how do i know if my old system is compatible with the honeywell t9 before i buy it?

Check three things: voltage, wire type, and C-wire.

Compatibility checklist:

• REQUIRED: A C-Wire (common wire) for 24 VAC power, OR use of the included C-Wire Adapter.
• NOT Compatible: Line voltage systems (thick black wires with wire nuts, labeled 120V or higher).
• NOT Compatible: Electric baseboard heat (120-240V).
• NOT Compatible: Millivolt systems.
• NOT Compatible: Systems requiring S terminals for indoor/outdoor sensors.

To check your existing system:

1. Turn off power at the breaker.
2. Remove your old thermostat from the wall plate.
3. Look at the wires. If they are thin (like thermostat wire), not thick black wires, you're likely okay on voltage.
4. Check for a wire in the C terminal on your old wall plate. If present, you have a C-wire.

IF THAT DIDN'T WORK: If you see wires in terminals not listed in the T9 guide (like A, L/A, E, K, U1/U2), you need additional wiring support. Visit honeywellhome.com/support.

HEADS UP: If you have a zoning panel (multiple thermostats, one furnace), C-Wire Adapter installation is complex. Plan on professional installation.