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Short on-off cycling of air conditioner or heat pump compressor/condenser unit diagnosis:
What causes the A/C or heat pump fan condenser unit to keep cycling on and off rapidly and how do we fix it? Here we explain the common causes of compressor unit short cycling.
This article series explains how to diagnose an air conditioner or heat pump that is not cooling: this article explains
how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperature, or an air conditioner that won't start.
Air Conditioner or Heat Pump Short Cycling Diagnosis
Reader Questions: My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems
Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!
Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old
(July 17, 2016) Mike said:
This was an interesting article. My central AC works perfectly during the night, but short cycles 5 seconds on, half second off on hot days. Any thoughts as to what would be causing this?
2016/09/12 Linda Jackson said:
Our Trane Ac unit keeps cycling on and off about every 10 seconds.
(July 19, 2016) Johnny Todd. said:
York 3 ton heat pump. Heatpump kick on and right back off? I changed board! Super heat is perfect!! I'm lost plz help! Unit is jumped and and cools great great pressures, but kicks off as soon as it kicks in when wired properly?
14 Causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair
(mod) said to Joe and Matthew, Mike, Linda and others (re questions just above):
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 but that's as far as I can get.
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 the compressor shuts off for just a few minutes, then re-starts, I suspect trouble. Causes include lost refrigerant, ice-blocked cooling coils, thermostat not working properly (blocked, in sun, in airflow, dust-crud clogged).
Loss of air conditioner refrigerant - 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.
A/C Coil Icing - 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.
See FROST BUILD-UP on AIR CONDITIONER COILS
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.
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 re-start 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 re-start (against low head pressure) the next time it turns on.
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.
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.
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 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
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.
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
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.
Poor A/C or Heat Pump Compressor/Condenser Location or Siting: depending on its operating temperature rating, a compresor/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.
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.
Unusual rate of building heat gain: A building that is poorly-insulated, has unsually 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. Do air conditioners dream of electric suns?
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).
This condition was reported in the FAQs by a reader commenting
at HEAT ANTICIPATOR Adjustment
But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting 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 need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.
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 re-starting 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?
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Questions & answers about why an outdoor compressor/condenser unit is turning itself on and off too frequently, posted originally in this article are now found at SHORT CYCLING AC COMPRESSOR FAQs
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Timothy Hemm, Yucala, CA, contributed photographs of electrical wiring and equipment installed in California buildings. Mr. Hemm can be contacted at TimHemm@yahoo.com
A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
Thanks to reader and research scientist Cyril Roberts, Barbados, for technical discussion and investigation of air conditioning system dehumidification problems (April 2009).
Thanks to readers Beth & Dennis for asking about how to improve an inadequate air conditioning system supplying cool air through crawl space ducts and floor registers. (May 2010).
Thanks to reader William Smith for discussing cooling coil leaks and lost cooling capacity diagnosis - June 2010
Thanks to reader Jacob Behrends, FL for discussing how a clogged condensate drain line can overflow condensate into a condensate pan that in turn may contain a safety switch that shuts down the whole air conditioning system. August 2010.
Determining Electric Motor Load and Efficiency, U.S. Department of Energy, web search 08/01/2011, original source: http://www.p2pays.org/ref/40/39569.pdf [copy on file at InspectAPedia.com]
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
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