HVAC heating, ventilation, air conditioning & refrigeration system component identification FAQs:

Frequently-asked questions on how to find & identify the components or parts of an air conditioning or heat pump system, including indoor and outdoor equipment.

This article lists and explains the function of the basic components of an air conditioning or heat pump systems and provides detailed inspection, diagnostic, and repair advice. We include photographs to assist readers inrecognizing cooling system defects.

FAQs on Air Conditioning & Heat Pump Components

These questions & answers about the components in an air conditioning or heat pump system were posted originally at AIR CONDITIONER COMPONENT PARTS - be sure to see that article.

On 2013-02-25 by billy

Great site! Very informative

On 2012-12-18 by webros_shr@yahoo.com

kood site most helpful for student like me. Navid

On 2012-12-17 by harry

some of the sketches or pictures are missing

On 2012-09-22 by House doctor -oliver

excellent info

On 2012-09-08 1 by (mod) -

Vinson, check for a blocked condensate drain

On 2012-09-08 by (mod) -

Pete,
The behavior you describe is normal for some AC / heat pump systems. As long ad the fan continues to run, is not making an unusual noise like humming or rattling you should be ok.

On 2012-09-08 by Pete H

When the a/c kicks on, is the fan on the outside condenser supposed to come on right away, or does itcome on a few minuets later

On 2012-09-02 by vinson

water leakes from the bottom of in door ac/heater , still cooling but water runs from the coners down on the floor about a half gallon in 2 hrs.in fast drips when starting and at the end, when blower shuts off

On 2012-07-07 by (mod) -

Jim,

Weak air flow sounds like a coil frost blockage problem. Temp fix that is also diagnostic is to turn the system off for a few hours to let all the ice melt.

If airflow rate is back to normal the coil was ice blocked - that still needs diagnosis and repair. Replace any dirty air filters and ask the tech if s/he agrees to check for a loss of refrigerant or a refrigerant metering device problem.

On 2012-07-07 by Jim Grammas

large ball of ice around the low pressure line at the plenum, water on floor, weak air flow through registers

On 2012-06-05 by (mod) -

Rickey, several problems can cause high current draw at an A/C system; some high current is normal at motor start-up at the compressor/condenser unit, for example. But if the run current is abnormally high, I'd suspect that the comopressor is binding, or there is a refrigerant metering valve control problem causing high head pressure.

On 2012-06-05 04:29:19.950344 by Rickey

My AC is rated at 8.5 ampere. But now it is consuming up to 13 amperes of current. Due to this stabilizer gets overheated within 1 hour. please tell me about the defects.

On 2012-05-29 by (mod) -

Matt, some fan motors may incorporate a reset button, or you may be talking about a different reset switch for the whole unit - it'd be described in the operating manual for the unit and is model dependent.

There are other failures, such as a bad contactor relay, that can prevent the outside unit from starting.

On 2012-05-27 by matt

i cant get my outside unit to work checked all wiring and fuses the only thing i cant find is
the reset pressure button according to the schematics its optional on my unit

On 2012-05-02 by (mod) -

Bobby, try the diagnostic starting point link(links found at the ARTICLE INDEX near the end of this page) titled DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP

On 2012-05-02 7 by bobby

when the uni t ,it starts and gets cold ,and alittle while,then the fan on the outside unit stops it stops cooling

On 2012-02-18 by (mod) -

Hema I agree;

On 2011-11-09 by Hema

If the A/C keeps running but never reaches the set temperature of the thermostat first be sure that y our air filter is clean or just replace it and that you have good airflow. If you have good airflow but the air never cools you need a service call.

On 2011-10-03 by (mod) -

Bryan if you have a single air handler feeding both zones, and if too much air is blowing out of one of them, you should be able to find or if necessary install a balancing damper in the ductwork. By partly closing the damper in the "too cool" zone, you will force more air into the other zone.

On 2011-09-28 by Bryan

Our central air system has been working fine all summer. It has 2 zone, upstair and down. WHen the down stairs thermostat call for cool air, the upstairs zone gets most of the air flow. This causes the system to run for a long time dropping the upper zone to be cooler than set for and the lower zone never reaches set temp?

Is there something I can fix or should I wait for the spring service to have this addressed being less than a month of the season left?

