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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 #5

Recent Q&A on Diagnosing the Outdoor AC unit fan operation

A/C or Heat Pump Fan Condenser Unit Fan diagnostic FAQs No. 5.

These questions & answers about the outdoor unit fan operation 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 article series 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.

Article Contents

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Miscellaneous HVAC Condenser Fan FAQs

Can you use your A/C as a fresh air vent fan?

All I want to know is how to get new air into a room without the AC running. Can the fan be used for this?

when the AC is off and only the fan is running, is the fan just moving air around in the room or is it replacing the air in the room? 2018-09-20 by Carolyn Reese

Mod reply:

Carolyn

Sorry to be vague but ... as Mark Cramer says: It Depends.

If you are running a window AC unit AND if the window unit has a control that will admit outdoor air, then yes you can run the unit in fan mode and blow outdoor air into the home.

If you are running central air conditioning AND if your central AC system includes an outdoor air source you can do the same. But that would be uncommon in residential air conditioning systems.

So in my third case, sorry, but no. If you have a typical home central AC system and you just run the blower fan alone, it will circulate air and thus may make the building a bit more even in temperature but it won't be bringing in fresh outside air.

What I do, when the weather is suitable, is open a window and stick a box fan in the window, blowing IN.

In a prior home that had a whole house exhaust fan we had a better solution: when it cooled off outside we'd open windows downstairs, then turn on the whole house exhaust fan that drew outside air in the windows, up through the house, and out through the attic and roof exhaust.


Can a motor can be bad but check out OK on the bench when it is not running

Is it possible for a condenser fan motor to be bad even if

1) the windings ohm out correctly and

2) there are no shorts of the leads/windings to the case?

Motor shaft turns freely, no wobble, slight in and out play on the shaft, maybe 1/4" to 3/8".

I was out of town when the AC quit. Tech told my wife the capacitor was bad ( it does check out bad) as well as the fan motor. She made him leave the old parts, so I checked them. Comments? 2018-09-05 by MPG

Mod reply: YES

MP

Regrettably yes, a motor can be bad but check out OK on the bench when it is not running. That's because when the motor gets up to speed a wire can open due to the centrifugal force exerted on the rotor - the rotating component in the motor.

If a motor hums and won't start a common inexpensive thing techs try is to just replace the starting capacitor or to install a hard-start capacitor. Search InspectApedia.com for HARD START CAPACITOR to see details.


What happens if compressor is intentionally kept Off for a few minutes while Fan stays ON?

What happens if compressor is intentionally kept Off for a few minutes and the condenser fan is running for that time? Is it detrimental to the system? 2018-05-24 by Jyoten

Mod reply: not harmful but

No not at all,

Watch out: however cycling air compressor off and then back on to quickly can damage the compressor motor by forcing it to try to start against the high head pressure.



What is the speed of the air that takes away the heat from the refrigerant in the condenser

what would be the speed of the air that takes away the heat from the refrigerant in the condenser? 2017-05-10 by Sagar Lokare

Mod reply: 175 to about 600 CFM

Air speed can vary considerably in a residential low pressure HVAC duct depending on system design.

The speed ranges from about 175 cubic feet per minute to 5900 cubic feet per minute. Most likely you'll be in the low end of that range.



Does the outside condenser fan stop running when thermostat is set to cool?

Does the Outside condenser fan stop running when thermostat is set to cool. 2019-06-10 by Arturo

Moderator reply:

Air conditioning and heat pump systems will in fact stop periodically to go through a defrost cycle.


Surface rust on the stator in my heat pump motor?

Hello and thank you for the information. I can't physically turn my motor on my heat pump last year a service tech told me there was some surface rust on the stator he was able to break free I was not there to see him do it. Does this sound like the likely problem again? I didn't want to put too much pressure on it for fear I might damage or break the motor itself. 2019-06-05 by Todd

Reply:

From what you described I suspect that the motor needs to be replaced.


Reader question: heat pump fan does not run in the winter

My heat pump fan does not run in the winter, is that normal? 2018-11-10 by Charlie

Moderator reply:

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.

Follow up:

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.

Reply:

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).


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