How do I figure out what these HVAC control wires do?
Without a wiring diagram or memory of how the old thermostat was connected, it is difficult to identify the functions of thin control wires running to the condenser. This lack of information stops people from confirming wire paths and troubleshooting why the condenser fails to start.
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 wire connections with the air handler powered off
- 2Replacing large wire nuts with smaller wire nuts or Wago connectors
- 3guessing the wire configuration
- 4asking for advice on online forums
- 5Opening up the HVAC unit to take a picture of the control terminal to inspect wire routing.
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“There is a thin wire that is coming from inside the house to the condenser. If anything touched this wire, the air handling unit turn on but the condenser doesn't start. I think the wire junction is bad.”source ↗
“Before touching it, I want confirm if this is coming from the thermostat and if there is any color codes or diagram available.”source ↗
“I was expecting something simple so I didn't pay attention to how the old thermostat was wired.”source ↗
“What would it be: (wire - thermostat)”source ↗
“I am in the midst of installing a new thermostat. Took off the old one and took a picture (attached) and also opened up the hvac to take a picture of the control terminal (also attached).”source ↗
“My question is can I just use the RC wire that was connected to the old thermostat (and already connected to C at the terminal)?”source ↗
“Also, W and Y wires are connected at the pigtailed together at the terminal. Any idea why?”source ↗