Wire letter guide

K Terminal on a Thermostat (Honeywell Wire Saver)

K is a combined fan-and-cooling signal used by the Honeywell wire saver (THP9045) and Amazon’s C-wire adapter. The module at the furnace splits K back into G and Y, which frees the old G wire to become a C wire.

Honeywell T-series and the Amazon Smart Thermostat understand K. Nest and ecobee don’t. Switching to one of those means removing the wire saver at the furnace and restoring the original G and Y connections (ecobee then uses its own PEK).

Removing a wire saver

To move from a K-wire setup to a thermostat that doesn’t support K, turn off power, find the small module at the furnace control board, and note where its wires go. Reconnect the original G and Y wires to the board’s G and Y terminals, and connect the freed wire to C. Then wire the thermostat as a normal R, C, W, Y, G system (or use the ecobee PEK if you are going to ecobee without a spare wire).

How to confirm you have a wire saver

A wire on K at the thermostat almost always means there is an adapter module at the furnace, usually a small box stuck to the furnace cabinet near the control board, with short wires going to the board’s R, C, G and Y terminals. The Honeywell module is labeled as a wire saver or with a THP9045 part number. If you find a K wire but no module, stop and check the old thermostat’s manual: something else is going on.

Staying with Honeywell or Amazon

If your new thermostat is a Honeywell T-series or the Amazon Smart Thermostat, you can usually keep the module: connect R, C, W and K to the matching terminals, as before. The new thermostat then gets a C wire without any work at the furnace. Check the new thermostat’s setup screens for a wire saver or C-wire adapter option.