FAQs about refrigerant pressures & charge quantity for air conditioners & heat pumps.

This air conditioning repair article series discusses the the diagnosis and correction of abnormal air conditioner refrigerant line pressures as a means for evaluating the condition of the air conditioner compressor motor, which in turn, is a step in how we evaluate and correct lost or reduced air conditioner cooling capacity.

Q&A on Air Conditioner or Heat Pump System Pressures

These questions & answers about refrigerant pressures & pressure charts or pressure measurements were posted originally at REFRIGERANT PRESSURE READINGS & CHARTS - home. Be sure to review that article.

Measuring the refrigerant pressure in air conditioning, heat pump or other refrigerant systems can diagnose a range of operating problems including a refrigerant leak, over charging or under charging.

Question: how does the system stay at 70 psi

(July 8, 2016) Matthew said:

I worked on buses using on 22 Freon, the low side pressure should always be around 70 psi.

My question is, how does the system stay at 70 psi, is it the compressor, expansion valve,evaporator coil, all three, ?

The articulated buses were 16 ton systems, using 35 pounds of Freon, six-cylinder Thermo king compressors, two evaporator setups with epr valves. The 40 foot buses used four cylinder compressors, thermo-king or trane, 10 ton systems, one evaporator, no epr valve.

All buses work at 70 psi low side. How do they make it happen, thanks.

Reply:

Matthew:

The low side pressure when the compressor is running is a feature of the compressor's own pumping power combined with the properties of whatever device is being used to meter refrigerant into the system. That creates the pressure drop between high and low sides.

The actual pressure when the system is running depends on

- the specific refrigerant being used (R22 in your case)

- the adjustment of the refrigerant metering device

- a proper refrigerant charge quantity

- and the ambient tempeature.

So 70psi is not an absolute number. The charging tables given for any refrigerant gas will give you a map of the pressure changes of the refrigerant gas as temperature varies.

Question: What is considered low pressure requiring a recharge.

Aug 3, 2016) Ernest said:
What is the normal Freon pressure in a home air conditioning system. What is considered low pressure requiring a recharge.

Reply:

The requirement to re-charge an air conditioner or heat pump system might be suggested by cooling coil icing or by complete loss of cooling, or by very low suction pressures, or very low head pressures, but short of those extremes, pressure readings won't tell you if the quantity of refrigerant that's in the system.

On 2013-02-2 by Mike Hanabol

It is true that Pressure = Ambient temperature + 15 can be use in finding pressure.

On 2013-02-23 by Anonymous

What is the standard pressure in charging refrigerant at low side and high side?

On 2013-01-10 by khel

low side 75 psi and high side 250 psi

On 2012-09-21 by peter

if the out side temprature is 25centigrade what shoul be the reading of the low pressure

On 2012-08-24 by Shane

I replaced a compressor vacuumed it charged it when to check pressures and low side went right to a vacuum what the hell happened

On 2012-07-08 by ZiggyMe

Most cars have a "wide open throttle" shutdown control so the compressor does not affect your acceleration.

When you press hard on pedal, the ECU in the car tells the clutch of the compressor to release until you back off the pedal or the car does not require the power need to move faster. It's all up to the manufacturer to control that.

On 2012-06-19 by (mod) - car AC compressor stops when car accelerates

Anon, the first thing to check is for a loose fan belt. Sometimes a combination of a failing compressor or compressor clutch (that is providing resistance) and a loose fan belt conspire together, but the fan belt is the first culprit I'd suspect.

On 2012-06-19 by Anonymous

when my car accelarates the gas the cluctch of the compressor stops and no cooling at all?

On 2012-05-26 by rick

have 410a heat pump the compressor trips the internal replaced compressor still same starts with good pressures 330 high 125 low then slowly drops to 225 high 25 low?

On 2012-04-28 by (mod) - high pressure on the suction side of an AC system

Not sure, but that hi pressure on the suction side sounds as if perhaps a refrigerant metering valve is stuck wide open.

On 2012-04-13 by Glenn

Have a 2 ton R22 compressor with low pressure at 120. High side is at 80. What might be the problem?

