Thermostat blowing hot instead of cold
After a new thermostat is installed, the HVAC system blows hot air when set to AC and cool air when set to heat.
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.
Thermostat blowing hot instead of cold
- 1Changing the reversing valve (O/B) designation in the thermostat settingsDid this help you?
- 2Checking the wiring connections at the thermostat and inside the furnaceDid this help you?
- 3Reinstalling the old thermostat to get the air conditioner working againDid this help you?
- 4Posting photos of the wiring setup to online HVAC forums to ask for helpDid 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.
“My mom had someone put in new thermostats and now it won’t blow cold consistently. When it’s on cool it will blow cool for a couple of minutes and then it just blows hot air. When I put it on heat it blows cool for a little as well.”source ↗
“I’m trying to install a Honeywell T5 thermostat, but the air conditioner does not work after installation. It blows warm air.”source ↗
“I figured the instruction manual would call out something so meaningful. They do not.”source ↗
Where this came up
People with this problem also raised
- 4Why is my thermostat stuck in heating mode with no cooling option?
- 12Why is my AC blowing warm air?
- 13Car heater blowing cold air even with good coolant and thermostat
- 2Why is my Samsung freezer running warm instead of freezing?
- 3How to turn off a thermostat when the system smells like burning hair
- 2How to override a locked apartment thermostat