What is air conditioner short cycling, what causes it, and how do I fix air conditioner or heat pump short cycling on and off?

Here we define air conditioner short cycling and we give a complete list of all of the causes of short cycling along with repair suggestions.

Air Conditioner or Heat Pump Short Cycling Diagnosis

Article Contents

• AIR CONDITIONER OR HEAT PUMP SHORT CYCLING DEFINITIONS
• 15 CAUSES OF SHORT CYCLING
• EXAMPLES OF NORMAL & ABNORMAL ON/OFF CYCLE TIMES

Definition of Air Conditioner Short Cycling

Short cycling means that your air conditioner turns off and on too often, too rapidly, or at irregular intervals. That frequent on-off switching will ultimately damage the compressor motor, fan motor, controls, or other components, leading to system failure and costly repairs.

These specifics will help you decide if your air conditioner is short cycling or operating normally.

Air conditioner short cycling in minutes:

If your air conditioner turns off and back on at intervals of less than five minutes it is short cycling and needs repair.

If your air conditioner turns on and off at intervals of less than ten minutes it is probably short cycling and also deserves a service call.

Air conditioner short cycling in seconds:

Watch out: A "short cycling" air conditioner or heat pump compressor/condenser unit that turns on and then back off after just a few seconds is in serious trouble.

You should turn off the system and call your HVAC service company for repair.

Irregular on-off A/C cycling:

A short cycling air conditioner may also be heard running at irregular intervals over the course of an hour or so even though the indoor thermostat settings have not changed.

The compressor/condenser unit is the air conditioner or heat pump equipment that is located outdoors, such as shown in our photos.

The good news is that some of these AC short cycling causes are trivial to repair and thus not costly. Some, like a dirty air filter or dust-clogging a thermostat can be addressed by the homeowner.

Other air conditioner short cycling problems require help from your HVAC service technician.

For each short cycling cause or condition we link to a more-detailed diagnosis and repair article.

Normal Air Conditioner On-Off Cycling Times

If your AC system is properly sized, on an average day during the cooling season your unit will be working about half-time, or at about 50% of its cooling capacity.

While the actual "on" time of your air conditioner will vary for other reasons such as the indoor air handler unit's fan speed and cooling coil cleanliness, outside you will typically hear or see the compressor/condenser motor turn on for about ten minutes, then remain off for about the same length of time (10 minutes) before it turns on again. A 20-minute on-off cycle would also be normal and just fine.

In sum, if your air conditioner or heat pump runs for ten minutes, twenty minutes, or longer, turns off, then after a similar time interval it turns back on, your air conditioner is not "short cycling".

During very hot weather your air conditioner system may run just about constantly. If it is producing cool air the system is probably working normally. If however, your AC unit seems to never turn off, see the diagnostic details at LONG-ON CYCLING AC COMPRESSOR..

If your air conditioner is running but your home is not getting cool, see LOST COOLING CAPACITY.

15 Causes of Short Cycling Air Conditioner

An air conditioner or heat pump that is cycling off and immediately back on or cycling off/on every 10 seconds is abnormal and is likely to damage the equipment. I would turn the system OFF while waiting for repair, as I worry that more components will be damaged. I suspect this is a control board or relay problem, possibly related to operating temperature or pressure.

Short-cycling on and off of an A/C compressor unit every few minutes (say 1-5 minutes) sounds to me like a diagnosis and repair are needed.

A second sort of "short cycling" of the compressor also suggests that diagnosis and repair are needed: if an air conditioner or heat pump compressor runs for a very long time (i.e. the cooling system is not able to reach the indoor set point on the thermostat) and the compressor shuts off for just a few minutes, then restarts, I suspect trouble.

Causes include lost refrigerant, ice-blocked cooling coils, thermostat not working properly (blocked, in sun, in airflow, dust-crud clogged).

1. Loss of air conditioner refrigerant:
   
   This is due to a refrigerant leak in the system.
   
   You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak.
   
   Otherwise you will have to keep repeating the costly service call to just add refrigerant.
   
   See REFRIGERANT LEAK DETECTION for more about diagnosing and fixing low refrigerant that may cause air conditioner short cycling.
   
