Q&A on hard-starting HVAC compressor motors:

These questions & answers about hard-starting air conditioner or heat pump compressor motors help diagnose why the motor may hesitage, be slow starting, or may continue to burn out the starting capacitor.

This air conditioning repair article series explains procedures for the diagnosis and repair of hard-starting air conditioning compressors.

FAQs on A/C Compressor Hard Start or Intermittent Run or Stutter

These questions and answers about HVAC compressor motors that have trouble starting were posted originally

at HARD STARTING COMPRESSOR MOTORS

Photo: a motor starting capacitor, a common repair effort for hard starting AC compressor motors. [Click to enlarge any image]

Why is my inside cooling coil freezing up as soon as the AC runs?

My AC outside unit comes on but the inside freezes up almost immediately..

I can feel cool air coming through the vents but the blower fan isn’t turning.. I can hear the motor humming, I believe..

I’ve checked the 3 amp fuse in the connection and it is still good.. the squirrel cage fan turns freely because I can spin it with no effort with the power shut off at the main breaker panel.

The fan will not come on in either cooling or heating mode even when the thermostat is switched into the ON position On 2018-10-14 by Shannon

Explanation by (mod) -

Often a freezing oil is due to low refrigerant, or to something simpler like reduced or no air flow due to a dirty filter or blower not running.

Search this site for or go to FROST BUILD-UP on AIR CONDITIONER COILS to see details

Compressor won't start on my window air conditioner

Purchased an OLD Freidrich 36,000 BTU window air conditioner. Seller fired it up and ran perfect (for 15 minutes before buying it), very cold air flow and relatively quiet for it's age.

Transported it right side up (not to upset fluids inside), got it home and placed it in the window, fired it up, ran for 5 minutes or so, then something growled and compressor kicked off sending out a smell like something electrical was burning.

I shut the unit down and unplugged it immediately. Pulled unit out of window enough to open and expose electrical components and found nothing visually.

Leaving all still exposed, I plugged it back in, fired it back up.

Fan/motor starts and runs good, but the compressor (after calling for it to start) does absolutely nothing. No click.., no hum..,

nothing.., as if the wires were not even hooked up to it. I tested all wires and have proper voltage to the compressor.

Does the start/run capacitor have anything to do with the compressor? Why would a smooth starting and running compressor just stop like that? On 2018-06-09 by Earl

Reply by (mod) -

Sounds like a shorted wire or control, not a capacitor problem.

Repairmen changed out the capacitor but two days later the AC stops working

We have had two repair men out from different companies in less then a month and both changed out the capacitor and then two days ago the unit blows cold air for maybe 2 mins and then stops completely then does it again 15 mins or so later. Any ideas? On 2017-07-20 by Heather

Explanation by (mod)

Heather,

A service tech changes of the capacitor as an inexpensive effort to get a compressor unit running.

However if the motor has essentially failed it will simply blow out replacement capacitor.

So I suspect that the compressor motor in yr unit is overheating or seizing or otherwise failing.

Your service tech can testing confirm this for example by measuring the current draw in amps when the compressor tries to start.

by Heather

Thank you so much for your helpful response;

I have yet another tech Coming hopefully today! It is miserable!

 

Roof Top RV AC DUO THERM used to run but now won't

A Roof Top RV AC DUO THERM unit was given to me.Guess unit is about 25 years old.Used to work when removed from last RV.

There are 3 speeds for fan that work. 3 speeds for AC.If switch is put on AC the compressor kicks on runs for about 5 minutes,

Pipes are getting hot and cold then you hear presser building up compressor slow down and stop trying to start fan compressor thumps and jumps around before thermal shut down.

30 seconds go by and fan will run air is cool, but not cold. then it will run for 15 min. before starting agian. I have put a new hard start compasitour on unit. On 2017-07-19 by Doc

Where to look by (mod)

Look first for a bad capillary tube or thermostatic expansion valve.

