Why is my thermostat voltage fluctuating?
The 24v power wire supplying the thermostat can fluctuate rapidly, jumping randomly between zero and over twenty-one volts. This erratic voltage causes random failures during cooling cycles, leaving the unit blowing dead air with the compressor off or triggering unexpected heat cycles.
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.
- 1checking the low side on the transformerDid this help you?
- 2Switching to a different thermostat brand, such as EcoBeeDid this help you?
- 3Cutting off the thermostat and waiting to clear the issueDid this help you?
- 4Installing a common wireDid this help you?
- 5Replacing the thermostat with a different brand like ecobee or HoneywellDid this help you?
- 6Checking to make sure the wiring is secured and tightening all connectionsDid 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.
“Last night my thermostat started cycling quickly, about every 0.5 seconds or so. I could see the "heating" on my thermostat kick on and immediately shut off only to repeat the cycle.”source ↗
“I'm having issues with my HVAC system. I had a brand new system installed, and since i've been having issues with my thermostat/thermostat wiring.”source ↗
“Right now if I measure the voltage on the power wire (24v) coming down to the thermostat it is very sporadic.”source ↗
“It will jump from 0 to 18 to .3 to 6 to 0 to 21.5 and all over the place very rapidly.”source ↗
“The unit has always had random failures during a cooling cycle. The unit reported running but it would be blowing dead air compressor would be off.”source ↗
“That plus the known ~8v voltage leak from Nest would get 22.7v on my B terminal. My assumption is that a random spike would go past the voltage needed to call for a heat cycle during a cooling cycle without shutdown.”source ↗
“Mine kept failing also. Garbage thermostat.”source ↗
Where this came up
People with this problem also raised
- 2Why is a brand new aftermarket thermostat already failing?
- 4Thermostat doesn't work after opening it up
- 3How to turn off a thermostat when the system smells like burning hair
- 2Why does my dimmer switch read 100v instead of 120v?
- 7Mini-split can't keep house warm when it freezes outside
- 2How to operate an unfamiliar commercial thermostat