On 2011-09-19 by Violet

What would cause an A/C to run non-stop, never reaching the temperature the thermostat is set for? Cold air is coming out, but the thermostat never goes below 80 degrees and we are worried about a really high power bill. Thanks!

On 2011-07-28 by (mod) -

Greg, over at the article titled DIAGNOSE AIR CONDITIONER PROBLEMS (linked in the ARTICLE INDEX of this page) you'll find some diagnostic suggestions for a compressor/condenser that won't run. If the unit has power and the thermostat is calling for cooling the problem could be a bad switch, control board, fan motor, or compressor motor or start/run capacitor.

On 2011-07-28 3 by greg

Compressor and fan make noise but fan won't spin and compressor won't run

On 2011-07-25 by murugesan

midsubishi mud13 split a/c indoor stopped with blinging

On 2011-07-19 by (mod) -

Zach take a look at our new article (link(links found at the ARTICLE INDEX near the end of this page) ) titled FAN, COMPRESSOR/CONDENSER UNIT

You may have a hard-starting condenser fan motor or one that needs a new start/run capacitor;

On 2011-07-19 by Zach

My air conditioner will kick on without a problem but the fan on top of it wont start running, the fan works but only when it feels like it. It usually works for 4-6 days then shuts off and refuses to start up again for 2 or so days then it goes back to working fine. can anyone tell me what the problem is? the model no. is 505BJ030 its an outside unit

On 2011-07-18 by (mod) -

Jan: can't say what you can fix yourselves, but consider that the cost of running the A/C continuously unnecessarily may be more than the cost of a service call.

When the A/C won't turn off, is temperature indoors dropping BELOW what the thermostat is calling for? If so then there is a thermostat or control board problem and you need a service call.

If the A/C keeps running but never reaches the set temperature of the thermostat first be sure that y our air filter is clean or just replace it and that you have good airflow. If you have good airflow but the air never cools you need a service call.

On 2011-07-17 by DennisDespain

What could happen if a 2ton condenser was put on a 1.5 ton evaporator?

On 2011-07-16 by Jan

The outside unit of our air conditioner runs continually. We have to shout the power off to make it stop. From inside our home it appears to be working fine and cools the house properly, with that one exception. We cannot afford to call someone out, is this something we could fix ourselves? Please help.

On 2011-07-13 by (mod) -

M. Cooper: typical central air system replacement costs (compressor/condenser unit and indoor air handler) if you are NOT replacing ductwork is $2,000 to $10,000. The variation is more than by geographic area - it's unit size and special installation requirements.

On 2011-07-13 by M. Cooper

How much should it cost to replace the Box Coil and Plenum in Texas?

On 2011-07-13 by (mod) -

D Wool: you must have been spinning the condenser fan unit, right? You wouldn't be able to spin a compressor motor shaft as it is enclosed in a hermetically sealed container. If your system still won't run I'd check out the start-run capacitor(s) or just replace them.

You should not have to move the indoor thermostat to get the air conditioner to come on unless your thermostat is located in an abnormally cool location like right under a cool air supply register.

Think of the air conditioning thermostat as basically an "on-off" switch. In cooling mode, when room temperature is higher than the set temperature on the thermostat, the switch closes to ask the air conditioning system to run. When room air drops to the thermostat set temperature the thermostat opens to tell the air conditioner to stop running.

On 2011-07-13 by D Wool

My condenser fan motor needs to be helped by spinning to start and then stops after approximately 30 minutes. I have replaced the motor, but not the capacitor.
Should it be thermostat issue; I do have to move it around to get the unit to come on?

On 2011-07-13 by Dalton Woolery

After experencing I could spin the outside unit condenser motor and it would start, I replaced the motor. I ordered a new conpasator also but it was the normal one, so it was not replaced.
The

On 2011-07-13 by (mod) -

Vibrating/Shaking compressor unit: I'd look for a loose mount - that may be easy to fix; if the vibration is due to parts INSIDE the sealed compressor motor, it's at the end of its life - in either case I'd turn it off to investigate as you risk something coming more loose and damaging other compessor/condenser parts.

