Short answer
For most homes, Auto is the usual recommendation. The fan runs only while the system is heating or cooling, which tends to use less electricity and avoids some humidity problems in summer. On can make sense if you want constant air filtration or more even temperatures, but it has trade-offs. The best setting depends on your home and priorities.
What On and Auto mean
- Auto: the indoor blower runs only when the thermostat calls for heating or cooling.
- On: the indoor blower runs continuously, even when the heat pump is not heating or cooling.
This setting controls the indoor blower (the fan in the air handler). It does not control the outdoor fan.
Pros and cons of Auto
Pros
- Generally lower electricity use because the blower is not running all the time.
- The filter may last longer since less air passes through it.
- Less chance of moisture being blown back into the house in summer.
Cons
- Temperatures in different rooms may be less even.
- Air is filtered only while the system is running.
Pros and cons of On
Pros
- Air circulates and passes through the filter continuously.
- It can reduce hot and cold spots between floors or rooms.
Cons
- Higher energy use, depending on the type of blower motor. Variable-speed (ECM) motors use less than older motors, but still use some.
- More wear and more filter changes.
- In summer, moisture on the cooling coil can be picked up by the moving air and the home may feel humid. Our guide to AC cooling but not removing humidity explains the same effect.
- In winter, constant airflow can feel cool or drafty between heating cycles, since heat pump air is often not as hot as furnace air.
Fan setting in summer vs winter
Many people use Auto in summer to help control humidity. In winter, some choose On for short periods to even out temperatures. If you want circulation without constant running, some thermostats offer a circulate or scheduled-fan option, which runs the fan for a set part of each hour. Check whether yours supports that.
Which thermostat mode should I use?
Use Heat in cold weather, Cool in hot weather, and Auto (the system mode, separate from the fan setting) if you want the thermostat to switch between the two. Avoid Emergency Heat unless the heat pump has failed. See when to turn on emergency heat for details. If the thermostat shows a recovery message, our article on recovery mode explains it.
Is the outside fan supposed to run when the heater is on?
Yes, usually. In heating mode, the outdoor unit pulls heat from outside air, so the outdoor fan normally runs together with the compressor. During a defrost cycle, the outdoor fan typically stops for a short time while the unit melts frost, and you may see steam. That is normal (see is it normal for a heat pump to steam). If the outdoor fan does not run while the compressor is running, or the unit is covered in ice, turn the system off and call a technician.
What is the 3-minute rule?
Many cooling systems and heat pumps have a short delay, often around three minutes, before the compressor restarts after shutting off. This protects the compressor from starting against high pressure. If you switch the thermostat off and on quickly, the outdoor unit may not start right away. Waiting a few minutes is normal. Do not repeatedly flip the breaker to force a restart.
If the fan behaves oddly
- Fan will not turn off in Auto: check the thermostat setting, then the batteries if applicable. A stuck relay or wiring problem needs a technician.
- Fan will not turn on: check the breaker, thermostat and filter. A failed motor or capacitor should be handled by a professional.
- Weak airflow: replace a dirty filter and check that vents are open.
When to call a professional
Turn the power off and call a licensed technician if you notice burning smells, a humming indoor or outdoor unit that does not start, repeated breaker trips, or ice on the coil or lines. Capacitors can hold a charge even when power is off, so do not open the unit yourself.