Air Conditioning & Heat Pump Temperature FAQs

Q&A on A/C or heat pump output too warm or too cold

Normal Temperature Differences In an A/C or Heat Pump System, FAQs

What's normal air conditioner temperature drop across the cooling coil ?

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? - Luke Ducote

Reply: Typically this temperature drop is 15 to 20 deg.F across the coil

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.

The pressure ranges you cite, depending on ambient tempereature and refrigerant type, are normal.

You don't cite a temperature difference across the coil - that should be measured across the cooling coil or close to those locations.

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.

Watch out: Refrigerant is not a consumable item. Your system is leaking refrigerant - that's abnormal, costly, and an environmental contaminant.

If there is a leak the best procedure is to find and fix the leak rather than to keep adding refrigerant.

 

Is a 40 degree temperature rise across the coil - normal?

I have a 40 degree rise across the coil with the outside temp at 60. In heating mode, Is this normal. Thanks On 2018-11-18 by Mike DobyI -

Reply by (mod) - signs that backup heat is running

It sounds to me as if you're in heating mode and possibly your backup heat is also running. In that case, yes that temperature rise could be normal.

What are the normal operating pressure range and temperature range in an A/C system?

I like to also know about the operating pressure range that is the maximum range to which the air is compressed in a normal 1 ton ac considering normal outside temperature - On 2017-01-15 by Anonymous

Reply by (mod) - typical TXC TEV temperature response

For a typical thermostatic expansion valve (TEV) you'll see that it monitors and responds to an

Evaporator coil temperature range from 0 to 50 degrees F.

or 10 to 60 degF, for smaller HVAC equipment,

maintaining a liquid subcooling temperature of 10 degrees F.

Pressure ranges may be from 5" Hg to 90 psig.

Energy audit says should see 20-25 degF temp drop between air in at return and air out at supply registers

A/C contractor said system is running as best as it can, but an energy audit found return vent temp of 80deg, and register vent temps of 70deg.

The closest vent is a mere 6 ft away from the coils.

Energy Auditor says the drop should be in the neighborhood of 20-25 degrees, not 10 degrees. Who is right/wrong? I don't know if the A/C contractor is being deceptive because he is contracted by the landlord, and not by me. - On 2011-08-02 by Eric

Reply by (mod) - Wrong! the temperature drop is more accurately measured right at the cooling coil -

Eric the temperature drop is more accurately measured right at the cooling coil - that tells us what the A/C core components are doing and ignores for starters the effects of duct length, insulation, leaks.

When you measure where your "energy auditor" did, you can't separate a problem with the air conditioner itself - the outdoor compressor / condenser unit and the indoor air handler / blower unit from problems in the HVAC duct system, so you won't know where to look or what to do to improve the system's performance.

Right at the cooling coil I expect to see a temperature drop from air into the coil to air out of the coil around 15 to 22 degF.

If you are seeing just 10 degrees difference in air temp when measuring far away from the cooling coil, say at the return air inlet and at the closest (to the air handler) supply air outlet, your measurement is, compared to the one I describe above, ambiguous since we don't know about duct insulation, run length, crimps, bends, leaks, etc.

By separating out those factors we know what to focus on in improving system performance.

Exit Temperature for a water cooled air conditioner

What is the ideal exit temp for a water cooled air conditioner for a 2000 sq ft home ? - by Jerry

Reply by (mod) - What is the ideal exit temp for a water cooled air conditioner ?

Design temperatures for water cooled Air Conditioner Chillers - 10 DegF

Because the inlet and outlet air temperatures at air conditioning systems depend on ambient temperatures and weather as well as other factors, there is no single "right" outlet temperature for the cool air supplied by the system.

Technicians wanting to assess the operating status of the A/C system will look at the temperature drop across the cooling coil - that's what's described in the article above.

However the delta T or temperature difference across the cooling coil will be less in a water-cooled AC or chiller system than in others.

Typically an industry standard is to expect to see about a 10 degF. temperature drop across the chiller (a bit less than in a refrigerant-based AC system.)

Here's a quote from a HVACR training company, edited slightly for clarity

The numbers for chilled water (44 out 54 in) and condenser water (85 out 95 in) are industry standard values at full load conditions.

Most chillers regardless of manufacturer will have a 10*F delta T on the condenser unit and evaporator unit. - Source: Orr, Bryan, Jeff Neiman DESIGN TEMPERATURE DIFFERENCE for CHILLERS, www.hvacrschool.com/design-temperature-difference-for-chillers-2/

Water cooled AC units are interesting as there has been a lot of work done on the effect of ambient temperature on the operating efficiency of the system. Here is a quote from one of our expert sources

105°F is used for water-cooled condenser condensing temperature.