   Some air conditioners and heat pumps include a low pressure switch that will turn off the compressor motor if there is inadequate refrigerant. This switch is there to protect your system from damage.
   
   So if your thermostat is calling for cooling and the compressor/condenser fan runs but the compressor stays off, this switch might be doing its job. You will need to call your HVAC repair company.
2. A/C Coil Icing:
   
   If the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.
   
   When power is off and air isn't moving through the system the ice melts, the coil clears, air can flow again, and the system will work again - for a while, until the coil is ice and frost-blocked again.
   
   Take a look at the cooling coil inside of your air handler - IF you can see it. On many air handlers the coil is not directly visible without at least removing an access panel (or making one). The cooling coil will be on the outlet end of the air handler or for a vertical unit, above the blower fan assembly.
   
   If you see ice or thick frost covering the cooling coil (aka evaporator coil) the coil is ice blocked. You can turn off your A/C system, let the ice melt, and then restart it, but most likely your system needs something further.
   
   What? If the air filter in your air conditioning system is very dirty, then air flow across the cooling coil is blocked and it'll ice up.
   
   So while you're waiting for the ice to melt on the cooling coil, check the air filter.
   
   See FROST BUILD-UP on AIR CONDITIONER COILS for more details.
3. Bad refrigerant metering device, cap tube or TEV:
   
   If a thermostatic expansion valve is sticking and not releasing refrigerant into the cooling coil the compressor will find abnormally high, even dangerously, head pressure on its outlet end - the "high side" of the system.
   
   Some compressor units include a safety switch or high-pressure cut-out switch that will then turn off the compressor motor. Any cause of abnormally high pressure can trip that safety control.
   
   When pressure falls the cut-out may re-set and allow the compressor to restart.
   
   See THERMOSTATIC EXPANSION VALVES
   
   Watch out: an air conditioner or heat pump system that has been overcharged with refrigerant can also show up the problem as short cycling - excess refrigerant is blocking proper operation of the TEV.
4. A/C control board or switch problem:
   
   It's less likely, but a damaged control board or switch could also be causing rapid equipment on-off cycling.
   
   For example, if a TEV is jammed shut, high pressure on the outlet side of the compressor could be short cycling or shutting the system down.
   
   Also a bad control relay with worn spring, burned points, or other defects can switch the equipment on and off when it shouldn't.
   
   See CHATTERING RELAY CONTROL SWITCHES
5. Compressor damage or compressor start-troubles: 
   
   If a compressor motor is overheating, a safety cut-out switch may turn off the motor, allowing it to restart when the motor has cooled-down. Check for debris-blocked condensing coils at the compressor/condenser unit and double check that the condenser unit's fan is operating properly.
   
   If someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor.
   
   Normally that head pressure bleeds off over time, making it easier for the compressor to restart (against low head pressure) the next time it turns on.
   
   See HARD STARTING COMPRESSOR MOTORS and also
   
   see CAPACITORS for HARD STARTING MOTORS
   also
   
   See REFRIGERANT HIGH HEAD PRESSURE DIAGNOSIS
   also
   
   See BURNED-OUT COMPRESSOR
6. Bad condenser unit fan:
   
   If the condenser unit fan is not running or not running at full speed or has intermittent start trouble the compressor/condenser unit may be shutting down from overheating of the compressor motor.
   
   See FAN, COMPRESSOR / CONDENSER UNIT
   and
   
   See ELECTRIC MOTOR DIAGNOSTIC GUIDE
7. Thermostat wiring or device defects:
   
   Check for loose thermostat wires, debris-clogging of a thermostat's bimetallic spring or thermocouple (and clean them).
   
   A thermostat that is not working properly or that has loose wiring connections anywhere in the thermostat or control circuitry can result in rapid and irregular switching on and off of the air conditioning system.
   
   Imagine how crazy you are driving your air conditioner system if jiggling wires are saying "I'm too HOT... NOOO I'm too COLD... OH HECK I'm too HOT" and so on every few seconds as vibration or anything else causes wires or a contact to keep opening and closing.
   
   The thermostat could be at the bottom of an air conditioner short cycling problem if the thermostat is improperly located.
   
