Thermostat relays are switches that control the heating, cooling and fan portions of a heating and air conditioning system. Each relay is connected to the thermostat by an individual wire and is made up of an electromagnet and a pair of contacts.
When the thermostat is powered on, the electromagnet is activated and it attracts a blue wire inside the relay called an armature. This pulls the armature down, completing the electrical circuit and activating the switch. When the thermostat shuts the HVAC unit off, the armature disconnects. In the case of a thermostatically controlled HVAC system, the electromagnet is often activated and deactivated multiple times to keep the home at the set temperature.