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Heating

Is A Two Stage Furnace Worth It?

 April 30, 2020

By  Robert Bradford

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.

  • Hello Robert,
    This is an interesting take on two stage furnaces. I particularly like your comments on finding a good contractor, one who doesn't "upsell".
    I am in a suburb of Minneapolis. For a multitude of reasons, (cold spots, filtration, etc.) many customers run their furnace fans constantly. Especially with the newer ECM variable speed motors. A 2- stage burner seems to make sense because with the limited anticipation swing in newer thermostats, and the wide variance in heating demand in my market. A typical 80k furnace can run most of the year as a 60k and maintain comfortable temperatures. The only need for high fire is if someone left a door open for an extended period of time or the outside temperature was well below zero and the home had poor insulation.
    One other statement I would like to point out. My biggest complaint about newer 80% furnaces is the noise factor from the burners. Because the newer burner design injects the flame into the burner tube of the exchanger it is much louder than the old style burner ring under the cast iron exchanger. I've had customers very upset with me due to the drastically increased noise level.
    Keep up the good work. I enjoy your information.
    Bill Webster, Mpls. Master Certified warm air / ventilation. # TLIC7457. 50+ years

  • Two stage furnace in my home has caused me nothing but problems keeping it running right from the first year I owned it. Ill gladly put a less efficient model in my home to get rid of the problems I'm having. Right now the motherboard went down and will cost 500 before its in. Im shopping for a used single stage furnace and it looks like I can buy a decent one for around 300 dollars.

  • Hi Robert,

    Further to my previous question, I would like to ask for your further advice on the furnace and AC as follows: I was so far advised by the HVAC contractor to buy a two-stage furnace 120,000 BTU, 97%, 5 TON and a two-stage 5 TON AC. Both are Trane. As I mentioned in the previous email, my house is 5,200 sf with 03 stories. Is it still good if I just go with single stage for both of furnace and AC? How many BTU or TON would be enough for my house size? Thank you.

    Yen

    • The requirements for heating and air conditioning obviously very greatly depending on your location. (Minneapolis or Phoenix ?) Other circumstances would play into the equation greatly as well. (Insulation, windows, N.S.E.W. ->, tree cover, etc.) A little more info please.

  • Hello Robert, it’s so interesting and helpful when reading your article today. I am doing renovation for my house and planning to replace the furnace and AC. My house is 5,200 sf, 03 stories on the mountain area. How many BTU for the furnace and AC would you recommend to me? So far, all of the contractors advised me to go with two-stage ones but I think I have to reconsider it after reading your article. I look forward to hearing from you soon. Thank you.

  • Great article. Have a question.
    My evaporator coil in furnace is leaking Freon . Told to replace the coil and outside unit at same time. Both are 10 years old. Replace coil is $2000 and outside unit with new coil is $4500.
    Does that make sense? Why not replace coil inside then replace outside unit when go bad?

  • I am very glad I found your article on the single stage and two stage furnace. Most of the people I speak with say different things about both furnaces. I am in the process of getting quotes from some companies in my area. I feel that I might be getting to high of estimates, I was wondering if someone can contact me so I can speak with them to help me understand what I should be looking for and if my estimates are in the ballpark. Thank you

  • Thoughtful article and I feel less stupid after reading it than before. Stupid because while I typically over research purchases I recently went with the first thing a contractor recommended. I’m afraid I have the opposite opinion of the author.

    The house I bought had high and everything in it including a two stage carrier furnace and air conditioner from 2004, top of the line at the time I’m told. Furnace had to be replaced guy recommended a Bryant system and I just went with his recommendation. I’m very disappointed to discover after the fact that he replaced the two stage with a single stage, and at least I now understand that he may not of been as unreasonable in doing so as I first thought.

    My disappointment is I’m used to a very quiet system that rarely ran above the medium setting. When it did run high we heard it and this is how the new system sounds at all times. It comes on hard and it goes off hard as you all have probably experienced when you were in a room what a noise stops and only then do you appreciate how loud it had been. We feel temperature swings whereas before temps felt more consistent.

    So I came here researching whether it’s possible to change a single stage motor to a dual stage. I suspect it isn’t.

    So I have a difference of opinion but really appreciate the thoughtful analysis.

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