Photograph of an air conditioning system suction line temperature chartAir Conditioning & Heat Pump Temperature Q&A
Set 2 of HVAC Temperature FAQs

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HVAC system temperature FAQs #2:

More Q&A on air conditioner or heat pump system temperatures at the equipment, at the cooling or heating coil, at the supply or return registers, & how those temperatures help diagnose HVAC system problems.

What are the the normal or abnormal operating temperatures of air conditioning equipment when measured at different locations.

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Air Conditioning / Heat Pump Temperatures Diagnostic Questions & Answers

Photograph of a dial thermometer reading a high output temperature at an air conditioning compressorAir conditioner temperatures that are too high or too low can indicate specific operating problems on central and portable or window air conditioners. These questions & answers about air conditioner or heat pump operating temperatures were posted originally at AIR CONDITIONER TEMPERATURES - the topic home.

On 2017-05-11 by (mod) re: is 180 degF air out at the condenser unit too hot?


Typically we measure the air temperature drop across the cooling coil in the air handler: Delta-T or temperature drop across the coil, as per the article above on this page, reports that a typical temperature drop is 15 to 20 deg .F.

Keep in mind that the temperature in the cooled space as well as air flow rates and coil clogging by dust and debris (or frost) will significantly affect this temperature reading. For example a much higher temperature drop across the cooling coil might indicate a dirt or ice blocked coil, a dirty air handler blower fan, dirty filter, reduced air flow for other reasons (crimped ductwork) or on occasion low refrigerant due to a leak.

Measuring temperature across the outdoor compressor/condenser unit is less useful as an indicator of the A/C condition because of the very wide range of outdoor temperatures.

Because the compressor is condensing low pressure refrigerant gas to a high pressure temperature also rises, often to well over 100 degrees F. and temperatures from 180 degF. to 200 degF are normal.

In fact if the high pressure refrigerant gas were not hotter than outdoor air the air blowing across the outdoor condensing coil wouldn't be accepting heat from the coil.

On 2017-05-11 by Ray Nickel

The air blowing out of the outside compressor / condenser is 180 degrees is that too high?

On 2017-03-23 by (mod) - re: what is the right outlet water temperature for an air chiller plant?


The quest for a simple "right answer" will be in vain.

The entering air at any air conditioning system is more or less the ambient air temperature at the return air inlet.

The outlet temperature will vary enormously depending on the duct system length, routing, insulation, diameter, environment (hot attic), air flow rate, filtration restrictions and other features.

That's why we usually measure the air temperature drop across the cooling coil.

Good luck on your test.

On 2017-03-23 by sb khan

i want to know that what is the intering and out let water temperature in air conditioner chiller plant thanks

On 2017-02-12 by (mod) re: why do air conditioners back freeze?

John please use the search box just above and search for AIR CONDITIONER ICING to see articles explaining the cause, implication, and repair for freezing or ice formation on air conditioning equipment. That will be more detail and more thorough than I could enter here in an off-the-cuff reply.

On 2017-02-12 by John

Why do air conditioner back freeze?

On 2017-02-12 by (mod) re: heating system fan won't stop


Watch out: if your furnace blower temperatures exceed the HI or FAN OFF limit and the blower doesn not stop and the furnace doesn't shut off it is overheating, is dangerous, and should be shut down and left off while you call for repair.

In other circumstances, please see FAN WONT STOP - LIMIT SWITCH if your heater uses gas or oil heat; if your system uses electric heat, such as the backup heat for a heat pump system, still check the article I recommend but also look for a bad relay or control board in the air handler.

On 2016-12-15 by Ricardo Trevino

My fan on my heating system never shuts off when it reaches its high limit. Is this normal?

On 2016-07-06 by (mod) re: typical refrigerant pressure when the system is off

With an HVACR unit OFF for 30 minutes or longer the pressures in the system normally equalize between high and low sides. If they don't there may be either
- a stuck or clogged refrigerant metering device, control, or valve
- or a design with which I'm not familiar

On 2016-07-06 by robert

With unit off there should be pressure on both high n low. But what if no pressure n low side

On 2016-05-30 by (mod)

Fred: I don't see a question; but beware: adding refrigerant usually means the leak wasn't fixed.

On 2016-05-30 by Fred

The compressor is about 35 feet from the cooling coil. The inlet/outlet air temp difference is about 12 to 14 degrees which has been the same for about 4 years that I remember. The cooling unit is above the garage and hard to get to. It has not been washed in years. Gas was added to it 3 years ago.

