btu @ 97%. The contractor who quoted me the 60,000 btu was sure thats all what my house needs. Neither did a load cal.

    Thank you in advance.

    1. Hi Tino – I wish I was able to tell you what size furnace you need but there are just too many factors that require a physical evaluation to make that determination. What I can tell you is this; the most important aspect with regard to the size of the equipment is a performance guarantee in writing from the contractor. As an example: “Your furnace will keep your home at 70° when the outdoor temperature is at 0° or we will replace it for free.” Go with the contractor that stands behind their work in writing.

  5. Is there any truth to what I’ve heard about a two stage furnace being better for really cold climates? I live in Chicago and need a new furnace but money is really tight and I don’t want to get sucked into features I don’t need.

    1. Good question Patti. First I would tell you that no one NEEDS two stage equipment. Your current system is most likely single stage and if it’s sized properly kept you comfortable in the winter. Second, a home in a colder climate will not increase the reasons to buy two stage equipment. If anything it would go the other way. Two stage furnaces are marketed to help durning milder weather when you don’t need the full BTU’s of your heating system. You’re more likely to need the full output of your furnace in a colder climate. Hope this helps you save some money!

  6. Thank you so much for this article. I have been searching in vain for objective information online about the pros and cons of single versus 2 stage gas furnaces, and yours is a lone but compelling voice of reason in the wilderness. It seems clear that the utilities, manufacturers and contractors are well aligned in promoting the “better efficiency” hype for 2 stage furnaces. It’s a real struggle for the homeowner/consumer to sort out fact from fiction, especially since 2 stage furnaces haven’t even been around long enough to assess their true lifetime/out of warranty repair costs. In my area (Pacific Northwest) our utility will not rebate for any efficiency below Energy Star level, even though a minimum 92% efficiency is already mandated by the Canadian government. HVAC quotes are structured so that the single stage is more expensive than the supposedly superior 2 stage! A single stage furnace has served my needs adequately for 30 years, and I’ve no desire to have a furnace and fan running continually in my house all day long no matter how quiet. The “better comfort” claim is purely subjective and depends entirely on the occupants’ personal heating habits and preferences, house size and layout, other heat sources, and so many other factors. If a contractor pushes the “higher efficiency” claim for 2 stage furnaces, show them this ASHRAE study whose results indicate that two-stage technology by itself does not save energy: https://aceee.org/files/proceedings/2006/data/papers/SS06_Panel1_Paper16.pdf

    1. Hi Margaret,

      Thank you for your kind words. I’m glad you found the information helpful. And thank you for the ASHRAE study link. Hopefully more studies like this will be published that debunk the marketing hype.

  7. hi Robert

    Good article, I’ve been researching furnaces for a little while now, a load calc of my house indicates I need a furnace with an output of approx. 63k btu’s. An 80k btu 92% furnace has an output of 73,600 btu’s 10k more than I need, and 60k btu 96% furnace has an output of 57,600 btu’s 6k less than i need. (80% AFUE furnaces are not sold here).
    So the question is do I go over or under? A 2 stage 80k btu 96% offers a little bit less but more if I need it. In this case a 2 stage seems to be the answer.
    What do you think? The 2 stage is only $300 more.

    1. Hi Glen,

      Glad you enjoyed the article. In answer to your question, I never recommend choosing a furnace sized lower than the calculated heat loss for your home. I also never recommend homeowners doing their own load calculations. They’re more complicated then you’d expect and free online programs that offer this service are terribly inaccurate. Make sure you choose a contractor that can do a proper load calculation for your home. Then the sizing of the equipment is their responsibility.

      Also, watch out for the seemingly low cost of $300 to “upgrade”. Manufacturers want you choosing 2 stage so down the road when it needs to be repaired more often and for a higher cost you’ll be forced to choose new equipment.

  8. Thank you for this detail, really helpful, i have qoutes for single and two stage options, the comfort claim has been touted but no real benefits other than that.

