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.
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 wire connections with the air handler powered offDid this help you?
- 2Replacing large wire nuts with smaller wire nuts or Wago connectorsDid this help you?
- 3guessing the wire configurationDid this help you?
- 4asking for advice on online forumsDid this help you?
- 5Opening up the HVAC unit to take a picture of the control terminal to inspect wire routing.Did this help you?
- 6Guessing based on standard color-coding conventions for wires.Did this help you?
- 7using a battery at the furnace to identify wiresDid this help you?
- 8checking voltage and matching wires one by one at the fan boardDid 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.
“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 ↗
“Like title says, I am hooking up a thermostat that only requires the W and R connectors but the wires look the same.”source ↗
“I think I know which is which but I’m not 100%”source ↗
“I watched a video about determining wires by continuity with a multimeter but not sure how you accomplish that with only two wires.”source ↗
“So unfortunately we won't know. We need to see what the connections look like in the outdoor unit and under the thermostat.”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 ↗