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Introduction to How Home Thermostats Work Dissection Making It Work Wired Digital Thermostats Lots More Information Compare Prices for Home Thermostats

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This thermostat is designed for a system with five wires -- the wire terminations are marked as follows:

RH - This wire comes from the 24VAC transformer on the heating system.
RC - This wire comes from the 24VAC transformer on the air-conditioning system.
W - This wire comes from the relay that turns on the heating system.
Y - This wire comes from the relay that turns on the cooling system.
G - This wire comes from the relay that turns on the fan.

This circuit board is located in the base of the thermostat. The five wires from the house hook up to the five screws.
The two transformers provide the power the thermostat uses to switch on the various relays. The relays in turn switch on the power to the fan and the air-conditioner or furnace. Let's see how this power flows through the thermostat when the air-conditioner is running.

Power from the air-conditioning transformer comes into the terminal labeled RC. The ball controlled by the mode switch jumps the current onto a trace that leads to the terminal in the lower-right corner of the circuit board.

This terminal connects to the top layer of the thermostat through a screw. It connects to the pink wire (see below), which leads to the bottom wire in the mercury switch. If the switch is tilted to the right (as it would be if the air-conditioning were on), the current travels through the mercury into the blue wire.

This is the top layer of the thermostat. The three screws visible in this photo connect to the circuit card in the bottom layer of the thermostat.
Through a screw, the blue wire (see above) connects to a lug in the lower-left corner of the circuit card.

From there, it goes through a trace on the circuit card to the other branch of the mode switch. The ball in the mode switch jumps the current onto a trace that connects to the terminal marked G, which energizes the fan, and the terminal marked Y, which energizes the air-conditioning.