Feb
07
2017

What’s in a Thermostat?

We’ve all seen them…some of us still have them, but do you know how that good ol’ round thermostat on your bedroom wall actually works?

Here’s how:

Inside the thermostat is a bi-metal coil. Attached to one end of the coil is a lever that sticks out of the casing (that’s the tab you move to adjust the temperature setting). Attached to the other end of the coil is a tiny vial containing mercury and three wires. Wire A stretches all the way across the vial. Wire B stops short of the left side of the vial. Wire C stops short of the right. If the vial is tipped one way or the other the mercury touches one of the short wires and completes a circuit.

How do you tip the vial? By adjusting the lever:

You want more heat so you slide the lever up. As you do so the coil is tightened and the vial is tipped to the left. The mercury touches Wire B and completes the circuit, which triggers a relay switch that activates the heating system. As the temperature around the thermostat rises, the coil loosens and the vial slowly returns to level. As soon as the mercury no longer touches Wire B, the circuit is broken and the heating system is deactivated.

If you want the AC to kick in you do the same in reverse. Switch the system from HEAT to COOL and then slide the lever down. As you do, the coil is loosened and the vial is tipped to the right. The mercury touches Wire C, completes the circuit, and triggers the relay switch to activate the AC. As the temperature around the thermostat falls, the coil tightens and the vial slowly returns to level. As soon as the mercury no longer touches Wire C, the circuit is broken and the AC is switched off again.

Next time…programmable thermostats and how to use them to save money!