The cooling tower water temperature into the condenser is assumed to be 85°F. ...

For every 10% decrease in initial temperature, there will be a 9% decrease in capacity.

For instance, the condensing temperature drops to 101°F. - Training materials provided by Industrial Controls Corp., 17 Christopher Way Eatontown, NJ 07724 USA, Tel: 1 (877) 867-1299 Web: https://www.industrialcontrolsonline.com/

Can the inlet temperature in the A/C compressor be 15 degree celsius

Can the inlet temperature in the compressor be 15 degree celsius or will it be too hot for the compressor? - Mohammad Reza

Reply by (mod) - Can the inlet temperature in the compressor be 15 degree celsius

Mohammed,

At the outdoor compressor/condenser unit, an outdoor ambient air inlet temperature of 15C (60F) is about the low limit at which a typical residential air conditioner is expected to operate. Below that there are risks of damage to the equipment.

If your outdoor compressor/condenser unit is a heat pump then it is expected to operate ( in heating mode) at lower temperatures, though a typical heat pump will not work efficiently if the ambient air inlet temperature is below about 25-30 deg F (-4C to -1C).

In summary

In heating mode the outdoor ambient air inlet temperature at the heat pump compressor/condenser unit is best at 35F (1.7C) or above.

In cooling mode the outdoor ambient air inlet temperature at the heat pump compressor/condenser unit is at 65F (18C) or above.

 

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

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 by Minh (HVAC repair technician): I typically get the units I work on down to 59 degrees coming out of the air vents

Carl: the 2 ton unit is within the range 77-61=16 degree different.

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 vents

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

 

Survey of A/C Temps & Pressures at Various Points

Do I need a new A/C system if mine is low on refrigerant?

Had two HVAC techs come out to look at my system. Paid both $175, both just tried to sell me new systems and one keep pushing the sale "to get a tax credit".

I looked into the credit, it applies to high seer systems (90 something) not the 80% they both tried to push.

I think my system is just low on R-22, the system worked fine until two years ago.

I checked the liquid line just out of compressor and just before the orifice restrictor before the evaporator coil, both were 78 degrees, just after the orifice the temp was 51, the suction line just outside the evaporator and just before the compressor was 73-74 degrees.

The duct work and a vent were 66 degrees.

Both techs looked at both pressures and said "they looked right," I am not completely positive with their assessments, they didn't even try to check for leaks or add some R-22.

I'm thinking about adding some A/C dye to check for leaks and some R-22. I know adding R-22 is a gamble, but it's either pay another tech $200 or buy some R-22. On 2018-05-30 by Will -

Reply by (mod) -

Normally before replacing HVAC equipment that's not working, I would insist on a credible diagnosis of the problem.

With the diagnosis I can get a repair cost estimate. That plus the age and general condition of the system would tell me to repair or replace.

Just adding R-22 is a bad idea: if there's a leak it would need to be found and fixed. Else the system is just contaminating the atmosphere and pouring your money after it.

Followup by Will

Here's some more information (numbers). With the air temperature around the condenser/compressor at 80F, the low side is 65 psi, the high side 175 psi. Do these numbers seem right?

Reply by (mod) -

That's probably low. Take a look at the refrigerant pressure articles at

REFRIGERANT PRESSURE DIAGNOSIS 

Temperature & Pressure Readings by Will

I see the linked article also tells about the where the Freon changing of state moves with the charges.

Oh yeah, I forgot to mention earlier that my system specs call for 4 lbs of R-22.

The tag also say 150 low side and 300 high side, the article says to ignore this as this is the leak pressure.

A last point of observation is that there is condensation towards the top of the coil but the bottom of the coil is dry. Thanks.

Hey thanks for helping.

Pressures at the Service Valves

I did another check of the system with ambient air temperature around the compressor at 88F.

• The low side read 70 psi (the gauge says I should have 42F at the evaporator).
• The high side read 210 psi (the gauge says I should have 105F at the high service port).
• The vapor line is 75°F at the service valve into the condenser and
• The liquid line is 90°F at the service port out of the compressor.

Temperatures at the "A" Coil

A check of the "A" coil with thirteen tubes (the side I could get to) shows the following temperatures.

• The top of the "A" coil was 47.3°F
• The middle read 64.5°F
• The bottom row measured 75.2°F
• The same temperature of the vapor line at the service port.

I vaguely remember reading, some time back, that if the system is low, the Freon starts to absorb the heat early in the coil, making the beginning of the coil colder than the end of the coil.

I am wondering if that is what is going on here a low charge, or the evaporator is plugged, or the restrictor orifice is partially blocked.

