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.