(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.