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Photograph of This new compressor was placed directly against a brick wall. One third of its condenser coil cooling ability was blocked. Heat Pump Compressor / Condenser Fan FAQs #4
Q&A about the outdoor AC unit fan operation

A/C or Heat Pump Fan Condenser Unit Fan diagnostic FAQs No. 4.

These questions & answers about the outdoor unit fan opeeration help diagnose problems with the air conditioner or heat pump condenser unit fan or with no-cooling or no heat (from a heat pump) system.

This series article discusses the diagnosis and repair of problems with the outdoor compressor / condenser fan and fan motor, including fans that wont' run, fans that run at slow speed, and fans that won't stop running.

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HVAC Compressor/Condenser Fan Diagnostic questions & answers

Air conditioning compressors overgrown with shrubs (C) Daniel Friedman

These questions and answers were posted originally at FAN, COMPRESSOR / CONDENSER UNIT - be sure to see the repair advice at that page.

If you are troubleshooting the indoor air handler blower fan that delivers cooled or heated air through the duct system and supply registers, see BLOWER FAN OPERATION & TESTING - home.

On 2019-05-02 by (mod) - confusion when new capacitor has different number of connectors or wires

Matt.

Please try tne search box at page top or bottom find our article on CAPACITOR WIRING

there you'll also find CAPACITORS SELECTION to be sure you have bought the proper type of cap.

On 2019-05-02 by Matt

My condensor fan went out it has 4 wires i bought a new fan and it came with a new single phase capicitor my unit has a dual capicitor ...and colors of wires are different but new fan has plug ins for capicitor ..can i get some help please

On 2019-01-17 18:39:14.116103 by Mark

I just replaced the outside (compressor unit) fan motor on my old Carrier heat pump. (The old motor was noisy from a worn bearing). The heat pump is now properly putting out heat - not just cold air. Any idea why?

On 2018-12-14 19:08:32.395386 by Lilo

Outsid fan will not run ore blow

On 2018-11-20 12:56:00.495978 by (mod) -

There may be a misbehaving defrost cycle control board.

On 2018-11-20 06:23:28.312379 by Paul jayson

Two stage cooling , and gas heating unit. This ac had an econonimzer that does not work anymore but, anyways during winter or in the night time once it’s around 65 degrees outside the compressor and indoor fan motor will run but not both out door fan motors, could this be a possible bad thermostat that is killing the power to both condenser fans? During summer they will run fine or during the day when it’s decenlty warm or hot.

On 2018-11-10 17:14:53.820227 by Charlie

Thank you

On 2018-11-10 17:00:05.367193 by (mod) -

Charlie:

The heat pump lowest operating temperature outdoors depends on the model and design of the unit. The most-modern, most capable residential heat pump in use currently can obtain heat from outdoor air at temperatures down to about - 4 degF.

However in MOST heat pump installations, a heat pump will be efficient and effective at providing heat from outdoor air down to temperatures around 25-30 degrees F (or above).

At that point the system will switch to backup heat.

You could turn power off completely, but in doing so any defrost or heater components in the outdoor unit are also left off - which means that when you switch power back ON to the unit it should probably not be asked to actually run for another 24 hours. Otherwise it could be damaged by frost, ice, or by the movement of liquid refrigerant into the compressor motor. (You need time for its heaters to do their work).

On 2018-11-10 14:34:03.216911 by Charlie

Danjoefriedman: Thank you for your answer. One other question is that, in winter is that okay that the unit and the heating is on, but the fan does not run. Also can you tell me how cold it is when the heat pump stops running. Thanks again. .

On 2018-11-10 03:02:51.380591 by (mod) -

The fan will stop during a defrost cycle when the heat pump is in heating mode and needs to defrost ice on the outer coil. But if it never runs then perhaps your heat pump is never running and you're not getting any use of it in cold weather.

On 2018-11-10 01:01:22.822774 by Charlie

My heat pump fan does not run in the winter, is that normal?

On 2018-11-05 20:59:13.741954 by (mod) -

Aaron

also check this article : SHORT CYCLING AC COMPRESSOR inspectapedia.com/aircond/Air_Conditioner_Short_Cycling.php

On 2018-11-05 20:58:05.085260 by (mod) -

Aaron;

Why and when a heat pump goes into a defrost cycle when in heating mode


When your heat pump is in heating the outdoor unit (compressor condenser) is extracting heat from (even chilly) outdoor air.

The outdoor unit fan runs, air moves across the outdoor coil, heat is absorbed from outdoor air into the coil and into the refrigerant inside the coil where ultimately that heat is delivered indoors through the indoor air handler.


Why does ice form on the outdoor coil?

In heating mode, as heat is extracted from the outdoor coil its surface becomes colder than the outdoor air temperature and can drop below freezing. Then moisture in outdoor air freezes on the coil surface.


Why is a heat pump defrost cycle needed?