    We are from overseas (UK) and the whole furnace thing is new to me so this has helped lots

    I will be going with the single stage for sure.

  9. What a wonderfully honest article! I am a Professional Engineer and when I purchase a product I really get into reviewing product brochures, specifications and studies. Which as this article has clearly confirmed is a bunch of Industry “flush” and unrealistic product expectations. In my 30+ years of Engineering I go right to the Contractor for the real deal! The National Grid rebate being $420 for Two Stage versus only $140 for Single stage is what drew me into the Two Stage. Acting as clueless home owner I requested two quotes for High Efficiency furnaces from local HVAC Contractors. Both pushed 2 stage and blew off the single stage even when I asked about them. The I called in and HVAC guy that I have worked with for commercial HVAC inspections. Not a friend, just “good old boy” Contractor with 40 years experience. Good News!! He stated exactly what this article and your comments reflect! He won’t even install a 2 stage unless the owner insists and he makes it very clear of impending potential issues. Thank you so much for your article and comments. I will be having have a single Stage 96% being installed in a week or so.. As soon as I choose a new Hot Water Heater as mine will be Orphaned with the new High Efficiency Furnace… Now all I need is help with choosing a Power Vent or Tankless Water Heater :-) Do you have an article or a good source for honest info on that? Thanks Again!

    1. Thank you Steve. I’m glad the article helped in your research.

      I don’t have an article about choosing a water heater but I’ll give you my short answer. If it’s an option my first choice would be to keep a standard water heater and install a liner in the chimney so it’s properly sized. If that’s not an option I’d recommend a power vent water heater over a tankless. Tankless have many disadvantages. 1) The savings and lifespan are GROSSLY overstated. 2) it requires annual maintenance (which can be costly) and 3) it WILL be more troublesome. There are other factors in your home that need to be considered so find a good contractor that you can trust. Hope this helps.

  10. Thank you for this article. I now have to replace my 20yr old gas furnace. My personal friend who is also a HVAC Contractor, altho what I would call a “green horn’, being in the business for only 5 yrs., and I am an old schooler. He strongly recommended against a 2 stage(fancy schmancy-his words), given his experience including his own home furnace. In researching gas furnace companies it seems I can’t get past the joys of 2 stage hype-which caused me to doubt my lovely green horn friend. You have confirmed his advice & provided me more schooling. Again, THANK YOU!!

  11. I actually think 2-stage would solve some problems here. We have a house on one level, super-long runs, poorly insulated. In the coldest days it is demanding huge BTUs. Shoulder season it is OK and therefore sensitive to “blasting heat” syndrome. I think longer run times would balance out air distribution much better, keep the air in those crawlspace runs from getting cold before the next run, and yet manage the blasting heat issue well. Just- the oddities of our system.

  12. ksmarkin@gmail.com
    I’ve had 4 contractors only one recommended a single stage furnace. We have an addition that never gets warm or cool enough. He said we need the stronger blower to get the heat into that room. Does this sound right to you? Do you recommend one single stage over another (Carrier, Bryant, American Standard).
    Thanks for your help – great article!!!

  13. I just purchased a 2 stage furnace from One Hour Heating and Cooling. So far I hate it and want it out. They are coming again tomorrow, because I don’t understand the concept of this thing going on and off every 6/7 minutes. All I see is dollar signs for the electric bill. I just replaced my 40 year old furnace that was working, but thought it was costing me to much in gas, but now I’m thinking I jumped the gun and shouldn’t have done it. The guy that came out once already told me that it’s not going to cost me me in electrity that it’s like turning on a light. Well old school me hears my parents yelling at me to quick switching that light switch on and off or opening and closing the refridgerator. I hope they can replace this.

