Single stage, two stage or Modulating furnace.

By Ian Stringer If you are thinking about replacing your old furnace and you've done your research, you may have heard the terms single stage, two stage and modulating and are wondering which is best. Well, let me first explain the differences between them.

A single stage furnace basically has an on or off mode with a multi speed blower which can be set to deliver air at a given CFM or cubic feet per minute. Which usually equates to loud, louder or loudest. Basically a builders grade furnace and one I wouldn't recommend.

A two stage furnace starts at a low heat output (low fire) and will "ramp up" to high fire as the demand of the house requires it to. These furnaces also come with an optional variable speed blower motor which will also speed up as the output of the furnace changes. Variable speed motors are typically more quiet.

A modulating furnace will initially start up on high fire much like a single stage furnace. But once the home is satisfied the furnace will modulate the output so that the temperature never varies more than 1 degree. Basically it will fire more frequently for shorter periods ( quick bursts) without the usual blower noise or whoosh of air. These furnaces are whisper quiet when firing on the lowest output.

So which is best? I have always been an advocate of the 2 stage furnace with variable speed motor. They are efficient and relatively quiet and really only about $500.00 more than the entry level unit and quite a bit less expensive that the modulating variant. However if "total comfort" is your goal then choose the modulating furnace. I have had one installed since November (2014) and absolutely love it. The house feels neither hot nor cold, just comfortable. It can get a bit loud on high fire, but it rarely gets there. It will save me money, but it wont pay for itself in quite a while. Compared to my old single stage clunker it's a Cadillac. Quiet and comfortable but a bit more expensive. Advanced HVAC Systems