Photograph of Photograph of a water pump pressure control switch with the cover offRelay Switches & Contactors
Causes & repairs for clicking chattering relay switches in Air Conditioners, Heat Pumps, Water Pumps, & other controls

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Control relay switch chattering noises:

This article discusses control relay switch chattering or rapid, repeated clicking noises, their cause, meaning, and repair. Relay switches are widely used to control equipment found in or on buildings including air conditioning system or heat pump compressor/condenser unit compressor motors, fans, and in an air handler the blower fan used in forced air heating and air conditioning systems.

Relays are used to switch on or off backup heat for heat pump systems. Control relays are also used to control water pumps in private well systems or in water pressure booster systems.

Shown at page top: the relay switch in a water pump pressure control switch.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Clicking or chattering electrical relay switches

White Rodgers ran relay switch (C) Article Contents

[Click to enlarge any image]

What is a Relay or Contactor Switch, How do They Work & Why Are They Used?

Honeywell contactor switch (C) InspectApedia

Shown here: A typical general-purpose relay switch or contactor produced by Honeywell. [Click to enlarge any image] If you hear buzzing or chattering at this relay it probably needs replacement.

An electrical relay is a device that changes the state of a circuit in response to it being energized or de-energized by that or another circuit. - (Roberts 2006, copy available at REFERENCES)

In typical mechanical systems used in buildings, such as air conditioners, heat pumps, electric heat, and water pumps, one or more electromagnetic relays are used to control the operation of these devices.

The relay is basically an on-off switch that uses a lower current circuit to switch on or off a higher current electrical circuit (and thus device). There are several reasons that these controls are used:

How a Typical Relay Switch Switch Works

Simple schematic of how an elecrical relay control works on a hot water heating system (C) Daniel Friedman

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A low voltage switch such as a wall thermostat used to turn heat or air conditioning on and off operates at 24VAC, at very low current (Amps), and cannot handle the switching of higher voltage (120V or 240V) to higher-current-using mechanical systems that might run at 20 to 40 Amps.The low-voltage 18-gauge wiring and the little "switch" in the thermostat would burn or melt at those higher currents.

So we use the little low-voltage, low-current switch (such as a wall thermostat) to operate a contactor or relay that in turn operates the higher current circuit.

Current through the electromagnetic coil in the relay operates a magnet in the relay (pink rectangle in the illustration) that in turn mechanically closes electrical contacts (blue) that are designed and rated to handle the voltage and current of the device being operated. In our image of a Honeywell brand relay control (earlier on this page) you can see the magnetic coil - those red-copper windings are part of the coil assembly.

In our sketch above we've simplified a schematic of the operation of an electrical relay, in this case we illustrate a circulator relay control used to control the circulator pump of a hot water heating system. When the thermostat is not calling for heat the low voltage switch in the thermostat is in the Open position and the circuit is not energized. This is a Normally Open or NO relay. Items are of course not shown to relative scale.

When a thermostat or similar switch "closes" to operate a circuit, that sends voltage, typically 24VAC through the coil assembly. That coil creates an electromagnetic field that in turn energizes a magnet in the center of the coil, causing the magnet to move up or down.

The moving magnet, in turn, moves contacts (not visible) that close the relay switch to turn on the higher current/amperage circuit.

Relay in the closed position, operating the electrical equipment (C) Daniel Friedman

In the case of our sketch of a relay switch operation shown above, the coil is energized, the magnet has moved the contactor in the relay "up", closing the 120VAC electrical circuit and thus permitting the circulator pump motor to run.

What's the Difference Between a Relay Switch and a Contactor?

Circulator Relay (C) Daniel Friedman\

Shown above, a hydronic heating system circulator pump relay control with its cover removed. The basic components and connections in the circulator relay include:

Troubleshooting an aquastat and its relays is at AQUASTAT TROUBLESHOOTING


We use the terms "relay switch" and "contactor" or "contactor switch" synonymously in this discussion. Reading literature on repair and maintenance of residential and light-commercial mechanical systems such as residential or light-commercial air conditioners, heat pumps, hot water heating system circulator pumps, and water pumps, we see that technicians regularly use the terms "relay" and "contactor" as if they were the same thing.

Both contactors and relays are switches used to turn on or off an electrical circuit in response to some condition (temperature, water pressure, other). And both types of devices use a type of electromagnetic coil to move the switch to cause it to open or close.

Technically these two devices are not identical. Most-basically relays operate lower-current devices and contactors are used to operate higher-current equipment. Everything else I add below is simply adding detail to the differences in current or load that the switch is controlling.

So in a higher-current air conditioner or heat pump system the service technician using the term "contactor" is correct, while in a lower-current hydronic heating system in which a room thermostat is operating a circulator pump, the heating service technician using the term "circulator pump relay" is correct.

