What’s the difference between a single-stage, two-stage and variable-speed furnace?

June 28, 2023
What’s the difference between a single-stage, two-stage and variable-speed furnace?

You need a furnace, but what kind? One that heats, right? Joking aside, sometimes homeowners are unsure of what the best type of furnace is for them. There are a lot of options out there!

One of the biggest decisions will be about whether to get a single-stage, two-stage (sometimes called dual-stage), or modulating furnace. The differences between those types can be huge, and it can be the difference between having an absurdly comfortable home and one that’s only sometimes the temperature you’d like it to be.

Single-stage vs. Two-Stage Furnaces

The classic furnace unit has two settings: ON or OFF. This is a single-stage furnace because it has only one heat setting. Two-stage furnaces will have a second setting that’s usually 60 or 70% of the maximum heating output (measured in BTUs, or British Thermal Units).

Why does this matter? After all, will one additional setting really make a ton of difference? As it turns out, yes.

The first is that it can reduce the number of times your furnace has to start and stop. This creates wear and tear on various parts. Over many years, this can add up and can result in needing repairs or having a noisier furnace.

Second, do you always need 100% heating capacity? For most of us, the answer is no. For example, a typical autumn evening in Texas could dip down to 40 degrees, and you might want to heat your home. However, the amount of heat needed to stay comfortable will be a lot less than during a 10 degree February cold snap.

These are the perfect times for that lower stage. The other upside is that fewer BTUs means less money spent on your monthly heating costs.

To be clear, single-stage furnaces are the right choice for some homes (which we’ll talk about shortly), but dual-stage has certain advantages over it.

Modulating Furnaces

Modulating (occasionally called variable-speed) furnaces have multiple heating stages, from 100% capacity to as low as around 40%. They modulate between these stages to provide optimal comfort to a home. The exact number of heating stages will vary depending on brand and model.

If two stages of heating are a noticeable improvement over single-stage in a few areas, the same is true of two stages vs. multiple stages. The same benefits lowered energy costs, increased comfort, and less starting and stopping, will be true here as well.

Additionally, the lower heat settings often allow modulating furnace units to more evenly heat a home more. Let’s say you have a multi-level home, and the second floor is consistently less comfortable (in the winter, this would mean it’s colder). Once the main floor is sufficiently heated, low-stage operation from your heater can keep that temperature steady while also allowing the heat to spread throughout the home.

The result is a home without hot and cold spots, or wild spikes in hot or cold as the furnace kicks on and off.

Difference Between Modulating Furnaces and Efficiency

There’s lots of jargon and terminology in HVAC, so I want to be clear about what modulating means, and just as importantly what it isn’t.

When the furnace changes from, say, 100% BTUs to 70% BTUs in a modulating system, it’s doing two things:

  1. Adjusting the amount of fuel or energy that’s being used (natural gas, electric, propane, etc.)
  2. Adjusting the blower fan inside the furnace is what actually moves the air through your ductwork and into your home

This is part of what makes a furnace efficient, but not the entirety of it.

You might hear the term “high-efficiency furnace,” and this relates to the percentage of fuel or energy that’s going directly toward heating your home. For example, a standard new furnace unit will be 80% efficient, meaning that 80% of the heat generated will be pumped into your home. The rest will be vented, usually through a chimney flue or PVC pipe.

The most efficient home furnaces can be up to 98% efficient. This means that very little heat is vented, while most is going to your home.

So those are two separate ideas, but both are related to efficiency.

In practice, you can get an 80% efficiency furnace that is single-stage, dual-stage or modulating, or ones that are 90% or higher of all three types. This will increase your options. Understanding what each option represents can make your decision less intimidating.

Equipment Matching Furnaces

With all those options, it should come as no surprise that not all HVAC equipment is compatible with all other types of equipment. This is important when picking a furnace.

This is most important when matching air conditioners or heat pumps with furnaces. If your existing air conditioner is two-stage, for example, it makes sense to try to match the furnace to this level of operation (or vice versa).

To be clear, you can have a single-stage or two-stage A/C with a modulating furnace, or vice-versa. But occasionally this is where you’ll start to run into trouble.

Let’s say you installed a single-stage A/C last year, and now you’d like a modulating furnace. Did you replace the thermostat at the same time? If so, it may not have the necessary complexity to handle modulating, or you may have to run extra wiring into the thermostat to allow for it to communicate with the furnace properly.

This is a problem that can usually be solved with a couple of extra installation steps. Other times, though, there is no simple solution.

For example, installing different brands of HVAC equipment can be tricky. Even matching modulating with similar variable-speed technology can be problematic when it’s separate brands. The reason for this is the sophistication of the equipment, which actively communicates with the thermostat and other HVAC equipment. It can be hard to find compatible equipment that utilizes all the functionality of your heating and cooling units, and hard to find a thermostat capable of communicating with both.

Most times, matching “like with like” both in terms of efficiency and brand is the simplest solution. But if you’re only replacing a furnace, but not the air conditioner, it may be something you need to keep in mind.

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