Residential Furnace Variable Speed Vs. Constant

Variable Speed

• A variable speed furnace is one that has two speed modes of operation. The speed determines how fast the motor of the furnace will operate. One stage of speed is full power, in which the furnace is exerting its all. The speed stage is half-power, which is used when the furnace doesn't need to use all of its power but just enough to maintain the desired temperature. Variable speed furnaces use the latest advancement: intelligent technology, which enables variable speed furnaces to "communicate" with your thermostat to detect when to use full power and when to convert to half-power. For example, early in the morning on a cold winter day, a variable speed furnace will start in the first speed stage. As the room warms, the furnace motor will run at the second speed stage -- just enough to keep the temperature consistent.

Constant Speed

• Constant speed furnaces are also referred to as "single speed" or "single stage" furnaces. A constant speed furnace gives you two choices: all or nothing. When the furnace comes on, it comes on with full power and continues to exert full power; it will not fluctuate or tell itself to simmer down. For example, if you have a 125,000 BTU constant speed furnace, it will remain on until it has depleted all 125,000 BTUs. While constant speed furnaces are generally found in older homes, some owners purchase them because they cost less than variable speed furnaces

Similarities

• Both types of furnaces work with your home's HVAC heating and cooling system. You can use natural gas, propane or electricity to power either a variable speed or constant speed furnace and find Energy Star models for each. Variable and constant speed furnaces also provide you with models that have different BTU capacities and efficiencies. The higher the BTU capacity, the more heat the furnace will produce.

Differences

• Furnaces are given an Annual Fuel Utilization Efficiency (AFUE) rating. For example, a 76,000 BTU constant speed furnace might have an AFUE of 80, and a 76,000 BTU variable speed might have an AFUE of 90. Even though they have the same BTU capacity, the variable speed is more efficient. The 10-point difference will show when you see the difference in your energy bill. The higher the AFUE, the lower your use of energy and the lower your gas, propane or electric bill will be. The other difference you will feel is the consistency of room temperature, which will feel more consistent when a variable speed furnace is used. Rooms heated with a constant speed furnace will have a tendency to vary between feeling nice and hot -- or not.