Ice on the outdoor coil blocks air flow across the coil; with no air flow, no heat can be extracted from the outdoor air and worse, the compressor motor itself could be damaged.

So periodically the outdoor unit will go through a defrost cycle. A control board in the outdoor unit is the control over when a defrost cycle occurs.

Just how often a defrost cycle will occur is determined either by a timer on the outdoor unit’s control board (when in heating mode) or by sensors that some heat pumps may use to detect the presence of ice on the coil.

Depending on your heat pump brand and model - thus the heat pump's design - the outdoor unit in heating mode will run a defrost cycle as often as once every 30-35 minutes OR (on some more sophisticated designs) the heat pump may only run a defrost cycle if sensors tell it that the outdoor coil is iced over.


Factors that affect the frequency of a heat pump (in heating mode) outdoor defrost cycle include:

- the heating load - amount of heat being delvered indoors

- the compressor and control design (timer versus ice sensors)

- outdoor temperature (closer to freezing, more frequent defrosting needed)

- outdoor humidity (more humidity, faster ice formation, more frequent defrost needed)


What happens at the outdoor heat pump unit during the heat pump defrost cycle?

Usual design:

The compressor motor will continue to run - compressing and thus warming refrigerant in the system.

The outdoor fan will stop until the ice has melted away.

In some designs both the compressor and the fan may stop and electric heaters may be used to perform the necessary defrost.


When does the defrost cycle stop (for a heat pump in heating mode)

The defrost cycle will typically stop after about ten minutes maximum, and often sooner.

The defrost cycle will stop when temperature sensors at the outdoor unit detect that the coil’s temperature is high enough that no ice would be expected to remain on the unit.

Watch out: do not turn off your heat pump if it’s in the middle of a defrost cycle, or if you must turn it off, leave it off for at least 30 minutes to avoid damaging the compressor motor.


What happens at the indoor air handler during the heat pump defrost cycle?

During a heat pump in heating mode defrost cycle the indoor air handler will normally stop blowing air into the occupied space - that's because the system is not receiving heat and the designer doesn't want to blow cool or cold air onto occupants.

Indoors at the air handler or on some systems right at the thermostat a colored or blinking indicator light may tell occupants that the delivery of heat has stopped for a few minutes to permit a defrost cycle.


Abnormal Outdoor Heat Pump Unit Fan-On Cycles

If your thermostat does not call for heat for an hour or more (perhaps because the indoor thermostat is satisfied) I would not expect the outdoor unit to require more than one additional defrost cycles and I'd not expect the outdoor compressor/condenser fan to continue to cycle.

So if that's happening I suspect a temperature sensor or a condenser/unit control board error that's worth a service call.

If the outdoor unit is defrosting too frequently (at intervals of less than 30 minutes) something is wrong.

If at the same outdoor temperature that frequent defrost cycle has occured since original installation, the unit may be under-sized and unable to provide the amount of heat being asked of it.

If the indoor air handler air filters or blower fan are dirty then the indoor air flow and heat delivery are reduced and the heat pump may be over-taxed (as it can’t satisfy the thermostat).

If the coil in the indoor air handler is dirty that too blocks air flow, reduces heat delivery, and over-taxes the heat pump.

On 2018-11-05 17:49:37.981949 by Aaron

Heater on heat pump works. But, the outside fan turns on several times per hour even when no heat is blowing inside. Is this normal?

On 2018-09-22 by (mod) - ac unit short cycles during the day.

Jessie

Please check this article : SHORT CYCLING AC COMPRESSOR https://inspectapedia.com/aircond/Air_Conditioner_Short_Cycling.php

and let us know if questions remain

On 2018-09-22 by Jessie -

Novice here....
The ac unit short cycles during the day. It runs fine at night. We are in southern humidity & 90°+ weather.
Can someone tell me what the typical culprit for this scenario is?

On 2018-09-07 by (mod) - a snake got caught in my compressor unit fan

Mark

Nice going. I'm glad the system re-set itself when the fan motor, probably overheated being bound up by a snake who was surely upset as well, cooled off and re-started.

Let me know if you or the snake have further questions.

On 2018-09-07 by Mark Kennedy

I posted too early. After I turned the ca off for about an hour it came back on.

A snake giot caught in my fan and stopped it for several hours it seems. Now fan will not spin and can't hear any power getting to outside unit?

On 2018-09-02 by (mod) - fan runs inside but not outside

If the indoor fan runs but the outdoor compressor/condenser doesn't COMPRESSOR / CONDENSER DIAGNOSTICS at https://inspectapedia.com/aircond/HVAC_Condenser_Unit_Diagnosis.php might help

On 2018-09-01 by M.long

Fan runes outside wount.

On 2018-07-23 by (mod) - bad fan relay or frozen fan motor.

Greg:

Sounds like a bad fan relay or frozen fan motor.

