2000-10-24

how a thermostat works

I woke up, once again, in a freezing cold room, pressed up against her in a sleepy attempt to maintain 98.6. The culprit (again): an anonymous housemate’s idiocy in setting the thermostat to 50.

There is a popular misconception regarding the workings of the typical house thermostat. Suppose your house is currently at 65 degrees, and you’d like to warm it to 75. Which do you think will heat the house faster, setting the thermostat to 75, or to 90?

They will both take exactly the same amount of time. Many people think that if you’d like to heat your house quickly, you should turn the thermostat all the way up for “maximum heating”. This thinking is based on an incorrect mental model of the thermostat. The thermostat does not control quantity of heat; it is a simple on-off switch. It turns the heater on, at “full power”, whenever the room temperature falls below the target temperature. When the target is reached, it shuts the heater off.

Along the same lines, then, my housemates (thinking the house is too hot) set the thermostat to its minimum (50F), presumably thinking that this will cool the house down faster than simply setting it to their real target. (I assume they don’t actually want the house at 50, but rather would prefer (say) 65 over the prevailing 75.) This is incorrect, for the reasons above. The house will only cool as fast as the insulation and weather will allow; having the thermostat set below the actual desired target does nothing.