The functional or usage difference between a contactor and a relay is that

What Noises Occur at Relay Switches or Contactors? Causes of clicking or chattering noises at a relay switch

While some noise is common at these relay controls, clicking or chattering noises are common complaints we hear from both consumers and repair technicians. While many sources pose causes and cures for chattering or clicking at electrical relay controls, none of them proved comprehensive and some pose more legend than objective data. Here we have collected, explained, and offer comments on causes and cures for relay clicking and chattering problems.

According to Schneider Electric, some humming noise is normal at contactors or relays used with HVAC relays, and in larger mechanical equipment and relays humming and vibration are normal.

Basically, all AC devices which incorporate a magnet and armature assembly produce a characteristic hum. This hum is largely due to the changing magnetic pull (known as the flux change) which induces mechanical vibrations. - Schneider Electric, retrieved 2016/08/01, original source:

But if your contactor or relay switch is clicking on and off rapidly or making a chattering noise, that's probably not normal and merits diagnosis and repair.

Watch out: testing and repairing electrical components or HVACR equipment involves potentially fatal shock hazards - you could be killed. If you are not trained and qualified to work on electrical equipment you should not touch it and should hire a trained professional such as a licensed electrician or a trained HVACR technician.


Watch out: for chattering or rattling noises at an air conditioner compressor motor itself, especially in cold weather the problem may be failure of the crankcase heater.


Causes of & Diagnostic Tips for Contactor or Relay Chattering / Rapid Clicking

Chattering or clicking on and off very rapidly, perhaps multiple clicks per second, occurs only on electromechanical relays.

The most-common cause of chattering in contactors or relays in HVACR equipment is imperfect contact between the two contact surfaces or points. That contact problem may be due to burning or pitting of the contact surfaces or less often due to mechanical damage to the contactor switch assembly. About equally common is an accumulation of dirt or debris interfering with good electrical contact between the contacts.

Causes of contactor relay chatter and clicking include these items, arranged in order alphabetically rather than in order of probability:

Solid state relays won't chatter, chiefly because they don't operate by using moving parts such as those shown in our sketches earlier in this article. Also, solid state relays (SSRs) include circuitry that provides a built-in switching delay, operating time and release time that will determine the maximum switching frequency of those relays.

Temporary or emergency repair of contactor or contact point surfaces

Before doing anything to the contactor / relay, try looking at it in view of the contactor chattering causes we list above. Look for dirt, debris, loose parts, loose wires. Some of these causes can be easily corrected without risking more aggressive cleaning steps that might damage the contactor.

  1. Try using compressed air (canned air from computer keyboard cleaner) to blow dust or debris out of both the contactor points assembly and, if exposed, the magnetically-operated coil and magnet.
  2. Check for and fix any loose wiring or loose mounting components.
  3. OK you want to clean the contactor's electrical contact points, particularly if they look badly burned.

Watch out: a common and widely chatted-up "repair" for burned relay switch contacts is to file or sand the points or surfaces. And indeed if you file or sand with care to keep the points surfaces parallel to one another this can work as a temporary repair to get the equipment working again.

I used to do this by removing the contact point assembly or at least removing its electrical connections, then inserting a double-folded piece of emory paper between the points. While holding the points closed I moved the sandpaper back and forth to clean the contact surfaces, thus keeping them in good, parallel contact. But here are some things to consider:

Air Conditioner or Heat Pump Relay Control Switch Chattering

Chattering or rattling noise in the A/C or heat pump compressor motor at start-up on some models of HVACR equipment whose compressor uses a compressor crankcase heater element if the equipment is started-up before allowing 24-hours of power-on time first.

Example from Carrier: "On 24ANA7 models, starting the compressor without a minimum of 12 hours of crankcase heat prior to initial start--up may result in a compressor chattering noise and possible damage to the compressor." Details about HVACR compressor motor crankcase heaters are found at CRANKCASE HEATERS

Shown here: a White Rodgers fan relay switch, #90-370 184-916 typical of fan relay switches used in air conditioner or heat pump systems to control a compressor/condenser fan or an air handler blower fan.

[Click to enlarge any image]

Watch out: a chattering electrical relay switch at any electrical equipment or appliance can be caused by a variety of problems such as burned contacts, low voltage, loose wiring, equipment malfunction, and a hard-starting motor.

If your air conditioner or heat pump compressor motor is having trouble starting that may show up as a buzzing or chattering relay. I'd turn off the equipment and call for an inspection and repair. A hard-start capacitor might get such a motor going but I suspect it's headed for replacement.