 

I repaired the A/C fan but now compressor and fan won't start

Inside fan on my240volt air-conditioning went bad. I changed it,n put new plug. now the compressor n fan won't start. What did I do wrong ? On 2017-06-06 by Justin

by (mod)

Justin

I can't say, but I'd start by tracing power to the fan and to the compressor.

You didn't move any electrical wire or switch connections, right?

All switches are on, right?

It's not an FPE Stab-Lok or Bulldog Pushmatic electrical panel powering the equipment, right?

You didn't short a control board, right?

 

Our A/C compressor has blown 4 capacitors in a year

3 ton straight ac,fixed orifice. The unit has blown 4 capacitors in 1 year. Both sides of the dual run capacitor read zero with no leaks or puffing out. I cant see a strart relay and none is indicated by the schematic. The unit runs strong untill it repeatedly blows the capacitor. On 2017-05-10 by Scott

Diagnostics by (mod) re: compressor has blown 4 capacitors in a year

I would try looking for an excessive current draw or an intermittent short such as a winding short when a motor is spinning.

Most likely you need a new compressor motor

 

Why does my Air Conditioner compressor shut off after 3 seconds?

My ac compressor comes on for like 3 seconds then shuts off ? Any advise will be thankfull On 2017-04-09 by James

Causes noted by (mod)

James:

Several problems can cause the quick-off A/C compressor problem you describe, such as

- a failing control relay or circuit board

- a seized compressor motor

- a compressor trying to start against high head pressure due to a stuck or failed thermostatic expansion valve TEV or cap tube

 

AC compressor comes on for like 3 seconds then shuts off

When I start my window AC the voltage of my house gets low On 2017-04-09 by Rahul

Explanation by (mod)

Rahul

If this is a new problem that is occurring on a window AC system that's been in service for some time then I suspect the compressor motor is having trouble starting - it may be failing; the unit could be repairable if the problem is only the need for a new start/run capacitor but I'm not optimistic.

A seizing compressor motor will draw high current (Amps) that can indeed dim lights.

 

ABQ Trane BTB 718A 1984 compressor diagnosis

A/C Unit: TRANE
MODEL#: BTB 718A 100A1
Manufacture Date: 7/84

This hard-working unit is located at our apartment in Albuquerque, NM.

For the second consecutive day, the compressor and fan for this unit simply shut off between 4-5P.

The 2 30-AMP circuit breakers for this unit were NOT tripped. I am able to restart the A/C by resetting the two circuit breakers, which starts up both fan and compressor, which then run normally in accordance to thermostat settings.

These are the first occurrences of this condition since we started renting this unit on November, 2014. In your opinion, what is the probable cause for this unit shutting off in the middle of the afternoon? On 2016-07-20 by Ted

by (mod) - check other compressor overload or overheat controls

Ted

The unit may have other internal safety controls that shut it down due to overheating, abnormal gas pressures, or another malfunction; your service tech will check those controls.

Try cooling down the system with a garden hose to see if that makes a difference.

 

Outside A/C condenser unti runs after we cool it with a garden hose (water)

Fan outside and I side is blowing put start up capita tor on and still having problems with the compressor kicking on.

When it finally kicks on with the help of the garden hose it will run for hours and then kick off and about 20 minutes later it start the process all 9ver again. By the way it's Lennox. I do hear the compressor trying to start. On 2016-07-01 by Stephanie Evans

Comment by (mod) still having compressor problems after added a capacitor

That sounds like a failing compressor that overheats and shuts down. The motor is seizing.

 

Our over-sized Air Conditioner compressor not only wastes energy, but it won't dehumidify the home

You bring up the main issue really! Our unit is indeed to big for our home and it has caused us $$$$ over the past 6 years.

When sitting at closing our builder off handedly said, " they delivered the wrong Ac unit to your house...it's bigger and better for the same amount" something to that affect.
How were we to know.

A year later he up sized all the duct work in the house because of issues with the house not cooling. thought all was well but at that point we were on our own.

Many hvac techs later and they all suggest we buy a brand new smaller unit at around 5k. On 2016-06-17 by Dd

by (mod)

An over-sized compressor not only wastes energy, but it won't dehumidify the home adequately as it'll cool down too fast.

