Many non-programmable mercury switch thermostats have a heat anticipator switch to help operate the furnace. The heat anticipator cuts off the furnace, but not the blower, in "anticipation" of the heat left in the furnace that has not come through the vents. This allows the furnace to operate efficiently. When the furnace is cutting on and off too often or the room is not reaching a desired temperature, it is necessary to adjust the anticipator. Setting the anticipator on a thermostat requires removing the front of the unit.
Pull the cover off the thermostat. Most mercury switch thermostat covers simply snap on and off. There might be two tabs on each side you squeeze with your fingers to remove the cover.
Find the thermostat heat anticipator. It is usually a brass piece of metal with a pointer on one end. The pointer end is over a scale with settings. The scale will have wording stamped on it like "longer" and have several numbers. These numbers are the amp scale.
Move the anticipator pointer to the "longer" setting by one calibration mark on the scale if the furnace is turning on and off frequently. This moves the amp setting down. Rotate the anticipator lever with your fingers from the opposite end of the pointer.
Turn the anticipator pointer to the opposite direction of the "longer" arrow by one calibration mark if your furnace is not reaching the room temperature setting.
Place the thermostat cover back onto the thermostat and allow the furnace to cycle for three hours. This gives enough time for the room to warm to temperature. Continue making adjustments if necessary.