Chattering electrical switch relays occur on other mechanical systems as well. See WATER PRESSURE SWITCH NOISES and also see CHATTERING RELAYS, PUMP SWITCH in that article

Chattering noises in relays or switches used on HVAC equipment as well as all sorts of other noises coming from air conditioning or heating equipment are discussed at NOISES, HVAC SYSTEM DIAGNOSIS

Chattering rattling noises at the air conditioner compressor motor itself

Watch out: for chattering or rattling noises at an air conditioner compressor motor itself, especially in cold weather. If the compressor motor's electric heating element is not working or if you are trying to start a compressor motor in cold weather before the crankcase heater has been on for at least 24 hours the inability of cold lubricant to circulate and lubricate moving parts in the compressor motor can cause more than chattering or rattling sounds. It also damages the motor and shortens its life.


Compressor Motor Relay Switches & Contactors

Mercury displacement contactor switch used with compressor motors at,

Above is an example of a mercury displacement contactor switch or relay used to control compressor motors. When ordering a replacement contactor you'll need to specify at least the following:

Question: air conditioner clicks on and off every few minutes

(Aug 6, 2016) Anonymous said:
My unit will click on for a few minutes and then click off.
I have changed the batteries on the thermostat and changed the filter. What else can I do?

This question was posted originally at AIR CONDITIONING & HEAT PUMP SYSTEMS


This sounds like a control or relay problem - your HVAC service tech will look at your AC unit - indoors or outside for a failing control that needs repair, or perhaps first for a loose wire.


Heating & Electric Heat System Relay Switches

Electric Heat Relay Switches

Below: a typical replacement relay switch used with electric heat systems.

This switch might be operated by a wall thermostat and used to turn on or off electric baseboard heat. Chattering at this relay control or contactor switch would be unusual; if it occurs you should shut off the circuit immediately to reduce the risk of an electrical fire.

Electric heat relay switch (C)

Water Pump Pressure Control Switch Relay Clicking or Chattering

Photograph of a water pump pressure control switch with the cover onWatch out: while a single on-off click is normal for pressure switch operation, a pump pressure control switch that buzzes or chatters is sick and needs repair or replacement.

Schneider, discussing maintenance of their Square-D pressure control switch series offers these explanations and cures for a chattering pressure control switch (adapted):

Reference: "Preventive Maintenance and Troubleshooting Guidelines for Class 9013F and 9013G Pressure Switches" (2007), Original source: Schneider Electric USA 8001 Knightdale Boulevard Knightdale, NC 27545 USA 1-888-SquareD (1-888-778-2733)

Well pump relay switch and power control © D Friedman at

See WATER PUMP PRESSURE CONTROL SWITCH for installation, adjustment, troubleshooting & repair of well or water pump control switches.

The water pump control relay switch is illustrated separately and in more detail at WATER PUMP RELAY SWITCH.

All types of noises traced to building plumbing systems or water systems are discussed at PLUMBING SYSTEM NOISE DIAGNOSIS & CURE - home.

Sources of Replacement Contactors & Relay Controls

Field Report: Chattering Buzzing Sound Repair at Compressor/Condenser Unit

Chattering relay in air conditioner compressor/condenser unit - (C) BHQuestion: intermittent chatter, swapped out by changing contactor, more trouble

Great site and thorough set of diagnostic resources! I wasn't able to find a noise similar to what my system is doing on your site, so I figured that you may be able to add it to your library...if someone knows what it could be, even better!

Prior to this set of sounds, I was getting an intermittent chatter, which was resolved by swapping out the contactor and cleaning all electrical contacts. Now I'm getting this noise (attached), which was not resolved by changing thermostats (Had a Nest, and was suspicious that it may be a software based issue - Installed honeywell rth221b).

Seems to be sporadic as to the timing, but I tend to hear it at night - perhaps primarily due to the fact that I'm in the house, and it's quiet.

[Click to enlarge any image]

Temperature is cool enough right now to get by with fans while I figure it out and before I break down and pay someone for a diagnosis! Any info you have is helpful. - Anonymous by private email 2016/07/19

A/C COMPRESSOR RATTLE CHATTER NOISE [.MOV file] sound recording (no image) [314 KB]


I'll look at the video, and post with comments and send you that link for comment. Thank you so much.

About remaining chatter (comment more after seeing video), it's easy enough to take the thermostat out of the equation: just disconnect it entirely at the appliance-end and jump the two TT wire terminals together at the appliance - that's the same as the thermostat calling for cooling (or heating or whatever).

Chattering relay in air conditioner compressor/condenser unit - (C) BHSee CHATTERING RELAY CONTROL SWITCHES


Also see FAN NOISES, HVAC as ticking and chattering may be traced to a fan problem.

Reader follow-up: suspect voltage issues

Thanks... - the thermostat was a necessary buy in the next 12 months anyways assuming our housing plans come through as we hope - we liked the nest and had no issues with it last year or all winter, and plan to take it with us!