  14. Hi there…really appreciate this article. We are dealing with a contractor who has an excellent reputation for integrity and customer service (and maintenance plan). He explained that the 2 stage furnace (versus the one stage) has a type of fan than can keep the air circulation going in the house for less cost (and that keeping a fan going is recommended for air quality?)…The choice is between a Goodman stage 1 furnace, or the lowest level stage 2 with the particular fan. I’d like to

    I appreciated reading that the single stage might be sufficient for comfort in our home. We’ll be replacing a 25 year old Carrier unit, so I am thinking anything will be better than what we have currently. We are in a poorly insulated 1 1/2 story, sub zero temps 5-6 months a year, and 70s slider windows. Baseboard heat on second level, plus one vent from furnace.

    Is air quality from a fan that runs frequently/constantly going to make it worth the extra $800 plus dollars?

    Hope to hear from you soon.

    1. Hi Jaylene – I’d recommend two things. 1) Take a look at this article here on why you would want to avoid a furnace with a DC motor (also called a variable speed motor). 2) Get a few more proposals and focus on finding the best contractor. They’ll help you choose the best furnace for your home.

  15. Further to my last email…the 2 stage goodman model (GMEC962 – 40,000 BTU) has a DC motor and is energy star rated…I’m not sure why this would be better than the 1 stage model (SS96 – not energy star rated and not DC motor) Claim is that we would save more in hydro bills than with single stage, and that the air quality would be better.

    Again, 1 1/2 story home, replacing 25 year Carrier model. Baseboard on second level (only one vent upstairs)


  16. So, So confusing. I just got an estimate for a 2 stage and single stage Trane furnace The 2 stage was highly recommended
    by the contractor. I do trust these guys as they have been around for many years. Here’s the scoop, it’s an 80,000 BTU and
    80% efficiency. My house is around 13 hundred square feet not including the unfinished basement. The house was built in the fifties and isn’t well insulated. It’s very drafty in the living area where we spend most of our time. The upstairs has two rooms with one vent in each room and no cold air returns. Would a 2 stage help keep a drafty room more comfortable, and
    would the lower speed not heat the upstairs well.As it is now, we need a space heater to supplement the upstairs room. I have an 18 year old Armstrong single stage now and it’s still kicking ass with zero repairs so far. It’s getting up in age and was a little concerned with the 25 below temps last week in Chicago. The only reason I’m considering a 2 stage is once my furnace shuts off it’s chilly in ten minutes because of the draft. The 2 stage is around 600.00 more than the single stage.
    Also, does 80,000 BTU’s sound about right for the info I gave? I would also get an AC unit with the new furnace. I’d like to
    keep going with the Armstrong but am concerned with an expensive repair. The price I got was 6,100 for the 2 stage with a 2 ton 13 ser AC unit. Of course, minus 600.00 for a single stage.

  17. Hey guys. Your website is very helpful. I have gotten a quote on a two stage. After reading your information I decided to go with a single stage. But I was surprised that there was only a 295.00 dollar difference. That does not seem right. What is your opinion

    1. Hi Stanley. Glad you found our information helpful. It’s true that there won’t be a huge difference in cost up front between single and two stage furnaces. The savings will add up down the road in the longer life of single stage equipment and lower costs to repair.

      1. Here in Winnipeg Canada, I was given a quote of $2900 for a single stage and $3900 for a dual stage with variable speed fan, so quite a bit more.

  18. After rereading the threads, I went with a 60.000 BTU single stage. I’m very happy I did because a 2 stage would have been a waste of money. I’m glad I found this site and many thanks for all the great info. If I can still get the 10 year parts and labor for the sale price of 389.00, I might do it. The company has been around for 80 + years and isn’t going anywhere.

  19. John, after a sleepless night, we’ve decided on a single stage furnace.
    EVERYTHING I’ve read on-line suggests that 2-stage is the way to go, but the man installing our furnace stands behind his recommendation of a single-stage furnace, so we’re staying the course.
    I’m concerned about cool spots, and about noise, but your article has convinced me that the 2-stage is not worth the additional investment.
    Thank you!

  20. Thank you for your article! it has helped me clear out my questions about single stage and 2 stage furnace. I’ll be going with a single stage.

