If your house has to get really cold before the furnace kicks in even though the thermostat is adjusted to a higher temperature, you probably don’t need to replace your thermostat. You just need to adjust the existing one, following three simple steps: (1) adjust the thermometer; (2) level the thermostat; and (3) adjust the anticipator.
To adjust the thermometer, remove the thermostat from the wall. On the backside of the thermostat, directly behind the mercury switch and coil on the front, you’ll find a small metal nib (some models have a slot for a screwdriver). Using a regular thermometer as a guide, adjust the thermostat’s thermometer. If you have a rectangular model, make sure it’s level when you replace it on the wall.
The anticipator controls the length of the on/off cycles of your furnace. For maximum comfort and efficiency, the anticipator should keep the temperature of the room from varying more than two or three degrees. To adjust it, remove the thermostat cover and look for a scale with the word “longer” on it and an arrow. If your furnace doesn’t come on unless the room is really cold, you’ll need to make the cycles shorter so the furnace will come on sooner. To do this, move the anticipator, in small increments, in the opposite direction of the arrow.