On 2016-03-31 by (mod)

Pius, you asked this question on other pages. It sure looks as if it's a query from someone taking an HVAC test, which made me nervous about answering, though a discussion about feeling refrigerant lines is indeed here at

On 2016-03-31 by pius itungi

how can feeling line temperature help when inspecting an air condition system?

On 2015-10-20 by (mod) re: impact on condenser life of various outdoor temperatures

Sam, I presume we're talking just about A/C not a heat pump; Your A/C condenser has to be installed outdoors; an indoor installation would just drop the heat that it has extracted from the refrigerant right back into the building.

The life of the unit will however be affected by its duty cycle - how much it runs - that in turn is affected by how close it has to run to the lowest outdoor temp that the manufacturer says it can handle.

On 2015-10-19 by SAM

Will the condenser's life be shortened if the condenser is outdoors in(ambient) air temperature of 40F degrees and the air handler is in an office with a room temperature of 80F degrees? The office has no windows that can be opened to cool the space . The office reaches 80F degrees because it is directly over a boiler room that is over 95F degrees. Will running the A/C in outdoor cold air shorten its life?

On 2015-09-06 by (mod)


At A/C school we were taught to bring the frost line to the coil outlet, figuring that at that point we're evaporating all of the refrigerant being metered into the coil. If the coil is quite warm at the outlet it sounds as if either the system is nearly out of refrigerant or the metering device ins not properly adjusted.

On 2015-09-05 by whelanjh

In a 4-ton home air conditioning system, using a hand-help non-contact thermometer,I find a great temperature variation along the A-coil from refrigerant input side to refrigerant discharge side ( 6 degF at the inlet, 62 degF at the outlet ).

Is this a normal variation over the extent of the A-coil? (Jerry Whelan )

On 2015-08-14 by (mod) re: positive refrigerant pressure found on both low and high sides of an HVACR system

Thanks, Charles.

You are right that in normal operation we see positive pressure on both the HIGH side (high pressures) and on the LOW side (low pressures) of an HVACR system. The low side is also referred to as the vacuum line or suction line as it's the compressor input side. Only during system installation or servicing would we actually pull a true vacuum on the system as part of evacuating it prior to installing a refrigerant charge. IN that case too it's "pressure" but negative pressure or vacuum.

I'll review the article text above - thanks for commenting.

On 2015-08-12 by charles

In "Air Conditioner Suction Line Pressure and Temperature - Typical Data and What It Means" says a vacuum is measured from the low side, not a pressure.
This is incorrect. Of course it is a pressure.

On 2015-09-21 by Anonymous

Sandy, send this AC company home. The cost of replacement, (even if need to be done) shouldn't get over $350.
But first he need to eliminate other problems, like blocked ducts and dirty coils.

On 2015-06-30 by (mod) re: don't replace parts before diagnosing the reason for cooling coil ice or frost blockage


If by TXZ valve you are referring to a thermostatic expansion valve (usually written TEV), that is a refrigerant metering device. The relationship between a TXV or TEV valve problem and low air flow would be indirect: a faulty thermostatic expansion valve MIGHT cause icing of the cooling coil in the indoor air handler unit. In turn, an ice-blocked cooling coil would reduce air flow through the system.

But that problem is easily and quickly discoverable by mere inspection of the cooling coil for icing after the system has been on for a time.

In teh ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS given above you'll find more diagnostic articles on blocked HVAC duct air flow, weak air flow, inadequate return.

Note that if only some of your ducts have poor air flow then it's not a TEV problem since an iced coil would reduce the air flow into ALL of the supply ducts.

I would not pay a thousand dollars for a TEV replacement before knowing that the problem has been correctly diagnosed.

On 2015-06-30 by Sandy

Does the blocked air ducts affect the reading of the return register and the supply register. Our differential reads at 12 which does not meet state standards for a home inspection. They say a minimum of 14. We had an AC guy come over and says we need a TXZ valve replacement.

The unit is only 6 yrs. old. The job will be $1070.00 just wondering if the two blocked ducts are the problem and not the TXZ.

Question: what does low pressure on the high side of my air conditioner mean?

My unit doesn' t go more than 90 psi in high side, the low stay around 45 psi, I had try more refrigerant, but didn't make any change it's a heat pump 6 ton - Kim 5/16/11


Low high side pressure sounds like either you've lost refrigerant or the compressor is damaged or an refrigerant metering device is failing to ever close as it should. Just "adding refrigerant" without a properly measured charge and without diagnosing and fixing a leak, even if it worked, is likely to be just a short term band-aid fix.

