Air conditioner or heat pump operating temperature diagnostic questions & answers: HVAC system temperature FAQs:

Questions & answers about 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.

This article series explains the normal or abnormal operating temperatures of air conditioning equipment and what temperatures can be expected when measured at different locations.

Air Conditioning / Heat Pump Temperatures Diagnostic Questions & Answers

Air 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 - you will enjoy reading that material.

Question: air conditioner temperature difference across the cooling coil - what's normal?

I've been having problems with a Freon leak for a couple of years now on an R22 system. A charge usually lasts several months, however the last charge only went a month before I was having problems again.

Upon placing my gauges on the unit, I noticed the pressures were normal (75/210).

Although the condenser coils did not appear real dirty I cleaned them anyway. Both pressures dropped considerably so I added Freon. Then unit began running better/colder but is still having problems keeping up.

I added a little more Freon (up to 85 psi) on the low side, however the high side is not getting any higher. Did I put too much Freon? Could there be air in the system? I'm wondering if the compressor has been damaged but would rather not replace it as a means of troubleshooting. Any suggestions? 15 July 2015 Luke Ducote

Reply:

Indeed, Luke, if an A/C system has lost enough refrigerant that it is no longer cooling air then adding refrigerant to the system may get it "working" again.

But the temperature difference you cite, if measured across the coil or close to that would be fine.

See AIR CONDITIONER TEMPERATURE MEASUREMENT to make sure you're measuring the A/C temperature drop correctly.

See HVAC TEMPERATURE INSTRUMENTS if you need a better A/C temperature measurement device.

Refrigerant is not a consumable item. If there is a leak the best procedure is to find and fix the leak rather than to keep adding refrigerant.

Question: ice on the coils of a window air conditioner

Hi, the window airconditioner i'm working on shows ice formation on the evaporator coils. It started to around 2 to 3 coils then it gradually increases until almost the entire coils are covered with ice.

I won't go with the idea that the thermostat should be set in a way to avoid the compressor running continously because before the trouble begun the unit has been running almost continously for 8hours for the last several years and there was no occurance of ice/frost build-up. We check the unit current and its ok, so we believe the charge is also ok.

We check other possible causes, like distorted coils fins, dirt, obstructions and blower speed and they seem to be all ok. What do you think should I do more? - Arnie 11/10/11

Reply:

Arnie no way can I be smarter than a trained service tech, but

Question - how did younfigure the charge is ok vs refrigerant is low?

Question 2- could you have a partly crimped or blocked cap tube ? that can result in low side pressure being abnormally low, uneven refrigerant metering; a blob of oil or dirt traveling in the system easily blocks a cap tube.

Is there a drier that could be clogged or has the unit never been opened ?

Is the blower fan itself dirty? This one can foolmyou. Dirt on squirrel cage blower fan blades fills in their curve and really cuts airflow

Keep me posted

Dan

Question: can my liquid refrigerant line freeze?

can my liquid line freez with frost if so why - Kevin Ohio 2/19/12

Reply:

Kevin Ohio, no the liquid refrigerant line does not freeze, but it can be come frost or even ice coated if it is missing insulation, or in some cases if the refrigerant charge is improper, or if a refrigerant metering device is not working properly. In cooling mode on a heat pump or in an air conditioning system you will see frost formation on the suction line - the larger diameter refrigerant pipe.

 

Question: is the outside compressor unit controlled by indoor temperature?

Is the outside compressor controlled by the temperature outdoors? At a outdoor temp. of 72F and the indoor temp around the same and I turn the thermostate to 70F the compressor will not run only the fan. When the outdoor temp is 82F or higher the compressor runs normally. - Compressor situation 6/11/12

Reply:

The outside compressor on a residential cooling system basically runs under control of the thermostat that is calling for cooling.

But the efficiency of the system is affected by outdoor air temperature. In hotter outdoor air the compressor/condenser has to work harder, and perhaps longer to condense the refirgerant back to a liquid.

 

 

 

Question: My A/C quit blowing cold air - the repairman said "give it a few days"

My ac quit blowing cold. My ac repair man checked for leaks (found none). He cleaned both outside and inside coils. Still not cooling good so he put a very small amount of coolant in it. My air blowing out of my vents is aaround 80 degrees.

