Low Voltage Thermostats

Intro

A thermostat senses when the air temperature moves above or below the desired level and switches the heating or cooling system on or off.

If a thermostat is attached to an electrical box and has standard house wires connected to it, it is a line-voltage unit. If it malfunctions, shut off power to the circuit, remove the thermostat, and take it to a dealer for a replacement.

A low-voltage thermostat receives power from a transformer, much like that for a door chime. Typically the transformer is located on or near the heating or cooling unit. Replace the transformer if it does not deliver power. In a standard setup two wires control the heater and two control the air-conditioner. If there are more wires, consult a heating and cooling pro.

Checklist

Time
Less than 1 hour for most repairs

Tools
Screwdriver, soft brush, strippers

Skills
Stripping wire, making connections

Materials
Short length of thin-gauge wire, perhaps a replacement thermostat or transformer

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Step 1

On a low-voltage thermostat, pull off the outer cover and loosen screws to remove the inner cover. Clear away dust with a soft brush. Gently lift the control lever and clean the contact beneath it. Replace the covers and test.

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Step 2

If the thermostat still doesn't work, test for power. Strip the ends of a short wire and touch the terminals marked W and R. If they spark and the heating unit comes on, the thermostat is broken and should be replaced. If nothing happens check the transformer.

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What If the room temperature is far different from the setting?

A thermostat that sends the wrong signal to the heating unit or air-conditioner may just be out of plumb. Use a string with a small weight near the two alignment marks. If they do not line up, loosen the mounting screws and twist the thermostat until the marks align.

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