 

Our Air Conditioner Trips the breaker unless we leave it off for a while

Long time issue with our hvac! Doing the same old thing for 6 years no solution but told to just buy a new unit$

Had the compressor replaced last year$$$$ (unit is only 6 years old...new house). Same issue that caused the first compressor to fail is still present.

Unit will kick on and cool home just fine. It will reach the desired temp then cycle off.

As soon as the house reaches just 1* below temp setting the A/C will kick back on and flip the breaker:/ it does have a 5 min start delay but this doesn't seem to help:/ form the breaker flipping

We had the breaker fused uped to a 40amp from a 25amp last year...but it didn't help.

I have figured out that if I run and set the thermostats up 10* after the Ac kicks off from cooling the house...the Ac doesn't kick on after the 1* and give my Ac time to rest....a few hours it will not trip the breaker.

This method works every time.
This is my life everyday...babysitting the Ac :/
Help On 2016-06-17 by DD

Diagnostics by (mod)

If the compressor itself is working normally and is the properly-sized unit for the home then indeed what you describe suggests a problem elsewhere.

I suspect that the motor is overheating under load, drawing high current and then tripping the breaker.

I'd give your service manager a call and ask for diagnostic help from their senior, very experienced technician.

Wacth out: while one-step over-fusing on AC compressor circuits is usually permitted (check the data tag on your unit), upping from 15 to 40A is improper, unsafe, indicates lack of expertise and worst, is just asking for a house fire.

Your tech will probably check the thermostat settings, wiring, controls, and relays in the system. Indeed the suggestion of some of your comments is that the compressor is trying to re-start too soon and is trying to start against a high head pressure: a condition thats a problem for many compressor units.

Something as simple as a sticking TEV (themostatic expansion valve) can cause failure of the head pressure to drop after the compressor shuts off.

 

A/C compressor cycles on and off rapidly

We have noticed that our air compressor will cut off and start right up and then cut off and start right up. It seems like it does it the longer it runs.

We have had coolant added and we have replaced our inside thermostat.

However we are still having the same problem . What should we try next? On 2016-06-16 by Donna

Diagnostic suggestion by (mod)

This sounds like a control or wiring problem at the compressor/condenser unit, possibly a bad sensor or relay. I'd get it fixed lest you damage the compressor.

Adding refrigerant or replacing the indoor thermostat will have no effect on that problem.

 

The AC condenser unit fan runs but the compressor does not & the system won't produce cool air

I have a carrier heat pump. When turning on the AC the out side unit fan comes on but the compressor does not. the compressor tries to kick on but then quits. The air blowing out of the vents isnt cold On 2015-09-02 by Ingrid

Diagnostic suggestion by (mod)

Ingrid

Indeed if the compressor does not run the system won't produce cool air. The problem could be a failed compressor motor (expensive) or perhaps a bad control or relay. You'll need to call your A/C service company for diagnosis and repair.

Some diagnostics for an A/.C comporessor that won't start at all are found at

AIR CONDITIONER WON'T START

 

Why would a water cooled HVAC compressor start and then shut off with high head pressure?

what would cause a water cooled compressor to start and than shut off on high head pressure immediately with no type off water control valve On 2015-07-28 by jim

Suggestionsby (mod) water cooled compressor to start and then shut off on high head pressure immediately

Perhaps a stuck or failed reed valve in the compressor.

High head pressure could also be a stuck TEV or TXV expansion valve that's not sending refrigerant on to the cooling coil.

 

Could Loose Electrical Wiring (or a bad A/C start relay) Explain a Hard-Starting Air Conditioner or Heat Pump Compressor Motor?

I had a tech come out and install a hard-start kit to keep a 4-year-old 3-1/2 ton Carrier compressor from tripping the circuit breaker. The compressor failed this summer. A second tech was suspicious that a compressor less than five years old would be tripping the breaker.

