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First, remove the cover from the thermostat.
Find and test the red and white terminals.
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Your home has a comfort system of some type. What they have in common is that they all require controls to measure and regulate temperature and, in some cases, humidity. Because controls can go awry—and because they are relatively easy to check and fix—let's take a look at comfort controls as a component.
The main comfort controller is a thermostat, which is basically a switch that turns a furnace or central air conditioner on or off at a preset temperature. Comfort controls may also include a humidistat, which is a device that senses changes in the moisture of your home.
A thermostat controls the temperature by sensing a temperature change at its location and turning the furnace on or off to maintain the preset temperature. Your comfort system's thermostat may be either mechanical or electronic. An automatic thermostat can be set to lower or raise the temperature in the home during preset times by using microprocessors and thermistor sensors.
A humidistat can change the humidity in your home by controlling a humidifier or dehumidifier in your comfort system.
Clean and adjust a mechanical thermostat:
- Remove the comfort control's cover by unscrewing or pulling on it.
- Verify that the wall plate is absolutely level. If it is not, loosen the mounting screws and level the unit.
- Use canned air or a small brush to clean all the parts.
- Insert coarse paper (grocery bag or other heavy, rough paper) under each lever and clean by moving the lever and sliding the paper around.
- If the furnace turns off and on too often or too seldom, move the anticipator pointer slightly toward or away from the longer setting.
- To determine if the thermostat is accurate, hold an accurate room thermometer nearby while adjusting the thermometer coil.
Test a thermostat:
- Remove the thermostat body.
- Clip an insulated jumper wire to the R (red) and W (white) terminals on the baseplate. If the furnace goes on, the thermostat is faulty; if the furnace doesn't go on, the problem is with the furnace, its relay, or the transformer.
- If you're testing a mechanical thermostat, clean and retest it. Replace it if necessary. If you're testing an electronic thermostat, replace it. You can find standard replacement models at larger hardware stores and home centers.
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