Remove the thermostat cover and check for problems
If you have a mechanical thermostat and your gas- or oil-burning furnace cycles on and off a lot, or the room gets too hot or too cold before the furnace kicks on, you may be able to fix the problem with a couple of simple adjustments.
First, remove the thermostat cover and make sure the subbase is mounted level on the wall. If it’s not, the mercury switch inside won’t work properly, which can affect the accuracy of the temperature readings. Your subbase will either have leveling posts where you can lay the level, or a flat area at the top or bottom where you can set the level (Photo 1). Move the subbase around until it’s level and then tighten the mounting screws.
Next, adjust the heat anticipator, which fine-tunes the point at which the thermostat turns off the furnace burners. Locate the heat anticipator adjustment lever arm (Photo 2). If the furnace is cycling on and off too often, move the lever away from the “longer” setting by one calibration mark. If the room temperature is either too hot or too cold for long periods, move the lever closer to the “longer” setting by one calibration mark. Let the furnace run for two to three hours and, if necessary, repeat the above adjustment. If the problem persists, you may need to replace your thermostat.