Low HVACR compressor head pressure diagnosis:
What are the causes of low head pressure at the air conditioner or heat pump compressor? What are the effects of low refrigerant pressure at the compressor or in the system?
Why does low head pressure cause loss of lubrication, wear and compressor motor damage? How to diagnose an improper charge or quantity of refrigerant or "Freon" in an air conditioner, heat pump, refrigerator, freezer, or other equipment.
In this article series we explain how overcharging or undercharging of refrigerant in an air conditioner or heat pump is detected and we list the effects of overcharged or undercharged refrigerant. We also explain the various causes of liquid slugging a compressor motor.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?
Low head pressure may occur simulataneously with high pressure on the suction side of an air conditioner, heat pump or refrigeration system compressor system.
If the system is continuing to cool it may be doing so at reduced capacity, observed as a loss of up to ten degrees of temperature drop across the coil. The two most common causes of low head pressure at an air conditioner or heat pump compressor motor are:
Less common causes of low head pressure
Effects of low head pressure in a refrigeration system incude more than reduced cooling capacity of the system. On commercial systems (and maybe some residential systems) the reduced head pressure can cause refrigerant oil migration or more specifically, the system may be unable to push oil through the cooling coil (the evaporator coil) until it accumulates as a bolus of oil that suddenly returns in one shot tothe compressor.
The effects of poor oil return to the compressor include first reduced cooling capacity and later damage to the compressor motor. If the liquid line is not warm on your system you may face these problems. (A-1 Compressors 2009).
Smaller air conditioners or refrigeration equipment whose design does not include a refrigerant accumulator are at increased risk of damage from overcharge, liquid slugging, or oil circulation problems. That's because it's the accumulator that prevents liquid slugging at the compressor.
Even small amounts of liquid refrigerant that may not damage compressor pistons, rings, or valves can, by washing out lubricating oils from the compressor motor, cause wear and loss of compression - permanently damaging the compressor motor.
If your air conditioning or heat pump system has lost its cooling capacity or won't start
see REPAIR GUIDE for AIR CONDITIONERS.
...
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
Good Low Pressure on Low Side but Very Low Pressure on High Side of my AC unit
Started out with low head pressure and not cooling good so changed out compressor and dryer added oil back to new unit put all together and pulled a vacuum added Freon and unit is cooling some and I have good low pressure but only 3 psi head hi pressure does anyone know my next move? Thanks - On 2018-05-26 by Jack
Reply by (mod) -
I might look for a thermostatic expansion valve that is not closing when it should or is not correctly adjusted. If the thermostatic expansion valve isn't closing it keeps sending so much refrigerant back to the low side that the high side never develops pressure.
If the TEV is, on the other hand, working, then the problem is probably the compressor motor itself.
Review the low head pressure causes above on this page for a more complete diagnosis
Cause of zero gauge head pressure on a heat pump, compressor running
Getting no guage pressure on York heat pump in cooling mode but compressor is running. Is compressor bad? - On 2020-06-11 by Anonymous
Reply by (mod) -
Anon
The compressor is running and producing no pressure it's possible that it's because you've lost the refrigerant charge from the system.
High side pressure is very low on Lennox Compressor - bad TXV?
I have a 3 ton 410a Lennox with correct suction pressure however my high side pressure is way low.
Pressures are 120headover 118 suction low side. What would be the cause. Bad txv or tev - On 2019-05-07 by Cory Calhoun
Reply by (mod) - bad compressor
Cory, thank you for an interesting question. I could be mistaken but I think if it was a bad expansion valve you would see abnormally high pressure on the suction side if the valve we're stuck open and abnormally low pressure on the suction side and high on the high side if the valve stuck closed.
So unless you've simply lost the refrigerant charge (so there's no gas to compress) I'm afraid I suspect the compressor.
Cause of equal refrigerant pressures on High & Low Sides
I just installed a 3 ton mini split unit pressures are equal the liquid line is not warm the compressor sits above the cassette up on the roof my line is 15 foot long I'd appreciate any help thank you - On 2019-07-16 by I just install a 3 ton mini split
Reply by (mod) - Normal operation - system is OFF
Its normal for the refrigerant high side and low side pressures to equalize when the equipment has been off for a time.
What is "normal" head pressure for a Two Ton heat pump?
What is the normal head pressure for a 2 ton heat pump? - On 2019-07-03 by Freeman
Head Pressure isn't a function of heat pump tonnage or size, its' refrigerant and temperature
@Freeman, it’s not about tonnage. On a 85 degree day:
R22 - 200 roughly
R410 - 300 roughly- On 2019-07-04 by Anonymous
What should be the low side refrigerant pressure in a 13 SEER condenser unit?
What should the low side pressure be if the condensing unit is 13 seer and the air handler is 10 seer - On 2016-07-30 by Rich olsen
Reply by (mod) -
Neither the AC unit's SEER rating nor its tonnage or size or capacity determine the low side or suction side refrigerant pressure. The pressure you should see depends principally on the specific refrigerant in use and the ambient temperature.
Take a look at the temperature/pressure chart for your HVACR system for the specific refrigerant that you are using; you'll need to know the ambient temperature as well.
...
Continue reading at REFRIGERANT PROBLEM TYPES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
Or see these
REFRIGERANT LOW HEAD PRESSURE DIAGNOSIS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
Or see this
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.
Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca
In addition to any citations in the article above, a full list is available on request.