On 2011-07-13 by R. Mark

My system is cooling just fine, but the outside compressor unit is visibly strongly vibrating. Is it likely OK to keep using until a repairman can inspect, or could it cause further damage? Could it be the fan is loose or out of balance causing this?
Thanks,

On 2011-07-13 by (mod) -

Nancy, your HVAC tech might be able to install equipment that uses R410 refrigerant to replace R22, but take a look at the refrigerant discussion in the article linked to(links found at the ARTICLE INDEX near the end of this page) and titled
REFRIGERANTS

On 2011-07-12 by nancy wood

My outdoor unit was stolen this week wires cut and whole unit taken off concrete slab. Problem now is that unit was installed just 2 years ago. The inside the house part of the unit is quite old but runs perfectly well. Our electric bills are very reasonable and we are also running a swim pool pump.

Cant I still replace just the outside portion even tho the inside unit is refridgerant rated 22-r. Cant I still find a unit for the outside that will support that and at the same time if the inside unit goes out I would only need to replace it as the new outside unit could handle a new inside unit

I hope this makes sense as money is a big factor right now for this unexpected loss. Bottem line I can only afford the expense of replacing the outside unit if that is possible.

On 2011-07-07 by (mod) -

Tim: about an air conditioner suction line that "never sweats" - if you don't see condensation on the suction line it could be that the level of relative humidity at that spot is rather low, or the suction line in that area may not be cold enough to cause condensation (same thing said two ways). As long as you're getting adequate cooling you should be OK.

Nicole:
If you mean that your A/C system sometimes blows adequate cool air but mysteriously suddenly switches intermittently to blowing hot air, then back to cool air - it sounds like a control circuit problem. I'd ask for a service call from an experienced HVAC tech.

On 2011-07-07 by Tim

My central A/C unit cools the space to setpoint and I have about a 10 to 12 deg differential between my supply and return air. But my suction line at my condensing unit never sweats.

On 2011-07-07 by NicoleKnaak

I need help! My A/C unit turns from cold to hot even though it's set to cool and it's hot outside. This problem has stumped 2 A/C guys already. I'm pregnant and I have a 2 yr old who keeps waking up and crying bc its 89 inside. Any ideas what could be going on?

On 2011-07-06 by (mod) -

Steve Hart: if your condenser unit fan won't run but the compressor does, I suspect a bad fan motor or perhaps a fan motor off on reset. Even if the fan will run again after finding (if there is one) and pressing the reset button, you need a service call as the fan is likely to fail again; other possible explanation for the fan could be a control or control board.

On 2011-07-06 by (mod) -

William & Steve: try taking a look at the air conditioner diagnosis article linked to(links found at the ARTICLE INDEX near the end of this page) and titled
LOST COOLING CAPACITY

Kathy:
$17,000. sounds a bit extreme for an air conditioner repair or replacement; your sister ought to get a different A/C repair company to diagnose her problem, explain what the heck a leaky King Pin is, and then give a repair estimate. Keep us posted.

Tim: sounds as if your cooling fan assembly or motor needs some diagnosis and repair - maybe an overheating fan motor or a bad fan bearing?

On 2011-07-05 by William Carter

My compressor start and stops after running about five menutes

On 2011-07-03 by Steve Hart

The outdoor cooling fan on my unit is not spinning. The system itself will run but the fan wont.

On 2011-06-26 by kathy simmons

my sister's ac went out a repairman told her it was the king pin that was leaking? Is this repairable as we live in Phoenix Az it's 110 degrees and she cannot afford to replace it he told her that replacement was between 5 to 17 thousand dollars.

On 2011-06-24 by Tim Wrye

I have dual units for my shop one unit works fine, the other starts running the cooling fan gradualy slows then stops, the pump keeps running until the pressure gets high then it quits. If you let it cool down it will repeat this scenario again. I switched the circuit board, capacitor, and contactor switch from the good one to the bad one and the results were the same. Any ideas?

On 2011-06-22 by Yolimar McCombs

My A/C condensor will not stop working even after reaching the set temperature. I have clean the Condensor's coils thinking that it could be a air restriction but it is still running all the time. Could I have a bad thermostat? Can my unit be low on freon? I cant afford to pay someone to come look at it so I would rather do as much as I can to fix it before calling someone....

On 2011-06-18 by Bernie Curtin

Sometimes my outside compressor unit wont start. When this happens I hear a slight hum. Then I tap on the contactor and it starts and cools nicely. I have replaced my contactor twice "thinking I bought a bad one" and starting coil. What else do you think I need to replace?