My ac repairmand knew the temp was not cool,he said to give it a few days and let him know. I called him and told him it is still not working. He said he has all the info and would tell me about it. Now he does not return my calls and we are burning up. What can I do to correct my ac problem? - Bill 7/5/12

Reply:

Bill it sounds from your note as if your repair tech is either overloaded with work ( after it's all during very hotvweather) or is stumped .it's time to call someone else who will respond to you.

 

Question: is a hot bathroom wall next to HVAC equipment dangerous?

I have a older unit, central air unit inside the condo. The wall of the unit is against my bathroom shower wall. It gets so hot in there, I can't keep it cool in the bathroom.

I rent and I don't know what kind it is. But I have to so something as it is a one bedroom and my cooling bill is outrageous. Help please? - Donna Van Wie 7/09/12

Reply:

Thanks for the question, Donna, but I'm sorry - I don't really understand the situation that you describe. The indoor air handler unit, if it's a cooling unit only, should not be running so hot - it sounds as if you need a service call from an experienced technician. Keep us posted on what you find - it will surely help other readers.

 

Question: how accurate is the operating temperature range of an air conditioner?

how accurate is the operating temp range, and if the outside air is hotter than said range, is the machine potentially going to over work itself and have issues?? - JD

Reply:

Nate I do not know a factual answer to your operating temperature range question, it would need to be answered by the specific manufacturer of the A/C system. My OPINION is that the range is the temperature range at which the equipment can adequately cool the space for which it was designed.

At higher outdoor temperatures the system may not be able to produce the same cooling capacity. I would not expect it to be ruined by that fact alone. In fact it is common to see cooling equipment run for very long periods during very hot weather.

 

Question: should I set the fan to constant ON on very hot days?

should the furnace fan be set to stay on constantly during very hot days, to keep cool air flowing?? JD - 7/09/11

Reply:

JD:
the answer is an OPINION as I don't think there is a single right answer: Often I like running the A/C fan continuously to improve air/dust filtration and give more even indoor temperatures;

 

Question: does a drop in the temperature of cool air in my system mean the compressor is damaged?

I recently replaced a fan relay. Before the fan relay went bad, the register cooling temp was at 62f. The fan relay was bad for a few days and the outside compressor was running for some number of hours throughout the day until we realized the inside wasn't running.

Now, after the fan relay has been replaced the register cool temp has sometimes hit 56.1f. What's up with that? Could it indicate a damaged compressor? Thanks, Chris K

Reply:

Chris, thanks for the interesting question. But I don't think I can give a straight answer evaluating an 8 degF drop in cool air output temperature after getting a fan relay replaced. That's because building and environmental conditions vary so widely that the temperature of the return air entering the cooling system also varies widely from time to time.

A cooling system generally produces a rather constant temperature drop across the cooling coil, so long as the air flow rate stays unchanged. Something that slows the air rate may cause a greater temperature drop - not always desirable, and vice versa.

So if the weather or your home were hotter when you made the first measurement than when you made the second, that condition alone could explain the change that you saw.

 

Question: air conditioner compressor unit fan relay problems?

I recently replaced a fan relay. Before the fan relay went bad, the register cooling temp was at 62f. The fan relay was bad for a few days and the outside compressor was running for some number of hours throughout the day until we realized the inside wasn't running.

Now, after the fan relay has been replaced the register cool temp has sometimes hit 56.1f. What's up with that? Could it indicate a damaged compressor? Thanks, Chris - CK 7/14/12

Reply:

CK thanks for the interesting question. But I don't think I can give a straight answer evaluating an 8 degF drop in cool air output temperature after getting a fan relay replaced. That's because building and environmental conditions vary so widely that the temperature of the return air entering the cooling system also varies widely from time to time.

A cooling system generally produces a rather constant temperature drop across the cooling coil, so long as the air flow rate stays unchanged. Something that slows the air rate may cause a greater temperature drop - not always desirable, and vice versa.

So if the weather or your home were hotter when you made the first measurement than when you made the second, that condition alone could explain the change that you saw.