He discovered a loose connection in the cut-out switch. Would the hard-start kit have been necessary had the loose connection been tightened prior to it's installation? I'm wondering if the failure could have been avoided with proper diagnosis by the first tech

In 2009 tech # one had come to respond to my problem of a 4-year-old compressor kicking the circuit breaker. Without even looking at the connections in the shut-off box adjacent to the unit, he went straight into control panel, checked compressor current draw at start-up and installed a hard-start kit

I thought it odd that a compressor less than five years old would be drawing too much current. Had no problems in 2010 but last month the circuit breaker started kicking off again late in the day.

Tech #one returned, checked coolant pressures and return temp, added some freon and started to pack up. I asked if he wasn't going to check the current draws.

He reluctantly removed the control cover, checked draw and voltages and pronounced the unit was running perfectly before leaving with my $168 check.

When the circuit breaker kicked off again the next day, I started to suspect the breaker might be worn out. Called an electrician a few days later.

He replaced the breaker switch, again without checking the connections at the shut-off box, and within eight hours my compressor had died - coils shorted to ground - confirmed by tech # one who quoted me a price to replace the unit.

Now I called a contractor I knew asking if he knew any good AC techs. He referred me to tech # 2 who, in my first call, was immediately suspicious that my original problem may have been in the wiring, not the unit. His reasoning was that a compressor less than 5 years old shouldn't have been giving me problems to begin with.

He came over to confirm my compressor was shorted out. He then did what no other tech had done since I owned the house - popped the cover off the shut-off box and tightened the leads at the terminals - black wire loose, white wire "very loose."

My question is whether that may have been my problem all along and would a hard-start kit have temporarily relieved the symptom -tripped circuit breaker- without curing the problem. Tx Red,

Reply:

Tx Red: you and your second HVACR Tech have come across a useful diagnostic step: check for loose, or even burnt or arced wiring connections. I can't be sure of the answer to your question, as time has passed and we can't know for sure what conditions the first tech saw.

A bad relay can cause compressor start problems, and I pose that its possible that arced, burned, or loose connections at the relay could make it misbehave.

Your thesis that a loose wire could have been misdiagnosed as a hard-starting A/C or heat pump compressor motor is reasonable but not, IMHO, absolutely proven by events.

A compressor start relay (or most other switches) that are loosely wired will typically experience arcing at the loose connection (an event that can, by the way, when the circuit is active or the switch closed, draw higher amps than normal due to the resistance at the point of arcing and corrosion.)

Adding a start-capacitor is adding a device that gives a big voltage surge just during motor start-up.

A voltage surge that normally is overcoming inertia in a still motor might also overcome a start relay contactor resistance, caused by damaged or burned connecting points, or by a loose connection that was in effect acting in that same fashion.

The start capacitor is delivering voltage and pushing the motor into rotation.

So without being an E.E. but with some experience around this topic, my reply is "could be" but not that we can say for sure.

Also FYI even a brand new A/C compressor could become hard-starting due to other system problems that cause internal damage to the device or even by (perhaps less likely) a manufacturing defect, events at the property, even general power delivery conditions such as abnormally low voltage. To research the topic you'd look for events that can damage the start winding on an electric motor.

Finally, it's not a surprise that the first tech didn't check nor re-check for loose wiring but I agree it would have been a useful step. Often service techs, contractors, and some investigators become habituated to a short cut of trying the "fix" that usually works successfully and fast.

IF the original start relay or contacts that were found to have been loose is still installed, one could investigate further by inspecting that device for evidence of arc burning or damage.

Followup by Texas Red

Thanks, Dan. I didn't communicate the timeline very well in my original post.

In 2009 tech # one had come to respond to my problem of a 4-year-old compressor kicking the circuit breaker. Without even looking at the connections in the shut-off box adjacent to the unit, he went straight into control panel, checked compressor current draw at start-up and installed a hard-start kit. I thought it odd that a compressor less than five years old would be drawing too much current.

Had no problems in 2010 but last month the circuit breaker started kicking off again late in the day. Tech #one returned, checked coolant pressures and return temp, added some freon and started to pack up.

