HVACR Refrigerant Pressure Reading & Chart FAQs

Q&A on refrigerant pressures & quantities

A/C / Heat Pump Refrigerant Low Side Presssure FAQs

What does it mean if there is no pressure on the low side?

With unit off there should be pressure on both high and low. But what if no pressure n low side On 2016-07-06 by robert

Reply 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 and the pressure you'd read on either test port would be given in a refrigerant pressure chart for the specific kind of refrigerant in your system for the particluar ambient temperature around the compressor/condenser unit.

If there is no pressure at all measured in the system then I suspect

- you've lost all of the refrigerant through a leak

- there's a stuck or clogged refrigerant metering device, control, or valve AND a refrigerant leak

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.

Watch out: 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.

Causes of Too-High pressure on the suction side

The house I am working on has two identical units.

The table of refrigerant pressures on the unit shows high side of 300 and low side of 70 for the low humidity high 105 degree temperature that we currently have.

One unit shows that and works well.

The other has a high side of 270 and a low side of 90, but is working as well.

My conclusion is that either the compressor is losing efficiency, or the expansion valve is worn. Could anything else cause this? 2016-06-05 by Richard Ohran

by (mod) -

Yes, Richard, a thermostatic expansion valve adjustment might be feeding refrigerant faster over to the low side on the lower HI & Higher LO pressure unit.

Tech says low side pressure is too high (at 102 psi)

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 2011-08-06 by Dean

Reply 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.

How does the refrigerant system stay at 70 psi on the low side?

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. (July 8, 2016) Matthew

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 70 psi 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.

Refrigerant Loss / Leak Indicator FAQs

What is considered low pressure requiring a recharge.

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

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.

What are the right R22 Pressures at 100°F outside

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 2011-07-31 by Edgardo

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

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.

See R22 REFRIGERANT PRESSURE / TEMPERATURE CHARTS 

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!

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.

Coleman EB12 heat pump not cooling. Normal vs low pressures mean my system is leaking?

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

Normal pressure: about 60 Low & 260 High.

I 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

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. On 2011-08-10 by Joe

Reply 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.

Both High and Low Sides read 120psi and I'm not getting cool air

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 2011-07-11 by Kevin

Reply 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 make those measurements.

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.

If the compressor is running and there's no pressure difference the TEV may be stuck wide open or the compressor may have failed OR there may be almost no refrigerant in the system.

Automotive Air Conditioning Pressure FAQs

Car A/C compressor stops during vehicle acceleration

when my car accelarates the gas the cluctch of the compressor stops and no cooling at all? On 2012-06-19 by Anonymous

Reply 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.

Comment 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.

 

After fixing a refrigeraent leak the system won't take more refrigerant, pressure not changing

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 2011-05-21 by mar

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.

High Side HVAC Pressure FAQs

High Side Pressure is Much Less than Low Side Pressure

Have a 2 ton R22 compressor with low pressure at 120.

High side is at 80.

What might be the problem? On 2012-04-13 by Glenn

Reply by (mod) - high pressure on the suction side of an AC system? Is this a heat pump in heat mode?

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

OR

Your system is a heat pump and it has switched to HEAT mode.

Why is the high side refrigerant line on my heat pump cold?

Why is my hi side line on the heat pump is getting cold On 2011-08-31 by Troy

REply by (mod) - is your reversing valve set to heating mode?

Troy: is your reversing valve set to heating mode?

If so that's normal. The role of the refrigerant lines swaps and what was your "high side" and "warm line" in cooling mode becomes the "low side" and "cold line" in heating mode.

Meaning of High side at 100 psi low side at 35 psi

What does it mean when the high side is at 100psi and the low side is at normal 35psi On 2011-09-01 by Anonymous

Reply 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.

 

Can Rerigerant Pressure = Ambient temperature + 15 can be used in finding the right pressure?

It is true that Pressure = Ambient temperature + 15 can be use in finding pressure. - 2013-02-2 by Mike Hanabol

Reply: No

No the refrigerant pressure you'll find at an ambient temperature varies depending on which specific refrigerant is being considered.

330 psi on the high side drops to 225 High Side, 25 psi Low Side R410A

I 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 2012-05-26 by rick

Reply by (mod)

You'll see that though these pressures are possible, you must interpret them by knowing the ambient temperature when you're making the measurement.

See R410A REFRIGERANT PRESSURE / TEMPERATURE CHARTS 

High pressure line is cool - is that bad?

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 2011-07-25 by Jason

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.

Other HVACR Refrigerant Pressure FAQs

What causes pressure to be the same on both the high and low side?

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 either On 2011-07-28 by kevin

Reply by (mod) - system has been shut down

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.