  21. Thanks for the useful information. Are there any circumstances (e.g. type of climate; structure of house) in which a two-stage furnace would make more sense than a one-stage?

    1. Sure. Staging is important on larger systems. Like those in commercial and industrial applications. There are also times that I’ve put a two stage furnace in a home but not for the sake of just that feature. For example, a homeowner with allergy problems needs a system that constantly cleans the air. The best furnaces for that application only come with two stage heating.

      1. Thanks for the info. I have separate furnaces for upstairs and downstairs that need replacing. With dual furnaces, one supplier I am talking to says that 1-stage furnaces will be fine and I should not spend the extra money. Another supplier recommends 2-stage because they will be “quieter and keep the house more comfortable.” I am guessing you would side with the first supplier.

        Also, do you have a view on the effectiveness of ultraviolet air purification vs high MERV filters? Is there any reason to do both?

  22. Hello — I am building a home and they include a Carrier 13 Seer AC and Carrier Comfort 95 furnace (both single speed) The HVAC rep is trying to upgrade be to a 17 Seer variable speed and the Performance 96 furnace, also variable speed. I’m amazed at the upgrade cost (2600 for AC, 2000 for furnace) as it seems outrageous. Home is 3k square feet, 2-two story. Stick with the single stage?

  23. That’s good to know that a single stage furnace would be able to be fixed for cheaper than a two-stage one. Plus having it cheaper to fix would also save you some money on repairs. I’ll have to make sure to get a single stage furnace if I decide to replace my current one.

  24. THANK YOU so much for HONEST info!!! Does the same go for 2 stage air conditioning? I presume it does, but wanted to double check!

  25. Got a question: I have heard that they can make single stage furnace with a variable speed ( aka DC ) blower. Would there be an advantage to this configuration?

  26. This has been very helpful. I didn’t understand the difference between single and two stage furnace. Being a woman with no knowledge of furnaces, I have to put a lot of trust in the sales person. The more I know the better. Thanks.

  27. Thank you so much for this article. Great help. My house need to change the air filter system and the central humidifier. Thus, thinking about a new furnace too. Current furnace has nothing wrong, 13 years old and it’s a mid-efficient or low-efficient Keeprite furnace. Do you think I should get a new furnace now? I am just thinking that the old one may die in a few years, but maybe I will save some money on utility bills?


  28. I wanted to add to this thread that my husband chose a two stage hvac system last year. I am a light sleeper and I hear the unit turn on and off when we have extreme weather. The unit seems to run all the time and is very annoying to me. Hope this helps someone in decision making!

  29. I have a Goodman 3 ton central ac unit with 2nd stage heating or i can set it at single stage. When using single stage only it takes a while to raise the temperature, at least 45 minutes for a degree or two. With 2nd stage heating it only takes 15 to 20 minutes.With 2nd stage the amount of air coming out the vents seems almost double the output into the home.My bill does go way up using 2nd stage vs single. In my case 2nd stage will heat the house way faster and more comfortable sooner.

  30. We have a 25 year old two stage (here when we bought the home) and are now replacing it. The cost for a same type seems ridiculous (we have a 1500 sq ft ranch that is well insulated). Glad I read this article – going to go with a standard type furnace.

  31. Dear Comfort Academy,

    Happy New Year!

    Yours is the ONLY article I have found on the internet recommending a single stage high efficiency furnace versus a two stage.

    I have a 2 stage furnace that has lasted 19 years. In those 19 years, I have had to replace the variable speed motor ($1200) and Motor Module ($1197).

    Any savings I made in fuel efficiency have disappeared in those repairs. Also, I did not notice any significant savings in my gas bill over the years from installing the 2 stage furnace.

    After 19 years, it is time to replace my furnace because I have a rusty secondary exchanger.

    Question: Will a single stage high efficiency vent through my chimney again or will it continue to be vented through the PCV pipes outside the wall of my home. I cannot find the answer to that anywhere on the internet

    Thank you for responding to my question.