   For example, placing the thermostat right in the path of cool air being delivered to a room will cause the thermostat to cool down and turn off the A/C before the rest of the room has been made comfortable.
   
   See THERMOSTAT CALIBRATION for a list of thermostat locations to be avoided.
   
   See THERMOSTATS, HEATING / COOLING for a complete article series on thermostat installation, troubleshooting, repair.
8. Oversized air conditioner:
   
   If the short compressor on-cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled.
   
   A symptom of an over-sized air conditioner may be that your home quickly gets plenty cool enough but your air conditioner is not reducing the level of indoor humidity.
   
   A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled.
   
   Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat.
   
   See AIR CONDITIONER BTU CHART and OPERATING TEMPERATURES HVAC
9. Bad air handler fan speed:
   
   Some air handlers use a multi-speed or variable speed fan.
   
   If the fan speed is set too high for the design of the duct system and/or the building requirements the space may cool down too rapidly, humidity may not be removed, and the cooling unit may cycle off too quickly.
   
   If the fan speed is inadequate we may see that the system runs continuously and cannot cool the occupied space, or we may see icing at the cooling coil.
   
   See BLOWER FAN OPERATION & TESTING - home
   and
   
   See FROST BUILD-UP on AIR CONDITIONER COILS
   
   A dirty squirrel cage fan in the air handler will reduce the effective airflow and can also cause long on-cycles, coil icing, abnormal cooling system operation.
   
   See DIRTY COOLING COIL / EVAPORATOR COIL
10. Overheating electric motors:
    
    Some compressor motors and some fan motors include circuitry that switch off the motor under conditions of overheating - or "thermal overload". The motor may be running hot because of improper voltage, other rapid cycling demands from its controls, a failing bearing, mechanical damage, a lubrication failure or something else I've not thought-of.
    
    Some motors will automagically re-set their thermal overload switch when the motor cools down and the motor starts again. Heats up, shuts off, cools down, starts up. Some readers have diagnosed this trouble at an outdoor compressor/condenser unit by giving it a cool-down shower with a garden hose.
    
    Other electric motors such as on some oil burners pop a thermal overload switch that has to be manually re-set before the motor will run again.
    
    See RESET BUTTON, ELECTRIC MOTOR
11. Poor A/C or Heat Pump Compressor/Condenser Location or Siting:
    
    Depending on its operating temperature rating, a compressor/condenser unit located in a very hot, very challenging condition (rooftop in Saudi Arabia) or one whose airflow is blocked by bushes, buildings, or neighbouring compressor/condenser units fighting over airflow can overheat and may cycle off.
    
    See CLEARANCE DISTANCE, HVAC
12. Electrical power supply problems:
    
    This is less likely as more often if you've got an intermittent power supply problem or wide voltage variations from the power company you'll notice the issue soon by seeing flickering lights or problems with other equipment besides just the A/C system.
    
    But a loose wire or defective circuit breaker feeding just the air conditioning or heat pump system could be at fault.
    
    See COMPRESSOR / CONDENSER DIAGNOSTICS
    
    and
    See ELECTRIC POWER LOSS / FLICKERING LIGHTS
13. Unusual rate of building heat gain:
    
    A building that is poorly insulated, has usually high solar gain, or from its location, construction, or other features such as air leakage, has a high rate of heat gain can also be a nightmare for HVAC compressors who dream.
    
    If your building has a high rate of heat gain, the occupied space may re-heat unusually quickly once the air conditioner has cooled the space down to the thermostat's set temperature. This problem may show up as a long ON cycle followed by a short OFF cycle of the heat pump or air conditioner.
    
    See HEAT LOSS (or GAIN) IN BUILDINGS
    and
    
    See HEAT LOSS (or GAIN) INDICATORS
14. Bad thermocouple or dirty thermistor on gas fired equipment:
    
    Short cycling or unexplained on-off cycling of heating equipment has also been traced (by one reader) to a dirty or failing thermocouple (possibly the reader meant a dirty thermistor).
    