On 2018-07-22 by Greg

I have also checked relay on board and fan gets 110 signal from it

Why would my condenser fan not run if I have checked capacitor and motor and all check good. I manually engage contacts and compressor comes on but fan doesn't run even with a nudge with a stick

On 2018-06-01 by (mod) - fan won't run

Check the fan motor removed from the unit - that will tell you if the problem is the fan motor, bearings, capacitor, or the control board, relay, wiring.

On 2018-06-01 by George

I replaced the fan motor and capacitor on my unit and now the the fan won't come on. I've checked wiring several times and can't find any reason it not to start.

On 2018-05-07 by (mod) -

Sorry but I don't see enough information in the question to speculate on a useful answer. Did you try calling an HVAC service tech? What were you told?

On 2018-05-05 18:08:45.635234 by Anonymous

Fan starts on outside not but stops after 15 or w0 and restarts but we feel cool air for a little and then nothing

On 2018-03-19 17:41:08.827631 by (mod) - ac condensor fan will not power on.

Since we trust that the fan motor is OK the problem is most likely the fan relay or in the control board or in a wiring connection.

On 2018-03-13 23:10:05.624600 by Alan

My ac condensor fan will not power on.
I

Replaced the capacitor

Swapped out the fan motor into another unit - it worked fine

Cleaned the contactor contacts

Compressor srarts/runs

Any ideas as what to do next? The unit is a ten year old carrier unit

On 2018-02-23 1 by Theo

Compressor run but no power to condenssor fan.

On 2018-02-01 by (mod) - heat pump (outdoor unit) cannot star

Sounds as if the indoor blower fan works but your heat pump (outdoor unit) cannot start - could be because of very low outdoor temperatures, combined with a failure of your backup heating system. I hope you called your HVAC tech for repairs.

On 2018-01-28 10:43:49.959093 by Anonymous

Fan does not run! Room blows out cold air and runs constantly, blowing cold air! Why

On 2018-01-27 23:43:59.611204 by (mod) -

With NO heat and thus no air blowing from the supply registers I suspect that your blower fan is not running.

That could be a fan failure (like a broken fan belt) or it could be that the heat is not working and as the supply plenum never heats up the fan never turns on. Watch the air handler when the thermostat calls for heat.

On 2018-01-27 22:53:40.724266 by Mike

My Carrier fan coil unit stopped producing heat. When you turn on the thermostat, the unit comes on(inside & outside), however no heat blows out from the vents. I just moved into the house and I have no maintenance records. Is this possibly a filter replacement issue ?

On 2018-01-27 00:48:49.941078 by (mod) -

Capacitor bleed into windings is not quite a diagnosis I understand but I infer that the start or run capacitor was defective.

On 2018-01-27 00:44:10.570967 by Mark

Well, they replaced the compressor and it is fixed now except we won't know why is was doing that. All they could say was the capacitor was bleeding into the windings. Since it is under warranty it will go straight back to Rheem so no finding out what this phenomenon was except that it can happen.
Thanks so much for your time and attention, though.
Best Regards,
Mark

On 2018-01-17 21:02:35.035801 by (mod) -

Recently-posted Q&As for this article are at https://inspectapedia.com/aircond/HVAC_Condenser_Fan_FAQs2.php

On 2018-01-17 20:51:18.046276 by (mod) -

Mark

Thanks for the updates. All of this sounds strange to me but probably we're seeing a communications gap between the technicians and the rest of us. There sounds like a confusion between volts and amps, or at the least, I'm unclear how a control board OR a bad compressor would change the VOLTAGE readings.

Central air conditioner compressors run at 120VAC or more-often 220 or 240 VAC nominal voltage.
Some units will specify an acceptable range such as 208-230VAC or, depending on your country and electrical service, up to 250VAC.

I agree that 246VAC is within acceptable limits for the supply voltage to many AC condenser units.

The AC circuit should be dedicated - it should not be also supporting other electrical equipment.

If the supply voltage (from the electrical utility) is exceeding what the equipment can handle that can damage the equipment. But you'd want to do more than just fix the equipment; you'd probably want surge protection and voltage conditioning equipment to protect the repaired compressor/condenser unit from new damage.

A typical voltage stabilizer will handle voltages ranging from 170-270VAC and will keep that voltage in the range needed by your equipment.

Research:

Ortega, JM Maza, M. Burgos Payán, JM Romero Gordón, and M. Pinilla Rodríguez. "Supply Voltage Effects on the Operation of Residential Air Conditioning Appliances: Theoretical Analysis."

On 2018-01-17 19:05:50.727657 by Mark

They changed the board but it still doing the same thing. They are going to replace the compressor next week. The system is running on emergency heat so the whole thing doesn't burn out in the meantime.They have never seen this problem before which is a little surprising since it's a company with 5 locations, not a mom & pop business.

On 2018-01-17 15:17:12.207444 by Mark

They are here now to fix it. What they are going to try first is replace the Board since it's easier/cheaper. They say the stored energy in the capacitor is discharging into the compressor and the fan on shut down. We'll see...


...

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