A simplified way to view understand gas furnaces would be a “Good, Better Best” approach. In this article we will describe the differences between a single-stage (good), two-stage (better) and modulating gas furnace (best) in the context of providing comfort.

The key to understanding furnace performance in relationship to comfort is furnace runtime. The longer a furnace runs the more mixing of indoor air occurs which means even temperatures throughout the house. In terms of comfort control, the more speeds a furnace has the better.

Single-stage furnaces

Maintenance on Gas FurnacesA single stage furnace has one speed. The thermostat in the house calls for heat, so the furnace comes on at its rated capacity. It will run at full capacity until the thermostat is satisfied, then shut itself off. Single stage equipment is very common because most builders install single stage equipment because of the lower up-front cost.

The single stage furnace does a reasonable job of maintaining comfort. Depending on a whole host of factors, like the number of windows in a room, type of window, thermostat location, etc., temperatures swings can be up to 4 degrees. The main benefit of a single stage unit is that it is the most affordable option to purchase. They are also less likely to break down and repair costs are usually low because they have been manufactured for a long time.

Two-stage furnaces

A two-stage gas furnace has a high and low speed / output. The burner in a two-stage furnace has two levels of heat output: high for cold winter days and low for milder days. Since the low setting is adequate to meet household heating demand 80% of the time, a two-stage gas furnace runs for longer periods at a lower energy usage than the high setting.

A two-stage furnace is quieter and generates more even heat through the house. The longer, slower heating cycle eliminates the kind of rapid warming that many people find uncomfortable from a single-stage furnace. Longer run times also permit better filtration if you have or need a high-quality filtration system.

Two-stage furnaces give buyers the best balance between cost and value. They are more expensive to purchase initially than a single stage but run more quietly, efficiently and provide more comfort than single stage equipment.

Modulating or Variable-Capacity furnaces

Modulating gas furnaces take the value of staged heat to the ultimate level. These furnaces have sixty plus speeds due to their modulating gas valves and variable speed blower motors. Some models can run as low as 35% capacity and increase by .5% increments if the thermostat calls for it. Because they can manage temperature so precisely in a home, they usually run continuously at a very low setting which creates even temperatures in every room of the house.

These furnaces cost the most of any models and for the expense, you get the most comfortable indoor climate currently possible. The Lennox Precise Comfort Furnace Technology video does a great job in just over two minutes highlighting the difference between a single stage gas furnace versus a modulating gas furnace.

Lennox Precise Comfort Furnace Technology video

Which furnace is right for you?

At Advantage Home Performance we encourage homeowners to consider two-stage equipment over single stage whenever their budget permits. We also tell customers if they invest in a modulating gas furnace that they are doing it for the enhanced comfort and not for a quick return on investment.


ACCA Standard 5: Residential Quality HVAC Installation link here

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