I have a regular furnace without a HVAC setup. When it's summer we usually have the fan running constantly. The furnace fan then runs at about half the speed as it would when the heat is one.

Recently I was at a friends house with a similar furnace and thermostat. When he sets his thermostat to cooling mode it would once in a while kick on the furnace fan at full speed without igniting the burners, thus pulling in cooler air from the basement.

Would this be a feature from the furnace or thermostat? Do I have to wire the thermostat in a certain way?

  • fans are controlled by the thermostat or get hard-wired to some kind of auto-on or heat sensors. I've not heard of a thermostat being able to control the fan speed, just on and off. On the finances I've seen, you have to move wires into different terminals to adjust the speed. Maybe newer ones are fancier. Still, it's unusual, so you should just ask your friend.
    – dandavis
    Commented May 6, 2018 at 3:59
  • Unfortunately he recently moved to a new place so I can't check myself anymore. But I remember we talked about it. The fan was blowing like in "fan on" mode and once in a while it would kick on in full blow mode too cool the house. There was definitely no AC connected to it.
    – Roger Far
    Commented May 6, 2018 at 4:18

1 Answer 1


The common relatively (30 years old or newer)newer furnace have blowers with multi speed taps on the motor. If it is a variable speed furnace it has dip switches. Typically the higher speed is for cooling and the lower speed is for heating. Most furnaces go to the high speed for fan only operation, yours apparently goes to a lower speed. Without knowing the model number it is difficult to tell you exactly how to wire it. It may be as simple as changing the speed taps on your blower or as complicated as hooking an extra wire to the Y terminals of the circuit board and thermostat and tricking the furnace into thinking it is cooling.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.