What Is a Two Stage Furnace?

This question comes up a lot when home owners are looking for a new furnace.  The “Two Stage” describes the heating output of a furnace.  Most furnaces are single stage, meaning on a call for heat it comes on at 100% of its capacity, runs till the thermostat is satisfied then shuts off.  This, while effective is not the most efficient or comfortable way to heat your home.

Think about how “jack rabbit” starts effect the gas mileage in your car.

A two stage furnace comes on at “low fire”, or about 70% of it’s capacity.  It then runs on low fire until one of these things happen.

1. The thermostat is satisfied – Furnace shuts off
2. A preset period of time passes with the thermostat still calling for heat – Furnace jumps to high fire
3. The temperature at the thermostat drops another degree – Furnace jumps to high fire

So on the mild days, the first or “low fire” stage will do most all the heating, then on the colder days the second or “high fire” stage steps in and provides the maximum heat to satisfy the heating demand.