Here's how to check a thermostat's accuracy and clean it if necessary:
Step 1: Tape glass tube thermometer to wall a few inches away from thermostat. Pad thermometer with paper towel to prevent it from touching wall. Make sure neither thermometer nor thermostat is affected by any outside temperature influences. In some homes, hole in wall behind thermostat through which wires enter is too large, allowing cold air to reach thermostat and affect its reading.
Step 2: Wait about 15 minutes for mercury to stabilize. Then compare reading on thermometer with reading of thermostat needle.
Step 3: If variation is more than a degree, check to see if thermostat is dirty. To examine thermostat, remove faceplate, usually held by a snap or friction catch. Blow away any dust inside it. Do not use a vacuum cleaner; its suction is too great. If thermostat has accessible contact points, rub new dollar bill between them to clean these spots. Do not use sandpaper or emery cloth. If element is coiled, use soft brush for cleaning.
Step 4: If thermostat has mercury vial inside, use level to make sure unit is straight. If thermostat is not straight, loosen mounting screws and adjust thermostat until it is level. Then retighten screws.
Step 5: After cleaning thermostat, check it again with glass thermometer, as detailed in Steps 1 and 2. If thermostat is still not calibrated properly, it should be replaced as detailed below.
Check out the next section for tips and techniques on how to replace a thermostat.