Modulating furnaces are more efficient than single- or double-stage furnaces and they aim to keep the temperature at one or two degrees of the target temperature. Modulating furnaces can be perceived as noisier, but that is because they run more frequently. In the end, buying a modulating furnace is a personal choice based on your needs and your own home.
Modulating vs. Single/Double Stage Furnaces
Modulating Furnace
A modulating furnace is a furnace that slowly ramps up and down in response to changes in the room’s temperature. With a modulating furnace, the flame increases or decreases in smaller, finer increments, so that room temperature never varies more than a few degrees from the thermostat’s target temperature. Using a vehicle analogy, the modulating furnace would be closest to the way that a car drives—able to range from a full stop to maximum speed, but with innumerable cruising speeds between the two.
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; there is no middle ground. “On” is the highest flame setting and thus blows at its hottest level. To understand a single-stage furnace’s operation, imagine traveling to a destination in a car whose accelerator regulates the speed at either of two settings: a full stop or at maximum. 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).
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 a while. 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 pushing lesser amounts of heat into your house but on a more frequent basis. It is a unique experience to feel the heat coming in, without the usual accompanying swoosh of a blower at full capacity.
Comfort
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 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, to maintain the heat before the furnace kicks on again.
Noise
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 that 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.
Cost
Expect to pay $6,000 to $10,000 for a 98-percent AFUE furnace, installed. Modulating furnaces have an AFUE energy efficiency rating of 80 to 98 percent. While this is beneficial, it is unlikely that you will recoup that cost for years. 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. Even then, the energy savings only off-set the difference between paying for a modulating furnace and a staged furnace. The savings will not off-set the entire cost of the furnace.
Resale Value
High efficiency, modulating gas furnaces are eco-friendly, green purchases. Yet the resale value of green home improvements remains 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.