My neighbor is a commercial hvac guy so we're going to climb under the house tonight and see if we can take a look - I'll update you if we get anywhere! - 2016/07/19

We believe that the issue is a low voltage power issue, as the sound is most definitely the contactor arcing and rapidly engaging/disengaging. We thought that we found the issue in a loose connection at a wire nut - when we unscrewed it, the connected wires were actually broken off and stayed in the nut when we took the cap off.

We unscrewed, cleaned, re-stripped, and re-connected all of the connections, but are still getting the issue. We've got a fan control unit (white rodgers 90-113 if I'm calling it the wrong thing here) on order, as a colleague of my neighbor suggested that as the next logical step in the circuit.

Some additional information about my unit - it's an older carrier system (~20 years or so), and has both a high and low pressure safety? (neighbor seemed surprised by that) so figured it worth mentioning. I may be getting some of this confused since I'm translating from memory, but I figure if you know what you're talking about you can probably make heads and tails of it all.

Thanks - I'll keep you posted once we get the fan control unit in. If I have a chance to get a video of the chattering contactor with the lid removed, I'll send it on over. - 2016/07/20

Moderator reply:

Chattering relay in air conditioner compressor/condenser unit - (C) BH Thanks for the update. The "safety" controls to which you refer are probably pressure sensors that shut off the compressor if the system pressure indicates malfunction or unsafe conditions. Those controls can also produce a hissing sound if they operate.

Look closely at the circuit board; even a non-E.E. can sometimes spot trouble such as a burned component on the board - dark brown or black areas. The bad relay you described can be either a root cause of trouble or it can itself be burned and damaged by something else that is too frequently trying to turn a piece of equipment on and off.

Do keep me posted, and stay safe.

Shown here: the data tag for the air conditioner compressor/condenser unit being discussed.

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Reader follow-up: hot seized compressor

Came home from a weekend away to a hot, seized compressor. Hoping for the best and giving it the night to cool down and try a restart in the AM. The low voltage issue we suspected sounds an awful lot like the info you have on hard starts, so it sounds like we may end up a bit pricier than I had hoped.

Moderator reply:

A working theory is that a relay or other component is chattering because it's trying to switch on and start a motor that just won't behave.

There is a tremendous industry selling hard-start capacitors to give recalcitrant motors an extra kick in the seat of the pants. I am usually suspicions that we're throwing good money after bad.

Provided we first rule out a simple failure of the actual normal start/run capacitor, then if a motor's bearings or some other parts are failing so that the motor is resistant to starting, giving it a bigger voltage jolt may get it going but it's still a motor headed to failure.

Once a motor has seized at all, even if we get it going again by cooling down, and with some motors, squirting in some oil, I think the damage is already done.

Installing a hard-start cap is relatively inexpensive and easy so it's not ridiculous to try that, and it may indeed give some extended life to a failing and expensive part, but I'm just sayin'.

Reader follow-up:

No Dice on the restart. Got the compressor, coolant, drier, etc. on order and will be doing the replacement this weekend. Fingers crossed this solves the chattering issue as well and that it's not a low voltage problem. - 2016/07/27

Moderator reply:

Someone checked voltage, right? That's easy enough to do. Also a check of the current draw when the compressor is trying to start is a powerful diagnostic. Very high amps might be read if a motor is seizing.

Also a check of pressures is important. E.g. if an expansion valve has failed a compressor can have trouble re-starting against high head pressure.

Reader follow-up: problem solved:

Thanks for the advice (and yeah, we checked voltage on day 2 when my neighbor brought his work toolbox home). I'll make sure to mention the pressure check also! As always, will keep you posted with updates until we get it figured out - hopefully this will ultimately be a helpful resource for your site - 2016/07/28

Switched out the compressor, drier, capacitor, and relay on the furnace that we hadn't switched out already (it was for the heat and shouldn't have had any impact, but while you're in there...), and had previously switched out the contactor and fan control unit. Cleaned up some wiring connections, Got it charged back up and we're blowing cold air again!

Total parts cost ~750, plus $300 labor...certainly cheaper than a new unit! Thanks for all of your help and advice, and I hope this helps someone else down the road! 0 2016/08/01

Moderator reply:

Congrats; those repair costs sound surprisingly low, especially including the compressor motor. I'll post your discussion at A/C CONDENSER CHATTER / BUZZ REPAIR SUCCESS as that will certainly help other readers

Special Thanks: to Alan Carson, Carson Dunlop Associates, for technical review, comments, questions that improved the content and accuracy of this article. August 2016.


Continue reading at NOISES, HVAC SYSTEM DIAGNOSIS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Or see FAN NOISES, HVAC as ticking and chattering may be traced to a fan problem.


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