It's not always clear whether you have to upgrade to a two-stage thermostat when you install a new two-stage furnace. Most new high-efficiency furnaces 92 percent or greater have two-stage burners. A two-stage furnace fires up the first-stage burner and in most conditions stops there. However, on very cold days or if you're manually boosting the temperature, the furnace will switch on the “afterburners,” or the second-stage burners. Second-stage operation really kicks out the heat, but sends energy efficiency right down the drain.
Furnace manufacturers use two methods to limit second-stage operation. High-end furnaces incorporate an internal sensor that monitors the return air temperature. If the first-stage burners are on and the temperature of the return air falls, the second-stage burners fire up. Less expensive models use a simple timer to control the second stage. So if the house doesn't reach the set temperature within 10 minutes, the timer automatically fires up the second stage. Unfortunately, it keeps the second stage running until the thermostat reaches the set temperature.
Here's how a programmable two-stage thermostat works. As the thermostat comes out of setback mode, it looks for a temperature increase of 6 to 8 degrees per hour. It “learns” how long it takes the system to reach the set point. Then it adjusts the start-up time on each consecutive day so the house is warm when you wake up—without firing up the second stage. The thermostat only kicks in the second stage if the furnace can't maintain 6 to 8 degrees per hour.
A two-stage thermostat may need more wires than your current programmable thermostat. So you may have to fish an additional four-strand cable from the furnace room to the thermostat location (Photo 1). Or use a special digital thermostat with the existing wiring (Photo 2).
To sum it up:
- If you plan to keep your home at a constant temperature, you don't need a two-stage thermostat.
- If you want to vary the temperature of your home during the day or night, a programmable two-stage thermostat is the way to go.
Figure A: Two-Stage Furnace
A two-stage thermostat increases efficiency by running the first-stage burners as long as possible. If the first-stage burners cannot maintain a 6- to 8-degree temperature rise in the house, the thermostat fires up the second-stage burners. Timer-controlled furnaces waste energy by firing up the second-stage burners after 10 minutes, even if that heat isn't needed.