The Single Stage vs Two Stage Furnace Debate | Beware of Being Fooled

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

Many homeowners pay very little attention to their furnace. When winter rolls around and outdoor temperatures drop, we’re just hoping it starts.

The first time we even hear the option of a two-stage central heating system is usually from a contractor’s salesman. Often these salesman wax poetic about how much energy bills will go down and the two-stage heating will pay for itself.

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 2 Stage Furnace?

Two-stage heating describes a furnace with two settings of heat output. The first setting is called First Stage (low fire or low power). It 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. In most models, the low setting will also run the blower at a lower speed as well.

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

At that point two stages of heat will fire, blower will increase slightly and the furnace will run at full capacity. Both conditions are often caused by very low outdoor temperatures.

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 the heating needs of the home.

The difference is a single stage is a one speed furnace unlike the two speeds of a 2 stage. If a one stage furnace is rated at 80,000 BTU’s, it will only operate at that capacity.

One stage and two-stage gas furnaces are not the only type of furnace. There are also multiple models of variable speed furnaces and modulating furnaces. But modulating furnaces or any other types of furnaces won’t be covered in this article because most homeowners tend to be deciding between single and two-stage.

2 Stage Furnaces Disadvantages

Certain heating contractors will rave about the claimed long term benefits of the dual stage furnace. “Better indoor air quality, saves money, quieter and provides your home with optimal comfort” they’ll claim.

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

Is A Two-Stage Furnace More Efficient?

A two-stage furnace is NOT more efficient than a single stage furnace. It will not save energy or money on your utility bills compared to a single stage gas furnace. In fact, until 2019, two-stage furnaces generally cost MORE to operate.

A 2006 study completed by ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers) said this: “…while the 2006 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%…”.

These motors were not new. You could buy a two-stage furnace with an ECM motor for over 15 years. By far though, most 2 stage furnaces were sold with the older less energy efficient motors.

But more importantly, remember this changed in 2019. And manufacturers and contractors have been claiming “uses less fuel” and “saves you money” for over 20 years. So that means for 20 years consumers that bought a two-stage furnace believing they’ll save money have not been told the truth.

Spending less money on natural gas comes down to the efficiency of the furnace. Energy consumption is determined by heat exchanger design. Single stage and 2 stage furnaces of the same energy efficiency use the exact same heat exchanger.

In other words, a 95% efficient furnace is 95% efficient whether it’s running at a lower capacity or not. No savings on energy bills.

How Much Quieter Is A Two-Stage Furnace?

A two stage furnace is fractionally quieter than a single stage furnace. But compared to the old clunkers we’ve had in our homes for the past 30 years, ALL furnaces are quieter. Two-stage does not reduce noise complaints compared to single stage.

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 nearly the same.

If you want less noise and quieter operation, focus on finding a contractor that knows their stuff when it comes to airflow. That tends to separate the good from the bad.

Does A Two-Stage Furnace Keep You More Comfortable?

Who doesn’t want a more comfortable home? 2 stage heating claims it creates this by helping prevent temperature fluctuations. How? By running at 70-80% of it’s rated capacity in first stage, the furnace will run for longer periods thus reducing temperature swings and heat your home evenly.

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 fan speed 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 energy 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 gas furnace ran for 10 minutes, the two stage furnace would run for 11 minutes. This would have little to no effect on temperature swings.

Can A 2 Stage Furnace Improve Indoor Air Quality In Your Home?

No. Indoor air quality is dependent on air flow and ventilation. Not on the BTU output of your furnace. 

The claim for improved indoor air quality is that the furnace runs longer. But for any significant change in indoor air quality you need to run your blower 24 hours a day. A variable speed furnace with a more efficient blower motor will have an impact but again, the heating output has nothing to do with it.

One vs Two: The Winner Is…

One-stage furnaces generally have two major advantages over a dual stage model. They are less prone to breakdown and price tag to repair is less.

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 gas 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 really use less energy, save me money or improve comfort in my home, 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 provide an optimal comfort level in your home.

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 HVAC system for your home, they install oversized equipment.

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 10-15 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 tend to fail earlier. They won’t fall apart sooner than a single stage furnace. But price tag on repairs for a two stage furnace in the 10-20 year mark will be considerably more.

Replace the furnace or pay for high mark up parts. Either way manufacturers and contractors win. You end up spending more money on repairs or more money on replacing comfort equipment.

Conclusion

You can avoid the manufacturers trap. When the weather turns cold and you’re in need for a new furnace installation, don’t focus on furnace and air conditioner options. Your main decision is to choose the right contractor. Nothing is more important on the installation of a new HVAC system.

A good contractor will determine the heating needs of your home and help you find the right type of furnace. And the right furnace will probably be the cheaper option.

Take that extra money for a two stage unit and buy a better thermostat, air filter or humidifier when you get a new furnace. Those can improve indoor air quality and you’ll be much happier with that upgrade.

You will be happy with any single stage model as long a quality contractor handles the installation. And they won’t recommend features that you don’t need.