Why does my HVAC fan run continuously and blow hot air?
The HVAC fan can run constantly during hot weather because of a stuck relay, a faulty control board, or a short in the wiring at the main unit rather than an issue with the thermostat. This failure prevents homeowners from shutting off the blower from inside the house, often forcing them to cut power to the system entirely.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1disconnecting power to the systemDid this help you?
- 2Tracing voltage with a multimeter to locate the sourceDid this help you?
- 3Checking for a bad board or open limit switchDid this help you?
- 4Buying an exact model replacement part for a plug-and-play fixDid this help you?
- 5Calling a professional service technicianDid this help you?
- 6Turn off the furnace unit manuallyDid this help you?
- 7Have the technician come back to fix the wiring or settingsDid this help you?
- 8Putting everything back to normal and leaving the fan running constantlyDid this help you?
- 9Letting the motor run until it fails since it does not draw much powerDid this help you?
- 10Pulling the thermostat body off the wall baseplate to test if it's sending a fan signal.Did this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I never did anything with AC units before and had no clue what I was looking at.”source ↗
“Contactor was welded and wouldn’t release. Fan and compressor just kept running forever.”source ↗
“Our HVAC system starts blowing hot air through the vents during hot weather, and there's no way for us to turn it off unless we disconnect some power”source ↗
“We replaced our antique thermostat with a Nest, but it’s still doing the same thing”source ↗
“My Air Handler runs the fan constantly no matter what. I cannot figure out why for sure, but my best guess right now is the Blower ECM control Module”source ↗
“I disconnected the green wire from the thermostat as well, and it still runs.”source ↗
“Recently the air blower runs almost constantly.”source ↗
“I disconnected the thermostat and the fan continues so I think I have it narrowed down to something in the main unit.”source ↗
“I noticed, after he finished and left, that the furnace unit fan is running at all times.”source ↗
“It’s not “on” on the thermostat; in fact, you can turn the thermostat off, and the fan will keep running.”source ↗
“Only turning off the furnace unit turns off the fan.”source ↗
“the air handler kicks on as soon as it gets power, even if I completely rip the thermostat off the wall.”source ↗
“The exact millisecond the heating cycle finishes, the fan immediately starts right back up and continues blowing room-temperature air indefinitely.”source ↗
“I completely pulled the thermostat body off the wall baseplate, but the fan still instantly roars back to life the moment the furnace has power.”source ↗
Where this came up
People with this problem also raised
- 2Why does my car cooling fan run constantly after starting?
- 2Why is my oil boiler running constantly in warm weather?
- 2Why does my AC compressor run continuously and never shut off?
- 4Washing machine runs continuously on all settings
- 4Why does the outdoor AC fan stop while the compressor keeps running?
- 3How to cool down a house when the AC breaks during a heatwave