Relays / Contactors are nothing more than a switch. In heating and cooling, these relay/contactors are used to turn on high voltage components such as motors, compressors, and other high voltage components.
When a thermostat reaches a set temperature, a transformer, usually 24 volts, energizes the contactor, which has a coil built into it. When the coil is energized, it acts like a magnet and pulls the legs of the contactor down, making contact on both sides, and completing a circuit. When there is no voltage from the transformer, the coil lets go of the legs opening the circuit.
This allows a thermostat to control all the high voltage components in an HVAC system with only 24 volts.
Contactors come in three styles single pole, double pole, and triple pole.
The picture below is a single pole 40 AMP contactor. The end leads will have AC voltage from a breaker or disconnect fed into one side, such as 120/220 volts, and will always be hot. The other side will only be hot when engaged. The legs on the side are for the transformer to connect to and feed 24 volts AC to activate it when necessary.
HVAC contactors can be pressed down manually, completing the circuit, and are often used to test them in the field. A plastic cover with small screws will need to be removed to press down on one.
Another way to test one is to see if voltage is going from one side to the other. AC voltage from a fuse box will go into one side and always be hot. The other side will only have voltage when the contactor is engaged and working. Measuring both sides with a multi meter should give the same voltage when it is on.
The third way to test a contractor (with the power off) is to check for continuity when it is engaged by the transformer.
A common problem with a bad contactor is pits and scaring builds up where the legs touch. This can be seen by black burn marks and pitting.
Also, always check the voltage from the transformer for the correct voltage. A bad transformer not sending out the correct voltage will never let the transformer switch on.
Here is a good video describing how a contactor works.
Hi Mike, I need to get a part which i can not locate anywhere. Can you help me please? It is AB-1108-3
It is a relay switch for air conditioner
I have a simple ICP Central AC unit in the house I bought last year. The AC is about 5 years old, and worked perfect. This year, i had trouble with the blower motor not always coming on when the unit started up. I found that if i turned of the breaker to the Air Handler, and would wait 10 minutes, then turn it back on, it works normally. I replaced the run capacitor and the contactor, and it ran perfect for a week, now back to it was. Runs all day, but after being less active at night, I have to reset power, then works again. Any ideas on what may be the cause? Thank you..
My new AC has a problem with the blower motor. It is a 3 speed motor set on mid speed, however when running it drops to a lower speed. AC installer can’t figure out why. Any help?
My old heat pump has 2 relays in the air handler. 1 appears to be a delay relay that switches on the fan after the heat coils warm. Can I use a regular relay to replace it. I don’t like the delay, I would like the fan to come on instantly. I’m not exactly sure what the other relay does. Can you explain it?
About a month ago we had a Contactor replaced on my outside unit. I turned off air for 4 hrs the other day to fumigate spiders in bacement and when I was done I turned back on. Air would not work. Blower air handler works but will not cool. Had new Contactor put on today and 24 volts in but no volts going out. Something is making my Contactor go out. It just hums. Even my air guy is stumped. He is ordering a new Contactor but will it happen again?
I have 2 contactors, my problem is blower comes on but no heat. Is it possible that I have a bad contactor even though the blower works?
Thank you for given people like me the opportunity to learn . Am doing the hvac program online and you’re teachings are great. This is all new to me , am a registered respiratory therapist but always loved tools and mechanical trades ,and desided to get it done. I have learned a lot am 50% done , but very grateful for teaching like this, that help me to better understand. Thank you God bless..