    See THERMISTORS
    Or
    
    see THERMOCOUPLE REPAIR / REPLACEMENT
    
    This condition was reported in the FAQs by a reader commenting
    
    at HEAT ANTICIPATOR ADJUSTMENT
15. Outdoor heat pump unit defrost cycle problems:
    
    These can cause compressor or fan short cycling. See details at HEAT PUMP OUTDOOR UNIT DEFROST CYCLE

But if someone is turning the system on and off quickly, the compressor may have a hard time restarting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.

We have also see or a damaged compressor internal refrigerant valve causing high head pressures.

In sum, you may need a service call from a professional to correctly diagnose and repair the problem.

Examples of Normal & Abnormal A/C Compressor or Heat Pump Compressor Unit On/Off Cycle Times

Overall: I'd like to see a compressor/condenser unit ON cycle time of 10-20 minutes.

The length of time before the next ON cycle of the compressor/condenser unit isn't standard but it ought to be more than 5 minutes, perhaps 10 or more. The rate of heat gain, air leakage and other factors determine how quickly a cooled space heats up again once the cool air flow stops.

If an A/C or heat pump compressor/condenser unit is on about half-time and is cycling on/off say 3 times in an hour in hot weather while cooling a residential building with typical insulation, air leakage rate, and heat gain rates for its climate, that's probably normal.

In very hot weather an air conditioner or heat pump system may run continuously or nearly continuously as long as the indoor thermostat is calling for cooling.

It's possible for a residential air conditioning system to be operating normally when a compressor shuts off, then turns back on depending on the system's controls. That's so if the "off" period is long - perhaps 10-15 minutes depending on weather conditions and system capacity. In moderate heat, 10 minutes on and 10 minutes off would not be abnormal but in my OPINION it's close to the edge.

A compressor motor that is cycling on and off rapidly is going to be damaged by that condition and depending on the system design, it may also ultimately have trouble restarting against high head pressure in the system. I think most HVAC techs (more expert than I) will agree that if we get down to 5 minutes or less on and off there is definitely a serious operating problem to be found and fixed.

In any event, 5 Minute on-off cycling or more rapid on-off cycling of an air conditioner or heat pump is very likely an indicator of trouble.

The article above suggests common explanations for on-off or short-cycling of an A/C compressor unit. Usually I think first of an overheating motor. That in turn can be due to a motor that's failing or to another cause such as low voltage; a bad control board or contactor are less-likely candidates but then your onsite tech may find something else.

What about a compressor that stays on too long instead of too short a time?

If your air conditioner or heat pump has the opposite problem, staying on too long, see LONG-ON CYCLING AC COMPRESSOR.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-09-13 by Daniel

In south tx have thermostat set to 70, outside unit runs for 7 minutes then shuts off for 20 minutes is that ok?

On 2020-09-13 - by (mod) -

Daniel

Check your room or control thermostat; if the AC unit is shutting off because the SET temperature on the thermostat has been reached, that's normal and OK;

If it has not gotten close to the set temperature then the compressor/condenser may be overheating.

On 2020-07-13 by Aaron

After my unit shuts off the air handler continues to run for a minute. Is that normal?

On 2020-07-1 - by (mod) -

Aaron

It is normal for an air handler to continue to run briefly at the end of an on cycle in heating mode. It's most common when the system is being used in as it's desirable to purge heat from the heat exchanger, avoiding the risk of damage to the equipment. They may be some systems the do the same thing at the end of a cooling cycle.

On 2020-07-04 by Dave - very short on-cycles

We have a one year old Lennox 2 stage A/C. We noticed suddenly that it began short cycling... running for maybe 3 seconds and shutting off, then doing the same thing about 5 minutes later, and again running just a couple of seconds. The temperature inside though was OK, at 75 degrees which was the thermostat setting.

The next day, it worked fine without short cycling. The day after that, it began short cycling again. A HVAC technician came the next day, when it was working fine, and he found nothing wrong. He surmised maybe the low stage is so quiet it was running continually, or something, that didn't make sense. It's started short cycling again, and this time the temp went up to 77. Then it started working again. It's very frustrating as it seems we might be damaging this one year old unit.

On 2020-07-04 - by (mod)

If you're really seeing very short on-cycles then I agree it's worth further troubleshooting. Perhaps a polite conversation with the HVAC service company's service manager will get you a tech experienced with the issue. Sometimes it's a dirty expansion valve or other control that misbehaves intermittently and of course behaves perfectly when the repair person is on site.

