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Thermostats and Control Basics

A thermostat switches heating and cooling equipment through low-voltage control terminals. This sheet explains conventional, heat pump and communicating systems and what each common terminal letter means.

Ref HV-0021NEMA DC 3

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Most residential and light commercial thermostats run on 24 VAC from the equipment's control transformer. The thermostat closes a circuit from R (power) to a call terminal, and the equipment responds. Conventional systems have a furnace or air handler for heat and an air conditioner for cooling. Heat pump systems use the same compressor for both, with a reversing valve and backup heat. Communicating controls send digital data over a few wires between a matched thermostat and equipment instead of simple on-off calls. Line-voltage thermostats switch 120 V or 240 V heaters directly and are a different product.

How to identify

Look at the equipment and the thermostat sub-base terminals. A heat pump has an O or B terminal for the reversing valve. A system with two stages uses W2 or Y2. Communicating equipment has terminals labeled by data names (often 1, 2, R, C or A, B) and needs a compatible thermostat. Wire colors are a convention only. See the thermostat wire color codes chart.

Common terminal letters

Terminal	Function
R	24 V power from the transformer
Rh and Rc	Separate power for heating and cooling transformers
C	Common; return side of 24 V, needed for powered thermostats
W or W1	First-stage heat (furnace or heat strips)
W2 or AUX	Second-stage or auxiliary heat
E	Emergency heat on heat pumps
Y or Y1	First-stage cooling or compressor
Y2	Second-stage compressor
G	Indoor blower fan
O	Reversing valve energized in cooling
B	Reversing valve energized in heating, on some brands

Key points

Check the outdoor unit to learn whether the reversing valve uses O or B. Set the thermostat to match.
Many smart thermostats need a C wire. If one is missing, use a spare conductor or a listed adapter.
Do not use a conventional thermostat on a communicating system unless the equipment allows a 24 V mode.
A jumper between Rh and Rc is used when one transformer powers both heat and cooling.
Mark the wires before removing an old thermostat.

Common mistakes

Setting O when the system needs B, so the unit heats when cooling is called.
Shorting R to C while working, which blows the board fuse.
Using a line-voltage thermostat on 24 V equipment or the reverse.
Ignoring the heat pump's auxiliary heat setup.

Governing standards: NEMA DC 3 (residential controls, electrical wall-mounted room thermostats). Equipment maker wiring diagrams.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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