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Photograph of an old burned-out air conditioning compressor unit Outdoor Unit AC / Heat Pump Diagnostic FAQs #4
Recent Outdoor compressor/condenser unit questions & answers

HVAC compressor / condenser diagnostic questions & answers or FAQs:

This air conditioning repair article provides diagnostic questions & answers for the outdoor compressor / condenser unit used on air conditioners & heat pumps. These questions and answers help diagnose an compressor motor that is not working, compressor noises, condensing coil performance, or for heat pumps, outdoor coil icing.

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HVAC Compressors & Condenser Unit FAQs

AC Compressor © D Friedman at InspectApedia.com These questions and answers about troubleshooting and fixing air conditioner or heat pump compressor/condenser units were posted originally at COMPRESSOR / CONDENSER DIAGNOSTICS - be sure to check out the diagnostic suggestions on that page.

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On 2018-08-08 by (mod) - outdoor compressor takes too long to turn on

Paul,

The delay you described could be normal. For example in my office where we use a Sanyo split system air conditioner, the indoor blower unit may keep running at low speed but the outdoor compressor will turn on and off depending on when the control board (with its own computer) figures that it's needed. That's an energy-saving feature.

But IF your AC blower is delivering warm air AND the room temperature is still above the thermostat setting, then yeah something's wrong, perhaps with a sensor or control relay on the unit. That may not be a major repair cost ( a new compressor would be a major cost).

IF you hear the compressor humming but not starting, that's not very good and means there's a more serious problem. Call your service tech. Let me know what you're told.

On 2018-08-08 by Paul

Howdy, im no HVAC expert but i understand how it all works. My problem is that the indoor blower comes on, the outdoor fan comes on, but the compressor seems to take a while to come on, sometimes up to 10 minutes, meanwhile its blowing non cold air around the house.

I go outside next to the compressor, when it comes on i hear the fan but not the compressor (no buzzing sound), but it does come on later and then the freon pipe gets cold and im all good. I am in Miami and it is getting pretty hot here this summer, is this delay normal ? or is it a prelude to a more major issue ?

Thanks.

On 2018-06-29 by Anonymous

Thanks for your advice. The way I have the fan situated so it is blowing in the same direction with the main fan on the top of the condensing unit. I am first a retired electrician but over the years I have acquainted myself on some of the mechanics of how an air conditioner works but obviously I am not an expert.

The auxiliary fan I am using is placed right by the condensing unit blowing into heat rejection coils so the air I am blowing in is blowing in the same direction as the units main fan. Thanks again.

On 2018-06-27 17:54:56.662663 by (mod) -

Interestibg idea. Sounds to me as if it could work *except*

Watch out: if the direction of your fan's airflow is blowing against the condenser unit's own internal fan direction you will in fact interfere with proper condenser operation and make matters worse not better.

You might however have a booster fan blowing into the air intake, often at the top of the condenser unit.

And the installation of your supplemental fan would also have to be appropriately weatherproof and wire for electrical safety. I'll do some further research to see if I can find manufacturers opinions on this or existing designs, and I'll report back here.

On 2018-06-27 by kimball t rader

Does using a stand alone fan blowing into the coils of a condensing unit reduce the units head pressure thus reducing the workload of the compressor? I am not too keen on using those water misters because of my concern of calcium build up on the condensing coils. Where I live, electrical power is very cheap so I am not concerned if my electric meter spins a little faster.

On 2018-05-29 by (mod) -

Steve,

I agree with your AC experience. If the compressor runs and the condenser unit fan does not, nothing is cooling the high pressure high temperature refrigerant gas down to a liquid and the system won't work. I'd look for a bad fan relay, motor, or run capacitor, or even a bad control board.

On 2018-05-29 by Steve

should my compressor ever run without the condensing fan running, I have a 2 ton Heil central air unit. I have worked on automotive AC, Automotive unit work done always had the condenser fan come on at the same time or shortly before and stop at the same time or stay on shortly after the compressor. does not seem normal to me.

On 2018-05-21 by (mod) -

Mark

It's normal for a compressor to stop while the indoor air handler continues to run in SOME situations, but if by your notes you mean that the compressor won't run enough to produce cool air, indeed you need repair. The cause is not something I can diagnose from a brief e-text. It's time to call an HVAC service technician.