 

Question: stolen A/C compressor unit, now problems converting to an R410 refrigerant system

My house had its condensor stolen while we were in escrow. Purchased a 410 system. My buddy replaced it and we left the existing furnace. Changed out blower and coil. Evacuated all freon with nee refrigerant. It was cooling fine then one day it could not keep up.

My friend can't find the reason why. He checked temp at register. It was in the mid 40s then jumps up to the 60s. Can this be a faulty t x v ? Even though it was new maybe a deffective piece? - J Flores 8/11/12

Reply:

J

You made a good guess. Check to see that the thermostat is oontinuing to call foes cooling (set it well below the room temp) and that the compressor is continuing to run. In that case if the air temp surges between cool and not cool I too suspect the refrigerant metering device - presuming it's not a heat pump system.

Also check for icing at the TEV and that the TEV is rated to handle the r410

Reader follow-up:

Spoke to my friend and he is convinced that the condensor is to far from the furnace and this why its not blowing cold air. Furnace is in garage in front of house. The condensor is in rear of house. Lines coming from condensor go up aprox 20 ft. Into attic. 2 story house then goes all the way across aprox 40-45 ft.

Then down to furnace. He says any line being over 50 ft is going to struggle. Hes planning on moving the condensor to the side of house. Bringing it closer to the furnace. Aprox 25-30 ft of line. He thinks this will cool properly.

Reply:

J
That distance determines the length of refrigerant piping and thus the amount of charge needed. If the charge s low that could be the issue. On moving the system the whole thing will be emptied and recharged anew, which might then look as if it were the move rather than the charge that fixed matters. Keep me posted.

Reader follow-up:

Dan we are back to step 1. My friend moved the confenser back to its original place. His gauge read blue gauge 42 degrees. Red gauge 370 degrees. Text book temps he said. Temp at coil 48.8 degrees. Air from vent. 68-70 F. So from the coil to the register a distance of about 3 ft the air heats up 18-19 degrees!

Why? We even climbed in attic no leaks. Punctured one duct with thermometer. 68.5 f nxt week hes going to temove coil to see if anything is disconnected. Hes determined to find out why.

Still took it over 6 hrs to drop it from 87 to 81. We concluded that it was not a tpv valve.

This system does not have one. It has a coolant regulator which he placed. It allowed the temp of coolant to drop to 42. The most it had come down to was 52.

 

Question: can my cooling compressor stay outdoors with no damage in cold weather?

i need to cool a very warm room all year round. Can my compressor stay outside and opperate in cold winter temps without harm? - Michigan 8/14/12

Reply:

Outdoor compressor/condenser units are designed for outdoor weather exposure; but that's not to say that your particular unit is designed for outdoor use in cold weather.

Watch out: do not turn on an un-known air conditioner or heat pump system in cold weather if you don't know about the temperature rating of your outdoor A/C system (or heat pump system) and if you have not checked with your HVAC service technician.

For example, if a system has no crankcase heater installed, or even if it has one installed but the system has been left turned off, trying to run the compressor without sufficient time for the crankcase heater to work can damage or even destroy the compressor motor.

 

 

 

Question: Trane A/C system repair questions - air handler speed is too high?

I recently had my traine system repair under warranty work. At the time the repair technician said my air handler speed was two high, and set it on low. recently I came home and the air was not cooling, so I called repair company who sent another technician, but before he got there I replaced air filter and reset unit. It seem to do the trick but air flow from vents seemed low.

When he arrived he said everything check out, But he said the fan was set to low. So know I don't know what to think as two technicians from same company had opposite recommendations. I live in Texas were is extremely hot in the summer - CB 9/12/12

Reply:

Sounds as if there was confusion about fan speed and a dirty air filter. A clogged air filter reduces air flow through the system.

 

 

 

Question: my window A/C unit fan speed is too high - how can I adjust it ?

HI there, The lowest fan setting on my window A/C is way to high. It blows like a hurricane. Is there any way to adjust the settings to make "low" really "low"? I appreciate any help or suggestions you can offer! Thanks much, Carrie - 11/17/12

Reply:

Carrie,
I'm doubtful that you can at reasonable cost change out the fan speed control on your window A/C unit as long as the original one is operating to factory spec.