Question: My central air conditioner won't turn off

My central unit will not turn off. The temp in the house is 92 degrees and I have set the thermostat to 78, but it will not cool the house and will not turn off. Do I need to add freon in the unit?

Also, I flushed the unit with bleach to clean the pipes out and the air conditioner cooled the house for about 4 to 5 hours only. After that it will not turn off or cool the house. What can I do before calling a technican? Thanks - Ossie 6/8/11


I'm not clear where you used bleach but it doesn't sound like a good idea.

Before calling a technician check your air filters and check for duct work damage, leaks, crushing, etc. See the article linked to at the "More Reading" links at the bottom of this article and titled LOST COOLING CAPACITY for detailed diagnostic help for an air conditioner that is not working.

Question: My central air conditioner never shuts off

help,help. i have no problems with the air being cool. the unit will just not shut off unless i shut it off - Anon 7/3/11


An A/C unit that is providing adequately cooled air at the supply registers but still won't shut off is either never satisfying the room thermostat or there is a problem with the thermostat or the control board. Or there's another problem we haven't thought of. I'd first check to see if the thermostat shows that the room temperature is at or below the set temperature. It it is, I'd call a service tech.

If the room temperature never reaches the set temperature then either the set temperature is lower than your system can deliver, or there is a problem to find and fix, such as a blocked air flow (dirty air filter or fan).

Question: our unit keeps on cooling and won't shut off

Hi. For some reason, our dual zone unit keeps cooling the downstairs zone to well below the set temperature -- when set at 79, very cold air is still blowing though the thermostat reads 71. Cold air still blows even with both thermostats turned to "OFF."

Not sure what is causing this, but I am sure it's gonna cost me $700/month if I can't get the air to shut off.

Any thoughts would be greatly appreciated! - EL 6/25/12


EL, check for crossed or bad thermostat wires as a starting point.

For air conditioners that "won't stop running" or "won't shut off" readers will want to see these articles:

Question: what is the maximum that an air conditioner can cool indoor air below outdoor air ?

I think I remember maybe that I might have read somewhere that a home central air conditiorne has a difficult time holding an inside air temperature more than 20 degrees F below the outside temperature. True or False? - Dennis 7/6/11

Reply: it depends...

Dennis: my OPINION, though we don't care for "true or false" questions, is that the answer is ... it depends. On a newly installed split system I was working with last week the unit had no trouble getting indoor temp down to 68 F when it was 90 F outside.

But building insulation, heat loss or gain rates, solar gain, air leaks, etc. all add up to having an enormous effect on just what a given heating or cooling system can attain.

Question: our new A/C doesn't cool as well as the old one

We replaced our 9 year old central air unit with a new one (both 4 tons) and also added another air return. The new unit does not cool as well as the old unit. The air output temp is 67 degrees. The house stays at 82 degrees. The outside temp is about 100 degrees. Any idea why the new unit is not doing at least as good a job as the old? - John Hazel 8/5/11


You need a more expert onsite diagnosis than I can guess from just the information below, John, but I'd start by looking at

- air flow at the supply registers
- temperature drop across the cooling coil (look for 15-20 degF as typical)
- the blower fan cleanliness and fan speed
- obstructions in the ductwork - including a dirty or obstructing air filter

Question: my air conditioner blower was replaced and now the new unit doesn't cool?

I had my blower replaced and the temperature in house is only 3 degree different from outide temp; it doesn't matter at what temp I set my themostrat. I never had that problem prio to the replacement motor. It always reached the set temp I set on the themostrat. Freon was added with the replacement motor, is there anything else I should be looking for beside over servicing. - Sepulveda 8/19/11


Sepulveda I'm sorry to say it sounds as if the blower replacement may have been done without adequate diagnosis, or perhaps it needed replacement but there is a second problem to be found and fixed. You need a service call. Bur first check the diagnostic suggestions and advice found at LOST COOLING CAPACITY.

Question: My air conditioner won't start

my unit will not start i just had someone who said he knew what he was doing recharged it and now if i manually close the relay the high side goes up to 300psi and the low side will go up to 150psi is this an overcharged system. - Kevin Goans 8/21/11

Reply: wrong question, wrong repair?

An A/C unit that won't start has more wrong with it that missing refrigerant. If the problem were only lost refrigerant the system would still start, it just wouldn't cool.

Question: Do you think that the evaporator is dirty / in need of cleaning; or was it just iced up due to the continuous running of the unit?