    1. Thanks Jay. I’m glad you found the information helpful.

      To answer your question, yes, any furnace over 90% efficient will vent with PVC through an outside wall.

      On a side note, the secondary heat exchanger is stainless steel. I’ve never seen one rust. I’ve seen them form a hole over time, carbon up from improper combustion or get plugged with debris from improper installation. A contractor that told you it’s rusted may be a red flag. Think about getting a second or third opinion.

  32. Hi! We need to replace our single stage furnace that is about 17-18 years old. Does single stage or 2-stage matter if each floor is zoned? Our home was a one story ranch and about 10 years ago we remodeled and added a 2nd story (now 2200 sq ft home approx). At that time, the contractor added zoning to our existing furnace. So each floor is zoned & has it’s own thermostat. We’ve only had one contractor in our area come out to give us an estimate, however he recommended a 2-stage (a Lennox & a Daikin). I came across this site & appreciated the info. I just don’t know anything about furnaces & want to make an informed decision. Any additional advice would be helpful & appreciated. Thanks!

  33. Thanks for this article.

    I have been told that a two stage furnace can operate “zones” and when paired with two separate ducting systems, each zone can choose and maintain their own temperature setting. This sounds ideal for our suited basement.

    Does this sound like something that is true? Is it necessary to have a two stage furnace to accomplish this or can a regular single stage have dual duct work hooked up, be controlled by two thermostats, and accomplish the same task that was presented as an option the two stage furnace could provide?

    Please let me know if you found any info on this topic in your experience and search since it was not touched on in your article.

    Thanks again!

  34. Colette, no, that doesn’t make any sense. If you have one furnace and it fires up, then unless you have a way of shutting off ducts and opening them according to zone need automatically, every duct on the system will get the hot air. The furnace will have one big duct coming out, a trunk line, and ducts coming off of it.

    Robert, after a lot of research, I decided that single-stage made sense in Iowa, where we have very cold winters and I have a house that has less-than-perfect ducting. Since I’m not going to rip up floors and ceilings to get at the ducts and I do want air coming through even the weaker vents (yes I’ve played the normal games with arranging dampers and duct cleaning), I don’t want warmish air seeping in. I also don’t want the blower on most of the day, because even at the lower wattage, that’s potentially a massive increase in electricity use here (if you normally have the blower on “fan”, it is a savings, but you probably shouldn’t be doing that anyway, because it’s hugely wasteful). So when it’s quite cold out I want the furnace to blast and push the air out, heat the place, and then shut off. While it’s possible the weaker airflow would work, if I get that one wrong, that’s a very expensive fix.

    Going from 80% to 95% and PSC to X13 will still be a substantial GHG emissions savings.

  35. I never found an answer to this question: We have free gas from a local well. The well is known to be “rich” or having some oil fog that has caused problems with the secondary heat exchanger, cleaned once but is likely due again. It is a contractor installed
    Goodman in a new home in 2005.
    No more Goodman furnaces.
    My question: since the fuel costs us nothing, does it make sense to buy an 80% rather than another high efficiency unit with no secondary exchanger?
    Thank you, Steve Allison in WV.

    1. The natural gas from your well isn’t the fully processed, clean-burning fuel that most people are used too. What you have is known as “wet gas” because it contains higher concentrations of liquids such as ethane and butane. This unprocessed gas can corrode any heat exchanger.

      So to answer your question, I would go with the least expensive furnace available. You’re probably always going to have to replace earlier than most regardless of the brand or efficiency. On the upside, with free gas, the 80% furnace should work fine. And you’re right, it may tolerate the “wet gas” better since it won’t have the secondary heat exchanger. Hope this helps.