On 2020-06-28 by Christine - Air Conditioner shuts down in very hot temperatures

A/C turns off during the hottest part of the day for two+ hours.

On 2020-06-28 - by (mod) -

Christine

Some AC systems include safety controls that will shut the system off to prevent more-costly damage from, for example, overheated components or excessive refrigerant pressure.

So I suspect your system, when it's working hardest, has a component that's overheating or otherwise causing a condition that shuts the system down.

An experienced A/C service tech can track down the failing part.

On 2020-06-15 by Teresa - air conditioner seems to be short cycling

My air conditioner seems to be short cycling. Yet it has a thermostat which has a automatic compressor protection delay to avoid potential damage to the system. At least that’s what the thermostat manual says . So why would it be short cycling ?

On 2020-06-15 - by (mod) -

Teresa,

With no more data I can but guess that the short cycling compressor is due to one of the causes in the article above, perhaps a bad expansion valve.

On 2020-05-22 by fred - heat pump compressor turns off randomly

heat pump compressor turns off randomly but fan keeps running

On 2020-05-22 - by (mod) -

If not one of the causes above on this page, perhaps a bad relay, control, or even a loose or bad electrical connection.

On 2019-07-12 by Susie - My electric bill is increasing 30-40 every month

My electric bill is increasing 30-40 every month. Has tapered off after four months. New home. New a/c. Six other new homes, same size or larger, with more people, thermostat set much lower with electric bill 40 or more dollars less. Ac company cannot find anything. Electric company said my cooling percentage is too high.

On 2019-07-14 by (mod)

Perhaps an expert electrician onsite making amps checks can find the problem.

Watch out, there may be safety or shock hazards.

On 2019-05-17 by Richard D.- burned wiring, now AC unit short cycles

Last year at the end of the summer my AC unit burnt the T-1 wire off the top of the Contactor. I didn't repair it until this year. I replaced the terminal end and started it up and now it's short cycling. I will run about 30-45 seconds then shut off for about 1 or 2 seconds then immediately start again an repeat the symptom. Any thoughts on this would be greatly appreciated.

On 2019-05-18 by (mod) -

Do you think that replacing the burn to terminal actually corrected the problem that caused the burn up? I can't tell from the text. You might want to go through this with Justin's for short cycling given in the article above on this page.

On 2018-12-07 by Gary- blower cycles every 30 seconds

What will cause a blower motor to cycle every 30 seconds yet run smooth when the aux heat is used. When the thermostat calls for a 1 or 2 degree rise in temperature the blower motor short cycles but when the thermostat call for 3 degrees or more it doesn't short cycle until the thermostat setting is within 2 degrees. Than the short cycling begins

On 2018-12-08 by (mod) -

probably one of the reasons given in the article above.

Watch the temperature dial in the fan limit control switch as the furnace goes through a cycle and let me know at what temperature the dial is pointing and which direction does it moves when you see the short cycling.

On 2018-11-14 by Terry - why does heat pump turn on and off too frequently?

What will make your fan on a heat pump cut on and off

On 2018-11-15 by (mod) -

Possibly a problem with the defrost cycle.

On 2018-08-30 by Joe- AC unit runs 5 minutes then I smell something burning and it shuts off

Ive been noticing the smell of something burning the past 3 days now it's hot as hell in here. I went outside to see if the unit was running and noticed the unit only runs 5 minutes then it stops.

On 2018-08-30 by (mod) -

Joe

On the chance that the burning odor is from an electrical or motor or HVAC equipment failure there is risk of a fire. Sorry to say you should turn the equipment off at its circuit breaker or fuse, then call for repair.

...

Continue reading at COMPRESSOR / CONDENSER DIAGNOSTICS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see SHORT CYCLING AC COMPRESSOR FAQs - answers to questions posted originally at this article.

Or see these

Recommended Articles

• BLOWER FAN SPEED SETTINGS - how to set the air handler fan speed
• LOST COOLING CAPACITY if your AC unit is not cooling
• AIR CONDITIONING & HEAT PUMP SYSTEMS

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SHORT CYCLING AC COMPRESSOR at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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