Basics of Owning a Modulating Furnace

Hot water heater and furnace in basement
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What is it like to purchase and own a modulating furnace in your home?  Is it annoying to have the furnace running more frequently?  Does it save energy and money?

Bottom Line

Modulating furnaces are more efficient than single- or double-stage furnaces.
Their aim is to keep temperature with one or two degrees of the target temperature.
They can be perceived as noisier because they run more frequently.
Higher home resale value is debatable.

Modulating vs. Single/Double Stage Furnaces

There are essentially two basic types of gas furnaces you can buy.  

Single/Double Stage Furnace

The single stage furnace is the kind that many people are already familiar with.  This furnace turns either on or off--no middle ground.  "On" is the highest flame setting and thus blows at its hottest level during this stage.  

To understand a single stage furnace's operation, imagine traveling to a destination in car whose accelerator regulates the speed at either of two settings:  a full stop or at maximum, pedal-to-the-metal top speed.

A double stage furnace is more closely akin to the single stage furnace than the modulating furnace.  Like the single stage, it too has maximum high and off settings.  But it does have one additional "low" stage that allows for a slower, less energy-gobbling stage of heat.  

With the vehicle analogy, the car would travel at either full stop (0 mph), maximum speed (120 mph), or local street speed (25 mph).

Modulating Furnace

With a modulating furnace, the flame increases or decreases in smaller, finer increments, so that room temperature never varies more than 1 or 2 degrees from the thermostat's target temperature.

Using the vehicle analogy, the modulating furnace would be closest to the way that a car really drives--able to range from a full stop to maximum speed, but with innumerable cruising speeds between the two.

1. General Operation

When the modulating furnace is initially called into action--for example, first thing in the morning--it begins at the highest flame and blower level possible. Thus, it sounds and feels very much like single-stage gas furnaces you have experienced before.

That is where the difference ends. Once enough heat has built up in the house, there is no more need for a huge influx of heat from the furnace, at least for awhile.

After that, you will hear and feel the modulating furnace turn on and off frequently--far more frequently than with single-stage furnaces--but at lower levels.

The modulating furnace is basically pushing lesser amounts of heat into your house, but on a more frequent basis.

It is a unique experience to feel heat coming in, without the usual accompanying swoosh of a blower at full capacity.

2. Comfort:  Better Temperature Targets

Comfort is the marked, immediate change that you will notice.  

Because the furnace does not wait for room temperature to dip to dramatically low levels before kicking on, you feel a more consistent room temperature.

The house never really feels hot or cold. If you have the thermostat set to your desired temperature, the house feels just right.

You do need to have a well-insulated house, though.

3. Noise:  Not Louder But More Frequent

Modulating furnaces can be perceived as noisier than single stage furnaces because they run more frequently.

Naturally, many modulating furnace manufacturers advertise their units as being "whisper-quiet."  But modulation does not inherently make for a quieter machine.  

One reason why modulating furnaces can appear to be noisier is because users may be accustomed to having dead silence between active stages of single-stage furnaces.

With modulating furnaces, you have less dead silence between maximum stages.  They do not run louder but they do run more frequently.

With many new furnace purchases, you may also be purchasing a higher volume unit.  This means that air is most likely coming through your vents at a higher volume.

 This results in whooshing from the vents.

4. Cost:  Higher Than Single/Double Stage

Purchasing a modulating gas furnace is done purely on faith. You are told that modulating furnaces have an AFUE energy efficiency rating of 80%-98% You read that you can reap big tax credits for buying these furnaces.

But they can be expensive. Expect to pay $5,000 to $6,000 for a 98% AFUE furnace, installed.

Despite the salesperson's claims, you will not recoup that cost in the first winter, not even close. Or the second or third.

Modulating furnaces are long-term purchases where you expect to be in the house for ten years or more before you realize any cost savings.

5. Resale Value:  Debatable

High efficiency, modulating gas furnaces are eco-friendly, green purchases. Yet the resale value of green home improvements remain debatable.

Theoretically, a better furnace should add value to your home upon sale. In practice, you could mention your high-efficiency furnace in your sales literature, but you may not see a much higher actual sales price because of it.