On 2012-01-12 by lopez

what happen if aircomprsor is runing witout refrigerant for half our?

On 2012-01-02 by george

Hi have a goodman h/p high side im getting 0 psi what do think it is.

On 2011-11-11 by Mamakgeme Comfort Mphahlele

What causes the residential aircon unit(split) to freeze coil(Evaporator),Low side and high side simultationusly.

On 2011-10-14 by (mod) -

Ethan, how are you measuring pressure, and under what conditions, and at which port? Are you controlling for temperature? Are you charging from the low side - gas, or high side - liquid?

On 2011-10-13 by ethan

Hi i had a problems with charging the refrigerant onto the Compressor which part of our 37 Kw HVAC system.I use Dupont R417 A,

when the pressure reach 60 PSI it rapidly increase to 90 PSI even with little charging, and when it come to decrease the pressure ( i want the pressure in range 60 - 70 PSI ) it rapidly drop when it reach 80 PSI. The Refrigerant drop until 40 PSI.

Any idea or solutions ???

On 2011-09-20 by rotsen

steps in charging freon 22 in power pack 5 compressor

On 2011-09-20 by bryan

sir,iam used freon 22 my compressor is 5 unit power pack if i started 1 comprssor the head pressure is normal but when i started second compressor the head pressure is 450psi and the controll injection cooling will open

when i stared the the compressor the head pressur is too high

On 2011-09-01 by (mod) - pressures in an HVACR system have meaning only if

Anon pressures in an HVACR system have meaning only if you know when they were measured with respect to the operating state of the system.

High side pressure when the compressor has reached stable operating condition reflects the output capability of the machine and if low, can indicate a problem within the compressor unit.

Low side pressure is typically the vacuum side or "suction" side of the compressor, but low side pressure begins to climb when the compressor motor stops running.

But because on many systems high side and low side pressures equalize when the equipment has been turned off, and because there is a transition in pressures between those two states (on and off, equalized and not equalize), I'm not sure when your measurements were made and thus what they mean.

If both high and low pressures are lower than normal in your machine - that is, if the measurements of 100 psi high and 35 psi were taken with the compressor running, then the system pressures are almost certainly low for most refrigerants (you didn't indicate what refrigerant gas is in use).

Your tech will perhaps check for a refrigerant leak in the system as well as checking and correcting the refrigerant charge.

On 2011-09-01 by Anonymous

what does it mean when the high side is at 100psi and the low side is at normal 35psi

On 2011-09-01 by (mod) - is your reversing valve set to heating mode?

Troy: is your reversing valve set to heating mode?

On 2011-08-31 by Troy

Why is my hi side line on the heat pump is getting cold

On 2011-08-12 by Joe

I'll be checking it out this weekend (I hope) & will post my findings.

On 2011-08-11 by (mod) - 60 psi low side pressure and 260 high side pressure

Joe, that 60 psi low side pressure and 260 high side pressure sound typical to me too, and just seeing 80 psi on the high side could be a problem ... or not. It depends on when and how measurements are made. High side pressure measurements make sense only if made when the compressor has been running for a while and is up to full operating outlet pressure.

I mean to say that when an A/C system or heat pump has been "OFF" for a while, refrigerant pressures (in most designs) will equalize between low and high sides, and will stabilize at the normal evaporation pressure for the particular refrigerant used at the particular ambient temperature around the system.

That could certainly be 80 psi, depending on refrigerant and temp.

As you are familiar with auto A/C systems you probably knew all that - I elaborated for other readers too.

It's difficult to know if you have exactly the right total charge amount in a small residential A/C system without evacuating the whole system and then putting in a measured charge.

But if you're pretty confident that you have close to an adequate charge (certainly 60 oz is a pretty big charge for a small heat pump unit, depending on the additional factor of refrigerant piping distances and diameters ), then I suspect either a bad TEV (Thermostatic expansion valve) or a compressor that is unable to pump to pressure.

On a small heat pump you may have just a cap tube instead of a TEV. If that's the case, the cap tube has no way of failing in the direction of passing too much refrigerant too quickly to the low side. But a TEV can stick open and cause that problem. Let us know what you see.

