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.

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 - please review that material too.

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

On 2017-05-19 by Anonymous

My apartment manager has shut off the heat in our building and is transitioning to A/C (only one works at a time).

They sent out an email stating "we have shut the heat down and now have to let the lines rest and come to temperature before we can start the AC."

I've never heard of lines "needing to rest" before using the air conditioner (I'm from the south but I moved to New England this year)

. Is there something different about A/C units here that makes this necessary? Does this sound legitimate, or is the manager pulling our chains?

On 2017-05-19 by (mod) - 24 hours would be more than enough to swap between heating/cooling modes,

Typically 24 hours would be more than enough to swap between heating/cooling modes, and even then the problem being avoided would be avoiding turning on an air conditioner or heat pump in cold weather.

On 2017-01-15 by Anonymous

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 (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.

On 2016-12-05 by Anonymous - Is it ok to set upstairs heat thermostat way down in winter?

I have a two story house with dual units. No one uses the upstairs while my daughter is in college.

I turned the thermostat way down in the winter, and way up in the summer, so that I'm not heating and cooling an entire floor of a house that isn't used.

I was told by a friend that this is really bad on my unit, which is relatively new, is this true?

Do I need to go back to the four degree difference between the two units?

On 2016-12-05 by (mod) - Yes

No you do not. An air conditioning system should not be damaged by leaving it turned off or set to temperatures reducing its run-time.

On 2016-08-24 by Anonymous - Thermostat doesn't read accurately

My AC thermostat does not read correctly when the unit is running.

As an example the unit will come on with the temp reaches 78 but the tempature reading will steadly increase to 80. After about 30 minutes I can bump the setting to 80 to get the unit to shut off and in about 4 or 5 minutes the reading will correct itself, in most cases 76 or 77.

I've changed the batteries and checked all the setting but nothing seems to fix it. The thermostate is a honeywell.

On 2016-08-28 by (mod) -

Anon please search InspectApedia for THERMOSTAT ACCURACY and also for THERMOSTAT CALIBRATION to see details about how to diagnose and fix the problem you describe. Most likely the problem is one of thermostat location or wall drafts or wall temperature, not the thermostat itself. But on occasion, yes we do have to replace a wall thermostat that has a defective sensor.

On 2018-07-05 by Ken

My home HVAC unit has a compressor temperature of 125 to 150 deg is that too hot?

On 2018-07-05 by (mod) - is my A/C compressor too hot?

Probably not, but then you need to explain where and how your measuring the temperature.

 

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 

On 2018-05-31 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. 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 75F at the service valve into the condenser and the liquid line is 90 F at the service port out of the compressor.

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 47.3F, the middle read 64.5F, and the bottom row measured 75.2F; 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.

On 2018-06-01 by (mod) - how much temperature change is across the cooling coil?

Generally you want to see 15deg 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

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.

Carrier AC Temperature Differential of 14.6- OK?

Just bought a newly constructed home. Measured temp differential in a number of rooms after the hvac system had been running for a couple of hours. Best reading we got was a differential of 14.6 degrees. Isn't that kind of ow for a brand new Carrier HVAC system? - On 2017-09-27 by arny -

Reply by (mod) -

Arny

We need to sort out the question of temperature differentials across different rooms in a home. I would not assume (to start) that there is any defect in the Carrier HVAC system itself. If there are very uneven temperatures in rooms all served by the same air conditioning system I would look at

1. the duct work: where are supply and return ducts, how have the ducts been balanced (or not), are there open, squashed, or inadequate ducts

2. the building's variation in heat gain: a room with lots of glass facing south on a sunny day gets more incoming heat

3. the thermostat location: in a zone that is comprised of multiple rooms controlled by one thermostat, that thermostat is really only going to talk to the Carrier HVAC system about what it - the thermostat - senses.

is 180 degF air out at the condenser unit too hot?

The air blowing out of the outside compressor / condenser is 180 degrees is that too high? On 2017-05-11 by Ray Nickel

Reply by (mod)

Ray:

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 2016-12-15 by Ricardo Trevino - heating system fan won't stop

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

On 2017-02-12 by (mod) re:

Ricardo:

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 2016-07-06 by robert

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

On 2016-07-06 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

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.

What's the Normal Temperature variation along the A-Coil (evaporator coil) between inlet end and outlet end

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 whelanjh@gmail.com ) On 2015-09-05 by whelanjh -

Reply by (mod)

Jerry

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.

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

Reply:

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

Reply:

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 ARTICLE INDEX 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

Reply:

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

Reply:

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:

• COOL OFF HEAT, THERMOSTAT SWITCH
• FROST BUILD-UP on AIR CONDITIONER COILS
• FAN, COMPRESSOR / CONDENSER UNIT
• HEAT WON'T TURN OFF - Stop Unwanted Heat

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

Reply:

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

Reply:

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

Reply:

BB

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?

Reply:

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

Reply:

Sibaji

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

Reply:

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 the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR CONDITIONER TEMPERATURE MEASUREMENT
• HVAC TEMPERATURE INSTRUMENTS

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AIR CONDITIONER TEMPERATURE FAQs-2 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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