I asked if he wasn't going to check the current draws. He reluctantly removed the control cover, checked draw and voltages and pronounced the unit was running perfectly before leaving with my $168 check. When the circuit breaker kicked off again the next day, I started to suspect the breaker might be worn out.

Called an electrician a few days later. He replaced the breaker switch, again without checking the connections at the shut-off box, and within eight hours my compressor had died - coils shorted to ground - confirmed by tech # one who quoted me a price to replace the unit.

Now I called a contractor I knew asking if he knew any good AC techs. He referred me to tech # 2 who, in my first call, was immediately suspicious that my original problem may have been in the wiring, not the unit. His reasoning was that a compressor less than 5 years old shouldn't have been giving me problems to begin with.

He came over to confimr my compressor was shorted out.

He then did what no other tech had done since I owned the house - popped the cover off the shut-off box and tightened the leads at the terminals - black wire loose, white wire "very loose."

My question is whether that may have been my problem all along and would a hard-start kit have temporarily releved the symptom -tripped circuit breaker- without curing the problem.

Reply by (mod) -

Your thesis is reasonable but not, IMHO, absolutely proven by events.

A compressor start relay (or most other switches) that are loosely wired will typically experience arcing at the loose connection (an event that can, by the way, when the circuit is active or the switch closed, draw higher amps than normal due to the resistance at the point of arcing and corrosion.)

I've posted our conversation in this article in the Q&A just above the Comments Box section.

by Texas Red - what's a compressor start relay?

...not exactly sure what a compressor start relay is. Is it wired into the compressor itself, or is it located in the control box among capacitors, terminals, etc?

If its contacts were burned or otherwise damaged, wouldn't the solution be to replace the relay rather than install a hard start kit? I watched the intallation of the hard start kit and am pretty sure he just confirmed that the compressor was drawing too much amperage at start up and therefore required the kit.

He attributed the high start up draw to slow pressure equalization somewhere between evaporator, condensor, compressor(??) Not sure specifically where or how the uneven pressure was creating a problem.

Explanation by (mod) -

TX Red: slow pressure equalization when the compressor is OFF indeed suggests a blockage that needs to be found and fixed; perhaps at a drier/filter or at a thermostatic expansion valve or cap tube.

 

How often to replace the hard start capacitor kit on an A/C compressor motor?

How often should Hard Start kit be replaced? What is contained in such a kit? What is an estimated cost of the kit? (May 24, 2014) Rich

Reply: Once is enough

Rich: in my opinion, once. If the kit is not working then the compressor motor probably needs replacement. An exception might be if a power surge or other electrical or mechanical event damaged the capacitor.

A hard start capacitor kit should not have to be regularly replaced on HVAC equipment. If the capacitor keeps failing I suspect a wiring error or a motor that is failing solid and needs replacement.

 

How do I fix my AC if I can't afford service

I have a Rheem 2ton central air unit. My dog months ago ate wires outside and blew the board, thermostat, and some fuses.

I had all fixed accept I just got a new thermostat and checked wiring and my problem is that the heater had worked but now that its hot the AC turns on all systems go but then it keeps shutting off and the new board is suppose to have a diagnostic light but tells me nothing.

Cold air comes out of registers, a little slower, I can see the blower motor spinning in unit/air handler and outside compressor is spinning and staying on even when the blower motor shuts off.

I looked at the only capacitor inside where blower motor is and it is not swollen or anything but unable to check voltage. Please help 98+ temps lately and cant afford service! (May 27, 2014) Kevin Bresee said:

Reply:

A homeowner can perform basic maintenance and checks to keep an AC or heat pump system working, such as checking and replacing dirty air filters.

A homeowner with more skill can check that power is being delivered to the equipment.

What you should not try to do is fool with the refrigeant charge nor the electrical wiring of the unit. Doing so would be unsafe.

...

...

Continue reading at HARD STARTING COMPRESSOR MOTORS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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HARD STARTING COMPRESSOR MOTOR FAQs-2 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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