5 components coveredField manual · Thermostat
24 V HVAC thermostat control wiring: field service manual
Covers: Residential 24 VAC forced-air furnaces, air conditioners, air handlers and heat pumps (single and multi-stage). Not for line-voltage or millivolt systems.
Almost every US central system is controlled by a 24 VAC loop: a transformer in the furnace or air handler supplies R, and the thermostat switches that voltage onto call wires (W, Y, G, O/B). Knowing what each terminal does lets a tech prove whether the fault is the thermostat, the wiring or the equipment.
Safety first
- Never bypass limit, rollout, door or condensate safety switches
- Gas-valve and burner diagnosis is refer-out to a licensed gas technician
- Refrigerant-side work requires EPA 608 certification
How it works
The equipment transformer steps 120/240 V down to about 24 VAC. One side is R (hot), the other is C (common).
The thermostat is a set of switches: it connects R to W for heat, R to Y for compressor, R to G for the fan, and R to O or B for the heat pump reversing valve.
Rh and Rc exist for systems with separate heating and cooling transformers. With one transformer, a single R wire powers both (on many thermostats a jumper or internal link ties Rh and Rc).
C completes the circuit back to the transformer so a smart thermostat can draw constant power; without it, the thermostat must steal power through the call wires or run on batteries.
On heat pumps, Y1 runs the compressor in both heating and cooling; W1/AUX brings on backup heat, W2/AUX2 a second stage, and E is used by some systems for emergency heat.
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Always confirm values against the tech sheet for the exact model.
OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Clothes dryer with its vent duct in a small laundry closet by Introspectivedsgn, Pexels License