On 2018-05-21 19:04:40.643029 by Mark

heat pump compressor won't keep running.....codeser fan still going

On 2018-05-21 19:00:23.076726 by Anonymous

Compressor runs for 30 minutes.....and stops, codeser fan still going

On 2018-03-23 03:14:32.602827 by (mod) -

George,

Bad fan relay

Shorted wires

Bad control board

On 2018-03-23 02:09:57.011428 by george Perry

Hi - My condensing fan continuously runs even with the thermostat off . What could cause that to happen ?

On 2017-12-05 14:18:18.828140 by cosmos Taiwo

I want to know why Ac will trip of Generator after working more than 30 minutes

On 2017-07-31 22:04:04.609319 by (mod) -

Jim

Watch out for death by electrocution - if you're not trained to fool around inside the equipment it's rather dangerous.

That said, your tech would probably start by measuring the current draws - if it's abnormally high the motor may be seized.
Else she'll typically try the inexpensive shotgun repair approach: change out the start/run capacitor.

On 2017-07-31 19:47:03.024852 by Jim

Condenser fan works but compressor will not start and contactor contacts are stuck closed what are the steps to solve

On 2017-07-27 02:53:02.008656 by Ken

Does thermal protection protect a compressor motor if the supply is undersized and cannot provide the starting amps needed?

On 2017-06-15 22:34:12.605446 by Marius

I have a 10yr old Amana heat pump with a/c. Outdoor condensing unit is a 5 ton and air handler is 6 ton. Indoor fan works fine. Condensing fan works fine but it will not heat or cool. I replaced the capacitor, contactor, and the outdoor control board. The compressor is warm to hot to the touch. Anyone have any ideas?? Thank you!

On 2017-05-10 14:59:18.592928 by (mod) -

Sorry but I don't understand what fans secondary to a /c means

On 2017-05-09 01:15:32.160164 by Charles Smith

On a/c heat pump secondary fan to a/c compresure won't kick in to run and cool

On 2017-04-27 22:18:12.229880 by Joey

It stays running and goes to auxiliary heat

On 2016-08-06 15:25:09.258705 by (mod) -

Susie to read a good starting point for diagnosing and fixing the trouble you cite, search InspectApedia.com for LOST COOLING CAPACITY

On 2016-08-06 01:22:19.855734 by Susie coomer

My air is not cool

On 2016-07-18 17:11:14.680496 by (mod) -

In sum, compressor/condenser unit hissing sounds are usually traced to

A failed or failing or sticking valve inside the compressor motor

Abnormally high pressures on the outlet side of the compressor motor activate an internal pressure-relief safety valve. Check that the compressor/condenser's cooling fan is running properly. If the condenser fan does not operate (to condense high pressure high temperature refrigerant gas back to a liquid) high pressures develop on the outlet side of the compressor. Check the fan start-run capacitor and the fan motor.

Abnormall and very-high pressures in the compressor motor, possibly heard as a combined hiss and shriek.
Watch out: if you have this condition turn off the unit and leave it off until it is examined and repaired by a professional, as high pressures risk a dangerous equipment explosion.

A refrigerant leak. This ought to show up soon as an inadequate refrigerant cahrge


Other HVAC system hissing sounds may be harmless such as

Hissing at the thermostatic expansion valve, typically low in volume and short in duration, and traced to the TEV or cap tube

Hissing sounds from ductwork: look for an air leak in both supply and return registers. Even a small air leak can cause loud noises. This sound will occur when the fan in the the indoor air handler unit is operating, and it may change depending on the position of various duct system registers and dampers, blower condition, even air filter condition.

Of course there are other sources of hissing sounds in buildings, many of them, but not specific to the HVAC system.

TO permit other readers to comment I'll continue this Q&A at the bottom of

https://inspectapedia.com/noise_diagnosis/HVAC_Noise_Descriptions_3.php#Hiss

On 2016-07-17 by Safie


I had used the search box and read that entire page, there wasn't anything on it like what I'm experiencing. Maybe I'm just wording my searches incorrectly on Google. Like I said I've never heard a heat pump make that extreme "air breaks" hissing sound after it stops. Just wanted to see if anyone else had had this experience.


...

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CAPACITORS for HARD STARTING MOTORS to try to kick start a reluctant compressor motor

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