I understand Lennox used screened restrictors with some their older units. I will read the link provided. Thanks.

Reply by (mod) - 15 degF temperature change is across the cooling coil?

Generally you want to see 15 deg or more across the cooling coil.

It's risky to compare data from two different houses - though interesting.

You can't measure the precise quantity of R22 in the system but the temperatures or more commonly the location of frost line at the cooling coil can be diagnostic of low refrigerant.

And if there's a leak it'll continue to leak until you have little or no cooling.

Followup by Will: temperatures on a working A/C system

I compared my A/C condenser inlet to my neighbor's, his works, and

• the low side temp was 47F.
• The high side was 110F.

His system is two years old.

I didn't look at the pressures because I didn't want to inconvenience him too much, but his low reading compared to my high seems to suggest either I do not have enough R-22.

Would it be safe to add a small can to see any difference. Like I said in earlier post the techs didn't do any of that.

All they did was look at the pressures drilled a hole in my evaporator case and check the temp.

Both didn't even tell me what temps they were looking for. Both techs were from establish companies. In fact, the first tech was from the company that installed the unit in mid 2000.

How can a system need to be replaced in 15 years, after all, Lennox is supposed to be the best (that what we were told, that they last a long time compared to others). It sure was expensive to get and the parts are way expensive.

My neighbors run Carrier, Armstrong, Rheem, and other known brands and have no trouble, their units are older, and my neighbors told me the parts are cheaper than Lennox.

A point of interest, when I called Lennox about parts availability and prices, they told me that they don't do that at all and to call a HVAC tech.

So, I'm kind of stuck, and in a way I think we were hosed but the company who installed the unit.

If memory serves, we spent around $5000 and were sold a "80% efficiency" unit.

When talking with the original installing company about a new more efficient unit, we were told around $6000-7000 for the furnace and A/C, but they could again sell us another "80%" unit for around $5000-6000. And of course, my present A/C is out of warranty.

I wish I could measure the R-22 in the system.

Reply by (mod): warning when comparing A/C system temperatures and pressures

Watch out: you may be comparing apples and oranges when comparing your old A/C system temperatures and pressures with your neighbour's new A/C system since his is almost certainly using a different refrigerant.

The "normal" pressure readings for refrigerant will vary depending on refrigerant type and of course ambient temperatures.

Diagnose Abnormal Temperature Differences in A/C or Heat Pump Systems, FAQs

Air temperature 13 Degree difference between air intake and supply - not enough?

Temperature difference between intake and supply was 13 degrees, It should be 15 - 20 degrees, how do I fix it? - Brian

Reply:

Not sure you have a problem; Nor did you make clear exactly where you took your measurements.

Some things to check are refrigerant level, insulation in the system, dirty coil.

See the article above and also see the Recommended Articles at the end of this page.

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.

 

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.

Other A/C or Heat Pump Temperature, AirFlow, Fan FAQs

Impact on condenser life of various outdoor temperatures

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 2015-10-19 by SAM

Reply by (mod)

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.

Can my liquid refrigerant line freeze?

Can my liquid line freez with frost if so why - Kevin Ohio

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.

 

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

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: 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

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;

 

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

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.

Problems converting A/C 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

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 next week he's going to remove the coil to see if anything is disconnected. He's 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 [sic - he really meant TEV or TXV - ed.] valve.

This system does not have one.

It has a coolant regulator which he replaced. [Perhaps a capillary tube? - ed.]

It allowed the temp of coolant to drop to 42.

The most it had come down to was 52.

 

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

Reply: Yes

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.

 

Trane A/C Air Handler Fan Speed Set on Low, then we lost cooling

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

Reply: don't confuse low air flow caused by dirty air filter or a dirty blower fan itself with low air flow due to fan speed setting

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

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

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.)

My Central Air heat pump with digital thermostat, blows cold air but the temperature gauge reads hotter than the temperature really is.

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. (May 4, 2015) Rod said:

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.

 

A/C output air is too cold

The air output temperature of the air conditioning is absolutely frigid. Can that be adjusted to a more moderate temperature? - Ray Myers

Reply:

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

Second, your tecnician may check the refrigerant charge level to be sure it's not low,

Third your technician should check the A/C system total capacty, for example in tonnage, against your buildng size and insulation levels and heat gain paramters.

Watch out: if your A/C system is over-sized, not only might the supply air be too cold, the system probably won't dehumidify the building interior properly - it cools the building down too rapidly, not running long enough to remove moisture from the building air.

Then, yes, if needed your HVAC technician can adjust the return air balance and supply air balance between supply lines, possibly also adjust the fan speed or install duct dampers.

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

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 . kenneth fletcher

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