On 2011-08-10 by Joe

I forgot to mention that I just did that (added the freon) & the temp outside was 78 & it was running from this afternoon till I started about 9:30ish.

I have a Coleman EB12 heat pump & noticed it wasnt cooling as much like it did last year in the mid afternoons, though 60deg out of the vents at 90deg outside so I asked an a/c tech what the pressures should be & he said about 60L & 260H. Got a reading of 30L & 80ish so I added 4-15oz cans & only brought it to 40 & 90 respectfully

. Can anyone tell me anything I should look for?

I don't have the manuals on it but I run an auto repair shop & do r-12 & 134a all day long.....Thanks in advance, Joe.....

On 2011-08-06 by Dean

Thanks I'll look into that.

--Dean

On 2011-08-06 by (mod) - high pressure on the suction side (over 100 psi) sounds wrong

Dean I agree that high pressure on the suction side (over 100 psi) sounds wrong on almost any air conditioner or heat pump. Also the temperature drop across your cooling coil was a bit low. We expect to see a temp drop of 15-20 deg.F in that location.

In addition to an overcharge, a refrigerant metering device that is not working properly (Thermostatic expansion valve or cap tube) could be at fault. Or something else we're not thinking-of.

On 2011-08-06 by Dean

We came back from vacation and our A/C wasn't cooling anymore. We had a repairman take a look, and he said the pressure on the high side is ok (a little high @ 225).

But he said the low side suction was high, 102 when it should be in the 70s. He also said there was an 11 deg temperature drop over the coils.

He suggested our problem might be caused by overcharging the system when we had it tuned up last. But we haven't had it charged in two years, and the problem just came up.

He was baffled. The unit is about 10 years old. Any ideas or suggestions?

On 2011-08-04 3 by Edgardo

I'm curious how much R22 freon by weight does it take to raise the low side pressure 10 psi on a typical 2 to 4 ton AC system that's running OK but a little low?

On 2011-08-01 by (mod) - how much refrigerant should be in the system?

Edgardo I agree completely with the object of having enough refrigerant in the system to get the liquid to extend to just about the end of the cooling coil. But if refrigerant is NOT extending to the end of the cooling coil we don't, without some diagnosis, know if the problem is with a bad metering valve (TEV) or a short charge.

I'm unclear on how monitoring amps would tell us that the compressor was seeing liquid refrigerant (and being damaged) before there was any actual damage. I'll look into that assertion further to see what we can learn.

In my view high amps means that the electric motor in the compressor is drawing a lot of current, which I'd expect to mean that the motor or the compressor pump were failing.

We discuss this in the article titled TIGHT or SEIZED AC COMPRESSORS found at the ARTICLE INDEX near the end of this page under COMPRESSOR & CONDENSING COIL, A/C.

Also see BURNED-OUT COMPRESSOR

But liquid passing into an A/C compressor has the effect of destroying the refrigerant valves (typically reed valves) or in a more extreme case simply destroying the piston assembly just as a gasoline engine in a car will be destroyed if water enters the engine while it's running.

Finally, it sounds as if you may have corrected a short charge in your A/C system, but unless the system was undercharged in the first place, there must also be a refrigerant leak to be found and fixed.

On 2011-08-01 by Edgardo

Looking at the above info and the pressure-temperature chart I checked the pressures of my system and at 95-100°F outdoor ambient temp low side was about 62, high about 230 psi. So I added enough freon to bring up low side to app'x 72 psi.

Hi side went up to about 250. Indoor air outlets temperatures became more evenly distributed. I've always suspected the evap coil wasn't getting full because the air register that comes off the top of the air plenum (AC system lays sideways in the attic) was always about 10° warmer than others in the house.

Now that the low side is up where it should be the air coming off different areas of the coil seems more equally cooled. Hope that makes sense. The AC guy that installed this system battled with this problem but never really solved it for me. This person I knew from work, he was the contractor that kept my employer's AC running. He mentored me enough to know if it's working properly or not, that's about it.

