Defining a "2 Stage" Furnace

Old 05-03-08, 05:09 PM
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Defining a "2 Stage" Furnace

I have a 2 Stage Furnace. Can someone define what this means and what the 2nd Stage does? Thank you!
Old 05-03-08, 05:27 PM
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A single stage furnace has only two rates of heat output, 100% and off.

What this means is that when the thermostat calls for heat the furnace fires at 100% output for as long as necessary to meet the thermostat set point and then shuts off. This often results in short run times (with increased wear and tear) and a resultant lowering of overall efficiency.

A two stage furnace has a special gas valve and control system that allows it to run at approximately 60 to 70 percent output or 100 percent output depending on a few variables. When paired with a two stage thermostat the two stage furnace will run at the lower heat output as long as it is able to maintain the thermostat's set point. This increases run time and increases the overall efficiency of the furnace. If it cannot maintain the thermostat setting on the first (low) stage setting then it will "ramp up" to the second stage and operate at 100 percent rating until it reaches a point slightly under the thermostat setting (called "differential") and then ramp down to the first stage setting.

Since the entire heating system needs to be designed to heat the house in the coldest weather it means that most of the time the furnace is oversized to the needs of the house. Having two distinct heat outputs the two stage furnace can better meet the requirements when full heat output is not needed.
Old 05-04-08, 06:46 PM
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Furd did a good job telling you about this..

Two stage is best for Comfort. Longer run time will allow the heat to get to hard to reach area. Also with longer run time in 1st stage allows more air "change" to balance out the home's temp..

I am a proud owner of two stage furnace with a two stage stat, every room in the house (up and down) are -/+1˚ from one another.

Also with the furnace running in 1st stage, hardly ever hear it running.. I bearly hear mine 8' from my couch.

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