How Everything Works
How Everything Works How Everything Works

Question 597

What does the heat anticipator do on a furnace thermostat? Does it have anything to do with the dwell (temperature rise) of the unit? — BV, Burton, MI
A simple thermostat turns the furnace on when the temperature it senses falls below a certain value and turns the furnace off when the temperature it senses rises above that value. Because it takes time for the furnace to respond to signals from the thermostat, for the heat from the furnace to travel to the thermostat, and for the thermostat to respond to changes in the temperature around it, the furnace tends to stay on for too long after the thermostat turns it on and then to stay off for too long after the thermostat turns it off. The result is an oscillation in temperature: the home or building alternately overheats and then overcools. To reduce this oscillation, a thermostat with a heat anticipator limits the amount of time that the furnace stays on. Since the furnace turns off earlier, the temperature doesn't overshoot as much on the high side and the furnace turns back on again more quickly once the home or building drifts below the set temperature of the thermostat. Overall, the temperature still oscillates above and below the set temperature, but those oscillations are smaller and faster.

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