A relay is one of the most versatile controls at our disposal. It allows an electric current of one voltage to switch the current on and off to another device or appliance of a different voltage. The most common example of a relay exits right at the thermostat of a central heating or air conditioning system. Low voltage (24 volts) is sent to the thermostat from a transformer. When the thermostat calls for heating , the 24 volts operates a relay that sends 120 volts or 240 volts to the furnace, depending on the type of heating system. In air conditioning system, the 24 volt current trips a relay at the air handler to run the blower ( which can be 120 volts or 240 volts) and a relay known as a contactor on the condenser to provide 240 volts for the compressor and fan.

The device itself is simple in design. Thin copper wire is wound around a hollow sleeve. Inside the sleeve is a steel rod with electrical contacts attached to one end. When current is passed through the windings of copper wire, the sleeve becomes magnetic, and pulls the rod into it. The electrical contacts unite, and current is transferred to the intended device. Relays are used in motor vehicles, appliances in the home, and in the work place. The click heard when a computer connects to a telephone line for the internet service is a relay engaging.

The fan center relay can be found inside the cabinet of a furnace or air handler. It has a transformer to reduce the line voltage (120 or 240 volts) to low voltage and low amperage (24 volts; and usually less than 40 milliamps). This reduces the risk of fire or injury from exposure to live circuits and allows control wiring to be of smaller gauge and lesser expense to install. The relay will have a switching capability to make a connection (normally open) and disconnect a circuit (normally closed). On an oil or gas fired furnace with central air, the relay will disconnect the burner and engage the blower and condenser simultaneously if the thermostat calls for cooling. This prevents the heat from coming on by mistake.

The protecto relay, also known as a burner relay on an oil fired furnace, is explained in the oil burner section.

In order to use 24 volts to control the condenser for a heat pump or central air conditioner, a relay known as a contactor is used. The contactor must be capable of switching a high voltage and high amperage load on and off. It can have many terminals on it. The crankcase heater, the condenser fan, and the compressor all tap into the current available at the contactor. It is constructed of materials that will allow it to function for years under stressed conditions, but eventually will need replacing. It is imperative that the power be disconnected at the main panel and at the service disconnect outside at the condenser before any access is made to the electrical connections at the condenser.