I just moved into a new 3-level townhome with Central air for the whole home PLUS a 2nd cooling unit (Space-Pak type) for the 3rd level only. Approximately 2 weeks in, noticed a drop in cooling (hot day); checked the air filter, was completely clogged, replaced it. A few weeks later, very hot day, noticed drop in cooling again.

Thermostat temperature never reached. Manually shut down unit, restarted, worked better. After reading this site, I understand now that the evaporator was likely iced/frosted. 3 days ago, very hot days, water backed up in the unit. Service call, drain backed up.

Cleaned the drain. My question is: Do you think that the evaporator is dirty / in need of cleaning; or was it just iced up due to the continuous running of the unit? I measured the air temperature at one of the outlet/vents and it is at 57 degrees.

I would think that a dirty evaporator would mean poor performance of the Space-Pak even before ice could build, whereas since I am still getting poor performance on very hot days only once the system is running for a very long time continuously (12-24 hours or so), I am assuming that the evaporator problem is/was ice and not dirt.

I am thinking of changing the thermostat to a more advanced model that I can program with a cycle time for both on and off (i.e. never on continuously more than 6 hours, never off continuously less than 30 minutes) as well as changing the fan (air handler) from "auto" to "on" (which I have already done.)

Since I have changed from Auto to On, I have noticed an improvement (assuming that the ice buildup is draining more quickly / evaporator coils are warming more in between cycles).

Also, the large pipe (copper, approx. 1" dia.) wrapped with insulation feeding the unit has had some small amount of ice very close to the unit where there is a small tear in the insulation. Not sure if this is left over water from the drain backup (the serviceman said that all the insulation was likely soaked as a result and could take 7-10 days to clear up). Thoughts? - BB 9/3/11



I'd have the system evaporator coil inspected for dirty conditions and cleaned if needed. The thermostat program you suggest is an interesting idea, but in my opinion a properly working AC system operating in hot weather should be able to run continuously without icing the evaporator coil.

A dirty evaporator coil should also show up as reducer airflow. Also the blower fan itself may be dirty, reducing airflow and so leading to coil icing.

Reader Follow-up:

Thanks. I had a chance to read the notes left behind from the technician that performed the service call: he noted that the evaporator coils were rusted. I am assuming that this is causing the icing... should I look to replace just the evaporator, or upgrade the entire Unico cooling unit at this point?

What would you recommend as the cost/benefit considerations for the decision? I am assuming that the condenser and air handler are in good shape since the cooling and air flow are quite good just after the unit starts. I am assuming that it would be more cost effective to replace only the evaporator. Any thoughts based on your experience?


Interesting. I've never seen rusted evaporator coils since they are usually made of aluminum or copper, though there may be steel parts around the coil itself that can indeed rust. Perhaps the tech meant corroded. Gross corrosion, enough to clog the coil fins, would block air flow and could contribute to coil icing, though I don't usually find that the root cause.

I'd ask for a price estimate on coil replacement; it's a big job in that the system has to be evacuated and recharged as well as cutting out and soldering in the new coil. Let us know what you're told and I'll comment further

Reader follow-up:

I asked a different company to come quote on "repairing" the problem, where the problem was simply stated as a lack of cooling after some length of time when the unit is running, with evidence of icing (and subsequent leaking resulting from the thaw).

They suspected a low refrigerant charge, and will come back and look for a leak (for which they are charging me, no pun intended.) They did indeed say that the evaporator coils are copper, not rusted, and that no evidence of poor airflow was seen. I will keep you posted to see if a leak was found. - BB

Question: what is the minimum temperature for central air conditioning?

what is the minimum temperature for central air conditioning system? - Sibaji 9/27/11



The output temp of a central air conditioning system depends on the temperature of the air entering the cooling system. Typically we see about a 15 to 23 deg F temp drop across the cooling coil inside the air handler. Length and insulation of and on ductwork will mean the temp you measure at a cool air supply register will be somewhat warmer.

Question: Air Conditioner Suction Line Pressure and Temperature - Typical Data and What It Means

Determining proper suction "pressure" (really we're measuring a vacuum, not a pressure) is critical for proper refrigeration equipment operation assessment. If the air conditioner system suction pressure is too low (and that probably means the temperature in the line also is "low") it could indicate - BH 11/10/11


Thanks BH we agree.

If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.


Continue reading at AIR CONDITIONER TEMPERATURES - normal, abnormal, rules of thumb, simple diagnostics - topic home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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