  36. I have Goodman two zone 94 percent furnaces . And two thirteen seer ac . Is Goodman a good brand . It seems since dalton bought them they are better quality . I am replacing upstairs ac should I replace both at same time or it does not matter . 13 year old units . I was looking at Lennox merit series . A little higher priced . Any thoughts

    1. Hi Chris. I’ve repaired and sold every brand. They all have their pros and cons and I wouldn’t put any above another. The best brand is the one installed by the best contractor. Focus on finding the best contractor and you’ll get a reliable system.

      You don’t NEED to replace both systems at the same time but one advantage would be you might get a better deal if you do.

      Hope this helps.

  37. Thank you! I found your article very helpful, especially given that everything else I’ve read suggests I should get a two-stage furnace. Until a couple of weeks ago, the 41-year old furnace in my house worked great, but now the fan won’t stop blowing, and it’s driving me nuts. My contractor says it would cost over $1,000 to fix, and that he doesn’t recommend putting that kind of money into such an old furnace. This company has been doing yearly maintenance on my furnace for the past three years and seems to be well-respected in Seattle. Each September, they mention that I’m probably going to need to replace my furnace soon, but have never pressured me to do it. This is all just to say that I’ve had a good experience with this company and trust them. Today, I spoke with one of their salespeople. I am planning to purchase a high efficiency furnace, and while he did not push me one way or the other toward a single- or two-stage furnace, I got the distinct impression that he thought a two-stage furnace would be better. He said that they almost never put single-stage furnaces in anything but rentals these days, and that his wife especially likes their two-stage system better than their old system because it never gets cold enough that she has to put on another layer at the end of the cycle. I probably would have gone with the two-stage if it weren’t for this article and that the two-stage furnace costs almost $2,000 more than the single-stage. Both quotes are for Trane furnaces. I’m surprised it costs so much more for the two-stage furnace than the single-stage–is there a difference in installation costs between the two? My estimate does not break down the cost of the furnace versus labor.

  38. Unfortunately I came across on this article much too late. My contractor replaced my old furnace with the 2 stage. I have the unit up in the attic. It gets very cold in the winter. I’m only get a luke warm air with the 2 stage furnace and it runs constantly (6-7X per hr). If it drops 1 degree, it will run again. It’s very sensitive. First you’ll feel the cool air coming in (hot air cools off as it travels through the duct in the cold attic) and it take about 12-15 seconds to feel warm air. That’s like wasting gas 2 minutes every hour. My gas bill was whopping 30% higher but my electric bill wasn’t noticeably higher. Basically what I tried to do was to raise the temperature high so that it will blast at 100% (stage 2) and then lower the temp. There’s less cycles and it keeps warm air much longer inside the house than just constantly receiving luke warm air hour after hour.

  39. Hi Robert,

    I live in western Washington where it rains and is usually in the lower 40’s in the winter. Our 11 year old Lennox has cracked heat exchanger, with 2900 PPM CO. So we are going to replace i with hhigh efficiency furnace. We have three bids and they all push the 2 stage. One however added a bid for a 1 stage, which was $500 less. I appreciated reading your article so probably will opt for the 1 stage Rheem. But how cold it gets has something to do with it too, doesn’t it. I have read that 2 stage is best for really cold winters. Do you think so?

  40. Hello Robert,

    I really enjoyed your article. I think I will stay with the Rheem single stage motor. I can order online a Rheem 4Ton A/C and a 95% efficient Rheem furnace + Collector for $3600 (about half of what the pros charge),not including labor. I am looking for quality licensed installer in the Denver/Boulder area. Do you have any recommendations? Also, is Rheem OK for this job? I am just looking for reliable stuff. I don’t need many bells and whistles.


    Wes Levin

  41. My HVAC situation proves the accuracy of your arguments to ” not” invest in a 2 stage system.
    In 2005 I had an addition put on to my house and the contractor convinced me a 2 stage would be a perfect fit. In 2010(warranty had expired) the main board needed replaced: $600 board plus labor. Now, 7/2020 the modular failed and the cost would be $1000+ to resolve. Our compressor burned up……new was inatalled. When we had no a/c for a day and The furnace was intermittent It kicked on and I noticed on the thermostat that 1+2 appeared- in 20 years I gave never seen that which leads me to believe the system was running on stage 1 only! I decided to replace it with a single stage furnace……and what a difference to feel the new a/c cooling!
    What a waist of up front money and energy costs over the course of 20 years. Don’t waist your money!