On 2011-06-07 by (mod) -

Todd:
Restricted air flow could cause icing on the evaporator coil in the air handler; if replacing a dirty filter and allowing the evaporator coil to thaw out (by leaving the power off or optionally by running just the fan) did not permanently fix the problem then further investigation is appropriate.

If your air flow is restricted even when the coil is not iced over then you have a problem with the ductwork - original design (it has always been poor airflow) or a duct that is crimped, disconnected, or a damper shut somewhere (air flow used to be good but is not now);

Low refrigerant can also cause coil icing;

Icing on the refrigerant lines alone could be a sign of an improper charge (for example icing at the coil outlet end suction line can mean either low refrigerant or overcharged refrigerant);

On 2011-06-07 by (mod) -

Alessa:
If the chewed wires were shorted by the puppy's biting or even if the chewing just allowed moisture to enter the wire cabling, it could be shorted and certainly is unsafe. It needs to be replaced. If there was a short circuit, other components such as a control board could have been damaged. Watch out: live electrical voltage is a shock hazard that could kill someone.

On 2011-06-06 by Alessa

My central air conditioner stopped working. My husband noticed an electrical wire that runs from the outside unit through the wall and inside which was slightly chewed by our puppy on the outside of the home. Can this be causing the air conditioner to not work? Please help us, as we know that the cost to fix air conditioners can be very expensive. Thanks.

On 2011-06-06 by Todd

My A/C unit is getting frosted over on the refrigerant lines. Cool air out and fan is on, but restriced air flow. Another site said to use a clean air filter and run the fan for a while with A/C off until it defrosts. Worked twice , but keeps happening. Do I have a refrigerant leak? Do I need an expert or can I fix it myself with parts?

On 2011-06-05 by (mod) -

Sabrina there may be no reset button for your outside compressor/condenser unit if it's the compressor motor that is not running. The fan motor MIGHT have a reset button on the fan motor as we describe in ELECTRIC MOTOR OVERLOAD RESET SWITCH - see our list of article links(links found at the ARTICLE INDEX near the end of this page) . Try turning the system off for an hour and then turning everything back on.

On 2011-06-04 by Sabrina

Where is the reset button on a Coleman Outside A/C unit?

On 2011-06-03 by (mod) -

The red reset button on electric motors, Lennox and otherwise, is described in ELECTRIC MOTOR OVERLOAD RESET SWITCH - see our list of article links(links found at the ARTICLE INDEX near the end of this page) .

Typically an electric motor thermal overload reset button is built into the motor itself and is found on the surface of the motor. You'll see photos of what this button looks like in the article I've referenced.

On 2011-06-03 by juan bowers

where woule the red reset button be located on the lennox units? inside or outside

On 2011-05-31 by Tony

What would make my central air conditioner "short cycle"?

It runs for just a few seconds/minutes then shuts off then begins to run again. Repair person replaced a "high speed coil" on the blower motor. This seemed to fix the problem but only for a few hours. I can't find any information about this coil or any part that looks like the one he replaced.

The motor runs fine for the furnace but not for the air conditioning. We have an old (15 - 20 years) Trane system.

On 2011-05-28 by (mod) -

Mary if your outside fan (condenser unit) is not working you need a service call from an expert to diagnose the problem. But first make sure power is on to the equipment and that the thermostat is calling for cooling.

On 2011-05-28 by (mod) -

Lisa if your system uses a two-wire thermostat just about any 14-V thermostat you'd find at a building supplier would work - UNLESS your system is using 120V thermostats and wiring.

Check the voltage at the wires if you are qualified to do so safely, or look at your A/C unit or heating and cooling system to see if you can trace your thermostat wires back to a low-voltage thermostat transformer. That will tell you what type of equipment you have.

Watch out: with power on there are shock and electrocution hazards - hire a qualified service tech.

On 2011-05-26 by mary

outside fan not moving/turning -- suggestions?

On 2011-05-25 by lisa

i have just a/c unit and trying to find just a thermostat just for a/c. i have tried to use the ones for heat and air and they wont work. i just got to wires white and red one, please help

On 2011-05-15 by (mod) -

Assuming you refer to an air conditioner compressor/condenser unit compressor motor that is overheating and shutting down, the bad news is that the compressor may be seizing and at end of life. There are other things the service tech will check out first including the voltage level and current draw, and the refrigerant charge. S/he may also check the starter and run capacitors and even the installation location for sunlight exposure and adequate airflow around the unit. Let us know what is decided as it may help mother readers.

