Wire letter guide
W and W1 Thermostat Wire: Heat (and Aux Heat on Heat Pumps)
On a conventional furnace or boiler, W / W1 is the first stage of heat. It is typically white, but again, go by the letter.
On a heat pump, the compressor provides heat through Y, and a W wire usually runs the backup auxiliary electric heat strips. That is why the same letter can land on a different terminal depending on your system, and why our tool asks whether you have a heat pump.
W wire troubleshooting
- No heat at all. With power on and heat called, measure from W to C at the furnace board. About 24 V means the thermostat is doing its job and the problem is in the furnace. No voltage points to the thermostat, its settings or the wire.
- Heat pump runs aux heat all the time. The W wire may be on the wrong terminal, or the thermostat is set up as a conventional system instead of a heat pump. Check the equipment settings in the app.
- Radiant or boiler systems. Many boilers only need R and W. That is a heat-only setup, and most smart thermostats need a C wire or an adapter there.
W on a dual-fuel system
If your heat pump’s backup heat is a gas or oil furnace, the wire that turns the furnace on is often labeled W or W1 at the thermostat. Here it matters that the thermostat is set up for dual fuel, not electric aux heat: otherwise it may run the heat pump and the furnace together, which overheats the indoor coil. Our heat pump guide explains why, and the wiring tool asks about it.
W1 and W2 together
Wires on both W1 and W2 mean a two-stage furnace (or two stages of aux heat on a heat pump). Most smart thermostats handle two heat stages; the Amazon Smart Thermostat has only one heat terminal. After wiring, tell the thermostat how many heat stages you have so it can use the second one only when needed.