From above "Some sources also cite higher compressor amps draw but that claim needs more research" He always used an amp-clamp meter to check current while he was adding freon. I bought one too.

As I recall he said it would tell him if he was getting too much freon in the system before any compressor damage, current would start going real high. I also called a friend who's in the AC business and he confirmed my 70-72 psi reading. I think it's doing pretty well today. Thanks for the info!

On 2011-08-01 by (mod) - you cannot know that refrigerant in a system is low based simply on a pressure reading

Edgardo, you cannot know that refrigerant in a system is low based simply on a pressure reading - as long as there is any liquid refrigerant in the system the pressures tend to be the same.

It does make sense to look at the pressure differences between the high and low sides during system operation for abnormal pressure levels. There are quite a few problem cases that an expert HVAC tech should recognize, most of which are listed in the article above. For example very high head pressure on the high side can indicate a blocked coil or dryer or TEV.

Low refrigerant but not total loss of refrigerant can on some systems actually produce an abnormally cold cooling coil and even coil icing.

You can indeed look at the air temp drop across the cooling coil - I'd expect to see between 15 and 20 degF drop.

On 2011-07-31 by Edgardo

Hello what pressures should I see with my system, R22 4 Ton Goodman unit installed in '97. Take into account Texas at 100°F outside ambient temp. I think it may be a bit low on refrigerant judging from the indoor air temps only drop about 15° in to out, it used to do better than that. Thanks!

On 2011-07-28 by (mod) - same pressure on the high and low side

Kevin, it can be perfectly normal for the AC or heat pump system to have the same refrigerant pressure on both HIGH and LOW sides - if the system has been shut off long enough for the refrigerant pressures to equalize through the system.

Then both sides of the system will read a pressure that you should find is very close to a table of static pressures for your particular refrigerant for the particular ambient temperature around the system at the time of the pressure reading.

When the system is running, of course, you'll see the HIGH side pressures go way up and the suction side pressures drop. If they do not split like that then there's a problem, perhaps with the thermostatic expansion valve or worse, with the compressor itself, or sometimes with a blockage in the system.

On 2011-07-28 by kevin

i was dignosing my ac and it has the same pressure on the high and low side 145 psi on low and 150 psi on hi what dose that mean and what else can i do to dignose this problem in the house it feels like just the fan is blowing not hot but not coled eather

On 2011-07-25 by (mod) - high pressure line may not be indicating a problem

Jason if the system is working properly the cooler high pressure line may not be indicating a problem; it depends in part on where you are sensing its temperature.

The line would certainly be hot coming out of the compressor and entering the condensing coil, but the same line close to the air handler/evaporator coil may be much cooler, depending on the length of run and other conditions.

On 2011-07-25 by Jason

My a/c seems to be cooling fine, but my high pressure line seems to cool, it supposed to be 20-30' hotter then the outside temp.

What could be wrong ?

On 2011-07-18 by (mod) - condenser fan is not cooling the system

Mike: I wonder if your condenser fan is not cooling the system, or if the compressor motor is otherwise overheating and going off on thermal reset. Try cooling the outside compressor/condenser with a garden hose and see what happens.

On 2011-07-14 by Mike

AC works fine initially putting out cold air, but compressor stops after about 4-5 minutes.

Any ideas would be appreciated - Thanks!

On 2011-07-11 by (mod) - good air flow but no cool air

Kevin: good air flow but no cool air typically means lost refrigerant or the compressor unit is not running;

check first to see if the compressor/condenser is running. When you see the SAME pressure on both low and high sides, the system is equalized - a condition that one finds only when the motor has been off for a while.

If the compressor won't start and run, that's the basic problem; check for power, check the contactor, and of course check the thermostat setting.

On 2011-07-11 by Kevin

My unit has stopped cooling. I am getting good air flow through the vents, but not cold air. I put a gauge on the unit and my low and high side both read around 120 psi. The outside unit comes on without delay or noise.

On 2011-07-06 by (mod) - hard-starting A/C compressor

Bob: I'm not sure but it sounds as if you may have a hard-starting A/C compressor.