On 2011-05-15 by Anonymous

what will make a motor heat up and turn off

Comments:

(Mar 10, 2014) Anonymous said:
presentation on air compressor

(May 22, 2014) Anonymous said:
These information were very useful.
they helped through my research work as as student of architecture.
thanks

(Sept 9, 2014) GUGULAM said:
thanks for the info on air conditioning, you guys are doing a great job

Reply:

Sure, see COMPRESSOR / CONDENSER REPAIR - home

Thanks for the nice comment Anon. We will always also welcome your questions, comments, critique - which helps us both.

Question: I have been cautioned to stay clear of variable-speed blower options

(Mar 14, 2014) Phil said:
I'm in the market for a replacement electric/gas pack unit. I have been cautioned to stay clear of variable-speed blower options and their sophisticated electronic controls for lack of durability. The reason given was sensitivity to power fluctuations - drop-outs and surges.

Likewise, that copper/aluminum fin condenser coils of today are more leak prone than their all-aluminum cousins. Do you agree and if so could you recommend some 13-seer models for their use of more durable/reliable components? Or, help me know what to look for? Thanks for your expertise and help cutting through the marketing confusion!

Reply:

Thanks for the "expertise" compliment Phil but with a little embarrassment, this question is beyond our expertise. While your surmise sounds sensible, in fact across thousands of readers and comments, I have not seen a particular pattern indicating that variable speed blowers fail more frequently than their simpler single speed units. Indeed for systems providing both heating and cooling we generally want a blower capable of operating at at least two different speeds for the two different modes.

I suspect that power fluctuations are more of a worry for controlling circuit boards in general.

Back in 2009, in "Comparing Motor Technologies", achrnews.com Brian Michael reported that constant torque motors, a comparatively new design in use in HVAC systems (dating from a Regal-Beloit (General Electric) design in 2006) offered high efficiency that helps meet the 13-SEER HVAC models in your question. That article does not cite the concerns you raise.

Question: what is meant by "high side" and "low side" on an air conditioner or heat pump system?

(July 23, 2014) Anonymous said:
sometimes they say high side and low side is this referring to high pressure and low pressure?

Reply:

Yes

High side pressure is on the compressor outlet side and low side pressure is on the compressor suction side. On the outlet side of the AC or heat pump compressor the refrigerant is at high pressure (over 100 psi) while on the suction side the refrigerant pressures are quite low.

See REFRIGERANT PRESSURE READINGS & CHARTS for details.

Question: problem with the unit blowing hot air

(Aug 28, 2014) donrebich said:
Question: We began having a problem with the unit blowing hot air and going outside to the unit finding the fan not working but after hitting a red button the unit came on and functioned properly. Calling a service co and detailing what had transpired he claimed it was the fan motor.

Another tech came out and claimed the system had excess frion, the high pressure side was at 280 and suction was 75 on the compressor. Being extreamly hot outside, perhaps causing either the fan motor to kick the system off or a pressure swithch.

Reply:

See the diagnostics beginning at AIR CONDITIONING & HEAT PUMP SYSTEMS - home

Question: can you repair a defective air conditioner compressor?

(Mar 4, 2015) Rick Regaspi said:
May I ask if the defective compressor motor of Air-con can be repaired? or is there any repair shop for compressor motor?

Reply:

Rick: residential HVAC compressor motors are typically a sealed unit that cannot be field-repaired.

Question: split system is not cooling and not dehumidfying well and it squeals

(May 3, 2015) Robert said:
My split system is not dehumidifying the air and is not cooling very well. There is also a squealing sound when the unit turns on and a lot of noise in the cabinet where the A-coils are when the unit is turned off.
The copper line that is not insulated is cold rather than warm like it usually is.
Could there be a leak or might the A-coils need to be cleaned?

Reply:

Robert
I would investigate the squealing first - look for a failing blower assembly, belt, or in a split system, more likely a motor.

...

Continue reading at AIR CONDITIONER COMPONENT PARTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Recommended Articles

• A/C INSPECTION CHECKLIST - OUTDOOR UNIT
• AIR CONDITIONING & HEAT PUMP SYSTEMS - home

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AIR CONDITIONER / HEAT PUMP COMPONENT FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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