But you can try adjusting the air-directing louvers to not blow the hurricane where it's not wanted, or even to leave them partly closed to slow down the air flow.

Question: how cold can it be outside and still get heat from my heat pump system?

How cold can the outside temp. go down to and still let the inside unit warm the inside room to a comfortable temp. on a split system - Jon Joynson 12/14/12

Reply:

John,

The lowest operating temperature of a heat pump varies by brand and model. System efficiency begins to drop when temperatures drop below the low 40's, and eventually it's cold enough that backup heat or alternative heat sources are required.

Tyically a modern high efficiency heat pump can extract some heat from cold outdoor air down to the high 30's. And there are some recent models of heat pumps designed to extract heat from air down into the 30's F and even to as low as 15F. (But we'd want to see the efficiency operating curve and operating cost data too.)

Question: What air temperatures should we see out of Trane 2 & 3 ton cooling units?

(June 8, 2014) Carl said:
My friend is trying to buy a townhouse and had a recent home inspection. The inspector noted that there are two Trane units, one 3-ton and one 2-ton. He did not note the model numbers but did note they were manufactured in 2004 and said R-22.

On the 3-ton unit the supply air temp was 60 degrees and the return was 72 and he said this is out of range and is a problem, but did not indicate if this just means the unit is low on freon, has a leak, coils and filters need cleaning, or the unit is at the end of its expected life and should be replaced.

On the 2-ton unit the supply air temp was 61 and return was 77, again he noted this was out of range.

Can anyone shed some light on what these numbers mean, what they should be on a properly working system, and does it sounds like these units need to be replaced or just serviced?
Thanks,
Carl

Reply:

(June 8, 2014) Minh said:
To Carl, the 2 ton unit is within the range 77-61=16 degree different.

(June 16, 2014) Anonymous said:
To Carl: On an R-22 system you should get at least a 67psi reading on the low side (suction line). Once I get this reading I go inside and check the return air temp and then the temp coming off the a-coil. I keep adding refrigerant till I get a 20 degree difference. I typically get the units I work on down to 59 degrees coming out of the air vent

s. The 16 degrees mentioned is what I consider bare minimum. Yes, it appears there is a leak somewhere. Check all brazed joints and look for, or feel for oil on the surface. Add some refrigerant and you are good to go. Also, most AC units last 10-14 years, although I worked on an oldie that was over 30 years old. It finally failed. Good luck. Floyd/HVAC Tech

Question: central air runs continuously, house is cooled fine but the fan won't stop

(July 6, 2014) Anonymous said:
my johnstone central air systems runs continuously. it cools the house down fine but it won't cycle. what's up?

Reply:

Anon,

Some diagnostics for an air conditioner that won't stop running are over at

inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

Your system may simply be unable to keep up with the heat gain of your building, or there may be another problem worth diagnosing. Check the temperature drop across the cooling coil and check the air flow CFM.

 

 

Question: My Central Air heat pump with digital thermostat, blows cold air but the temp gage reads hotter than the temp really is.

(May 4, 2015) Rod said:
My Central Air heat pump with digital thermostat, blows cold air but the temp gage reads hotter than the temp really is. I will set the temp on the thermostat at 80 the temp reads 82 but the house is cold. I manually turn off the air and ten minutes later the temp reads 78 deg. While the A/C is running the thermostat reads hotter? I replaced the batteries in the thermostat twice, I blow out with can air to clean dust, no change.

Reply:

I'd check the thermostat position - it may be in a location that does not properly sense room temperature, such as in direct sunlight.

 

Question: A/C output air is too cold

(June 10, 2015) Ray Myers said:
The air output temperature of the air conditioning is absolutely frigid. Can that be adjusted to a more moderate temperature?

Reply:

First I'd check that the return air is adequate and that the blower fan is moving air properly.

Question: RTU heat pump outdoor condenser turns cold when in cooling mode

(June 10, 2015) kenneth fletcher said:
RTU heat pump outdoor condenser turns cold when in cooling mode ... condenser fan blows out cold air... unit has a reversing valve...what might be the problem ... thanks

Reply:

Sounds like a thermostat or control board or wiring problem putting you in heating mode when you want to be in cooling mode.

Also see REVERSING VALVE on HEAT PUMPS

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