A hard start kit (capacitor) might help if this is the problem. A hard starting compressor has an even tougher time re-starting against head pressure. That's why the compressor can re-start only after you've waited a while. Or the compressor is overheating.

On 2011-07-03 by Bob

I installed used trane high eff propane heater and ac unit in garage shop ac unit starts to cool runs 10 to 20 minutes and compressor shuts off air handler stays running stops cooling compressor will not start if I wait for hours with unit shut off it will start again only for same time period and comp shuts off again any suggestions?

On 2011-07-03 by Anonymous

why my aircondition is blows warm air.

On 2011-07-03 by Anonymous

is bad when the pressure is high or is it low.

tell me what and how do you determent good or bad compressor. is it bad when the pressure is or low pressure.

On 2011-06-23 by Laura

My residential ac unit was low on coolant and was icing up on the A coil. We removed the filter and it quit icing. This was a short term fix to stay cool. Then it was sweating purfusly and leaking water in the floor from under the wall behind the unit.

Husband checked presure and it was low on coolant. He added coolant and the gauges went haywire they are up and down up and down. He thought he had it at the correct pressure and the unit was squealing.

He went back and checked pressure and it had gone way up. So he let some out but squealing continued and gauges were still going up and down some but not as bad. System is still continously squealing every few minutes and is not blowing as cold in the house as it should be.

He cleaned the coils and ran a long wire brush down the drain hole to help with the water leaking. But its just not cooling well. Any ideas?

On 2011-06-05 by JOE

Have an 35yr old GE 4 ton unit still working, when checking the superheat range what should it be between, 10-15 degrees? and what does below 10 degrees mean or above 15 degrees mean in the way of charge, too much or too little?

.My low psi side reads 72 at 88 degrees amb..

I did a sub-cooling check at 88 degrees amb. and came up with 11.5 degrees..the correct range for sub cooling 10-12 degrees, I believe is correct or what ?? Thank you...

On 2011-05-21 by (mod) - If the A/C system includes a receiver then your total refrigerant charge can be imprecise

If the A/C system includes a receiver then your total refrigerant charge can be imprecise and the system will still work normally as long as the refrigerant metering device or TEV (thermal expansion valve or capillary tube) is working properly. If the problem is the metering device adding refrigerant won't change its behavior.

If there is no receiver then when I worked on such systems we had to empty the refrigerant, pull a vacuum on the system to clear it out, then put in the precisely correct measured refrigerant charge.

Of course a failing compressor internal valve can also affect low or high side pressures too.

On 2011-05-21 by mar

I am working on an air conditioner, fixed the leak, charged system, low side pressure at 62 psi, high side at 150 psi, ambient temperature at 74f,I add more refrigerant, low side at 72 psi and high side a 170 psi, system won't take more refrigerant,if I add more low pressure stays the same, help

On 2011-04-25 by Vidyadhara Seethur

Thanks.

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 REFRIGERANT PRESSURE DIAGNOSIS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see REFRIGERANT PRESSURE READING & CHART FAQs-2 - more-recent Q&A about refrigeration gas pressures in HVACR equipment

Or see these

Recommended Articles

• REFRIGERANT GASES & PIPING - home
  • CAR / TRUCK AC PRESSURE SWITCH
  • FROST BUILD-UP on AIR CONDITIONER COILS
  • GAUGE, REFRIGERATION PRESSURE TEST
  • REFRIGERANT CHARGING PROCEDURE
    • OVER CHARGED of REFRIGERANT, EFFECTS
    • UNDER CHARGED REFRIGERANT, EFFECTS
  • REFRIGERANT DRIERS & FILTERS
  • REFRIGERANT PRESSURE DIAGNOSTICS
  • REFRIGERANT CHARGE QUANTITY
  • REFRIGERANT QUANTITY on DATA TAG
  • REFRIGERANT HIGH HEAD PRESSURE DIAGNOSIS
  • REFRIGERANT LOW HEAD PRESSURE DIAGNOSIS
  • REFRIGERANT PRESSURE READINGS & CHARTS

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