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

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.

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.

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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