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.
- 1checking the low side on the transformer
- 2Switching to a different thermostat brand, such as EcoBee
- 3Cutting off the thermostat and waiting to clear the issue
- 4Installing a common wire
- 5Replacing the thermostat with a different brand like ecobee or Honeywell
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“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 ↗