  42. Wow Brad. This was so helpful. I was about to put myself on a waiting list for a 2 stage Payne and $1800 in added costs.
    Thanks for your insight.

  43. I've had a 95+ single stage 75k btu Armstrong for ~ 15 yrs, no issues. Cycles maybe every hour at < 45F outside, tstat set to 70F. Wanted a 40k but contractor said it was too small, I think it would have been more even heat with a longer run time. Glad I didn't get the 2stage with variable DC motor after reading this.

  44. Hello Robert:
    Thanks heavens for honest people like you.
    Great learning article, we have changed our contractor and are happy with the result of having single stage furnace installed in November 2020.
    Everything what you have pointed our in your clearly written article happened to us. Our original contractor served us well for 25 years, however, with the cracked heat exchanger in our 28nyear old single stage Lennox unit made us consider a new furn ace. The contractor pushed for two stage unit, however, we usually get two quotes for major expenditures. The second contractorundefinedowner came by himself and suggested one stage furnace and explained all in details.
    We jave done a lot of reading on line and we believe that your article is the BEST THING THAT HAPPENED in order to finalize our decision.
    Viola Woodhouse

  45. Excellent article! I’ve had to heating contractors give me quotes. Both pushed the two stage over the single. Everything you said in your article, is what they pushed!

  46. Robert,
    thanks so much for putting all of this information onto your website! I appreciate your straightforward approach, getting to the point quickly, and linking your sources of information.
    I was overwhelmed with information when I was making a decision for replacement of both my furnace and AC, and I definitely didn't like the "sales pitch" I was given by several local companies. I prefer to have the more technical information, so I can make my own decisions based on information and not hype.

    Your advice saved me a lot of money, and allowed me to choose a local HVAC contractor who offered a fair price and a quality installation. I prefer to use a contractor who spends less money on advertising and fancy vehicles, and more time on answering my questions honestly. I'm much happier to spend my money by paying skilled technicians money, rather than skilled salespeople.

    Thanks again!

  47. I recently got sold a Trane 2 stage furnace and question if it is running as it are suppose to. I will turn up my furnace 4 degrees. It will turn on at stage 1 and run for 20 to 40 minutes on that stage. Then will go to 2nd stage for 5 to 6 minutes then go back to 1 st stage and run 5 to 6 minutes this goes on for about 100 to 110 minutes before it reaches set temp reading. Out side temp is between 30 to 40 . Is this how my furnace is suppose to operateundefined

  48. I had no idea that single stage furnaces are less prone to breaking down and repairs are cheaper. I want to get a furnace that will last me for years. I'd have to talk to a professional first to see if they recommend a single-stage or a two-stage furnace for my home specifically.

  49. Thank you for the heads up about single-stage furnaces and how they can still manage to efficiently heat up a house. Efficiency is something I want to work on since my house currently has beat-up old appliances that cost a lot to run, so I wanted to make sure whatever appliance I buy helps toward this goal. Reading your article gave me a better idea of what to look for when I find an HVAC contractor that can help me get a new furnace.

  50. This information is very helpful to me in making a decision between one stage or two stage furnace, obviously two stage is more expensive without much of benefit. I don't want to install something with likely hood of breaking down more frequently and dealing with repair costs. Thanks for this information.

  51. Thank you !!!! Living in Chicagoland area . Sound reasoning and advice. Our carrier is close to 20 years old , still works but will change out before giving home to our daughters.

  52. I appreciate the info. I've recently had one of my zones ( upstairs)updated from a single stage to a 2 stage system and did not work out at all. The system is actually louder than the single stage and my electric bill did go up quite a bit. I'm at wits end on how to rectify the situation in that zone.

