What you will learn from Control Circuits for Air Conditioning & Heating article:

  1. step-down transformers and their function in a control circuit.
  2. the importance of electrical ratings
  3. how to protect the step-down transformer
  4. typical relay in a control circuit.
  5. diagrams to illustrate basic electrical circuits to help you understand it.
  6. lots of related and resource links for in depth related topics

Control Circuits for Air Conditioning & Heating Systems

Control Circuits for Air Conditioning & Heating - Have you ever wondered what happens when you turn on your thermostat? All the sequences and things in the system that need to work electrically and mechanically to make the system work properly. It all begins with the controls to make it work properly. In a basic HVAC control circuit, transformers are used to step-down the voltage from the line voltage to 24 volts.

Control Circuits for Air Conditioning & Heating

Control Circuits for Air Conditioning & Heating - How It All Works - Transformers

Step down transformers are used for air conditioning and heating systems to step down the voltage from a line voltage to a safer and more efficient voltage for use in the control of the system. Step down transformers can be 120V to 24V, 240V to 24V and many other combinations. Some control transformers are multi-tap transformers that can be used on either 120V or 240V to step down the voltage to 24 volts. Control transformers always come with a VA rating and that gives the maximum rating for which the maximum number of loads can be used for that transformer.

Each relay or load in the control system has a VA rating. The sum of these VA ratings for all the loads in the circuit cannot exceed the VA rating on the transformer. If the VA rating on the transformer is exceeded by the number of loads in the system then the control circuit will not work properly and the transformer will fail. Additionally, it is important that an inline fuse is placed on the hot leg or 24V hot side of the transformer. In case there is a dead short in the control circuit the fuse will prevent the transformer from blowing.

Basic Control Circuits for Air Conditioning & Heating - Relay for air conditioning and heating

Relays and Contactors

Relays and contactors are used in HVAC control circuits to turn things on and off such as blower motor, condenser fan motor or a compressor. Relays and contactors work in much the same way in a typical HVAC control circuit. They are both normally controlled by 24 volts and they typically pass line voltage to a motor or compressor. Relays and contactors have a few very important ratings on them. One is the voltage that controls the coil on the relay or contactor and the other is the amp rating of the contacts.
A relay or contactor with a 24 volt coil will blow out if 120 volts is used on the coil. The same with the contacts in the relay. If a relay with a 5 amp rating for the contacts is used to control a motor with a 10 amp rating then the contacts will burn out and the relay or contactor will be useless. If a relay or contactor needs to be replaced it needs to be replaced with an exact match relay or contactor.

Control Circuits for Air Conditioning & Heating - How It All Works

The relay or contactor is nothing more than a switch that turns on a motor or compressor. It works through the 24-volt control circuit. Some relays are controlled by other relays depending on the components in the system and the sequence of operation.

Blower Motor Schematic

The Basic Circuit

The relay in the above control diagram controls a blower motor as shown. This particular relay is controlled by the thermostat. The 24-volt neutral leg will run from the transformer. Then go to one side of a coil on the relay. The 24 hot leg of the transformer will go through the thermostat. And then to the other side of the relay coil. When the switch in the thermostat closes it completes the circuit. This energizes the coil closing the contacts and turning on the blower motor on. The thermostat controls the circuit for the blower fan. Whether the system is in air conditioning mode or if the manual switch for the fan is turned to the on position. If you have a gas furnace or other type of heating system the fan will be controlled differently either by time delay or temperature.

Source Path and Load

In the above control drawing is a basic electric circuit. The source is the source of power or electricity. The path is the wires coming from the source going to the load. The load is a light or a motor or something that will consume that power or electricity. It works the same as described above. The transformer being the source, the wires being the path, and coil in the relay being the load.

Control Circuits for Air Conditioning & Heating Systems additional resources:


Related posts to help further your knowledge. Learn the basics of electrical circuits include Ohms Law and HVAC and articles found here at High Performance HVAC.


Do you have a heat pump instead of an air conditioner? See our comprehensive heat pump category for help with heat pumps:


Furthermore, additional heat pump resources here:

High Performance HVAC Air Conditioning and Heating SystemsControl Circuits for Air Conditioning & Heating Systems