  53. Great no-nonsense article and well-explained. Wish there were more sites like this, where you don't have to read between the lines to figure out what the incentive or sales pitch is. Thanks!

  54. Great article. It helped me understand why I wasn’t even offered a single-stage furnace.
    They just kept on pushing the most expensive 2-stage furnace and my house is only 1100 square feet.

  55. 2-22-21
    This is a follow up to my 1-1-19 post on the Comfort Academy.
    First, I would like to correct an error I made in my post. I did not have a rusted secondary exchanger but a small crack in it as verified by another HVAC company.
    Since reading this article, I decided I would keep calling HVAC installers until I found one that recommended the installation of a single stage furnace. Every company I contacted, except one, recommended I go with the two stage furnace with a variable speed motor, since this was the type of furnace already installed in my home.
    I finally found an old school guy who basically said the same thing to me that is stated in your article. He recommended we get a BASIC furnace–a 95% AFUE single stage furnace with a multi-speed ECM motor. He echoed what you said: “More complex furnaces with “bells and whistles” will have problems down the road; their replacement parts are expensive, and they will cost you in labor to install (which I already knew from experience). With a BASIC furnace, your parts and labor will be less expensive down the line.”
    He recommended we install an ICP N95ESN single stage furnace and even asked me what “label” I wanted attached on the furnace cabinet (Day and Night, Comfortmaker, Heil, Tempstar, Arcoaire) because these were all the same furnace made by ICP.  He said this furnace should last us 20 years.
    One thing that surprised us was his advising us to purchase a 100,000 BTU versus our current 120,000 BTU furnace. We live in the Chicago area and had a polar vortex winter in 2019 and my wife was worried a 100,000 BTU furnace would not give us sufficient heat in such a situation. His response was the two stage furnace was 120,000 x.95 x.67 = 76,380 btuh delivered in low heat; the new furnace 100,000 x .96 = 96,000 btuh delivered constant. He said it would deliver enough heat in polar vortex weather.
    The furnace was installed in July, 2020 and we waited for winter to see how it would perform. We had a mild early 2020 winter, and the furnace ran fine with no problems (we kept our thermostat constant at 68 degrees) and kept us cozy and warm. There was no significant increase in our heating bills as they were about the same as last winter. It could just be us, but my wife and I could tell no difference between the single stage and two stage furnaces.
    Still, before we submitted this post to the Comfort Academy, we waited for “polar vortex” type weather to see how our smaller furnace would perform. In February, an arctic cold hit Chicago for 2 weeks (along with 18 inches of snowfall in some areas in two days’ time), and although we did not experience 2019 “polar vortex” temperatures, the wind chill was as low as -23F outside. We were comfortable, the temperature in the house never dropped and was holding steady at 68 degrees, and all rooms were comfortable. We have a two story home and our bedroom is on the opposite side of the house where our furnace is located in the basement, and we felt cozy during this entire time (we also ran a room humidifier in our bedroom). Looks like our contractor’s recommendation of a 100,000 btu furnace versus the 120,000 btu we had previously was spot on. I think this confirms your site telling everyone “You will be happy with any single stage furnace as long as it's installed by a good contractor. And a good contractor won’t recommend features that you don’t need.”
    Another interesting tidbit. The price we paid for our single stage ICP 96% AFUE ECM motor furnace was the SAME that we paid for our two stage variable speed motor 96% AFUE Bryant furnace back in June, 2001, 19 years ago. The estimates we received from contractors selling “name brand” 96% AFUE two stage variable speed motor furnaces with the added “bells and whistles” they recommended was MORE THAN TWICE what we paid for the single stage furnace.
    Thank you for advising us to get a single stage high efficiency furnace!

    1. That’s great news Jay. I’m so glad that you followed the advice of an honest contractor rather than the mindless marketing hype. I wish more people were like you!

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