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How to diagnose high head pressures at the air conditioning or heat pump compressor:
What are the causes of high head pressure at an air conditioner or heat pump compressor motor? This article lists twelve causes of high head pressure and explains their causes, effects, and diagnosis. Some of these, such as a bad TXV or clogs in the refrigeration system are cited as the most common causes of high compressor head pressure.
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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
A/C or Heat Pump Compressor High Head Pressure Diagnostic Checklist
12 Causes of High Head Pressure at an Air Conditioner, Heat Pump or other Refrigeration System
Below we list a dozen causes of high head pressure in an air conditioner or heat pump compressor motor. Of these, debris clogging or malfunctioning TXVs seem to be the most common problems.
Air contamination in the refrigerant system
Air flow blocked across the condensing coil. Low condenser airflow will show up as high head pressure, normal suction line pressure, normal superheat, and normal to high sub-cooling (See SUPERHEAT DEFINITION)
Air flow across the cooling coil: blocked air flow in the air handler, causing the TXV and its temp sensor to remain closed
Check valve inoperative on the refrigerant line at the condenser (look for "fluttering" pressure on the suction side).
A bad check valve on an air conditioner, heat pump or similar HVACR equipment can show up as abnormally low or abnormally high suction line pressures, or as abnormally low or high superheat. Experts suggest using an IR heat gun or similar tempeature measuring tool to check the tempeature of the refrigerant line on each side of the valve.
The readings can tell you that a check valve is stuck open or stuck closed.
Clogged filter drier. Check suction line temperatures across the filter/drier at the start of an on-cycle. If the filter is not clogged the temperatures will be the same.
See REFRIGERANT DRIERS & FILTERS
Contaminants on the compressor valves can cause both low suction pressure and high head pressure.
Crimp or block in the refrigerant piping on the high side or low side. (Symptoms similar to an improperl-set or debris-clogged TXV that is not passing enough refrigerant to the low side. In turn, dirt or debris in the system can clog the TXV (aka TEV) or cap tube.
Debris clogging or crimped refrigerant line at the condensing coil - Tim this would support your HVAC tech's insistence on replacing the condensing coil
High outdoor ambient temperature
Overcharging of refrigerant, liquid slugging of the compressor, oil contamination and other sources can damage compressor valves. Refrigerant overcharging will show up as high head pressure, normal suction line pressure, normal superheat, high sub-cooling.
TXV (or in some texts TEV) - Thermostatic Expansion Valve - improperly set, iced, contaminated, or clogged or crimpled capillary tube, or having lost power to a TXV power head can cause high pressure and can also cause valve damage by flooding the compressor if instead the valve sticks wide open.
Water (moisture) contamination in the refrigerant system. Air or water contamination in an HVACR system are referred to as "non-condensibles"
Note: air or water contamination in HVACR systems is common and can be introduced by a tech who does not purge air from gauge hoses before attaching the refrigerant gauge or by failure to evacuate the system when refrigerant has been lost and the system is to be repaired and re-charged. Installation of a filter/drier should also always be included in such repairs. Experts note that the symptoms of air contamination and refrigerant overcharge can be similar. ACHR NEWS calls these "non-condensibles" in the refrigeration system.
HVACR Compressore High Head Pressure Diagnosis References
"Btu Buddy 40: High Head Pressure, Air in System", Air Conditioning, Heating and Refrigeration NEWS Magazine, July 2006, 2401 W. Big Beaver Rd., Suite 700 Troy, MI 48084
Tel: 248-362-3700 Website: http://www.achrnews.com/
"High Head Pressure", HVACSite, orignal source: www.hvacsite.com/f2/high-head-pressure-800/
"How to minimise head
pressure in refrigeration", Carbon Trust, Website www.carbontrust.co.uk/
refrigeration, retrieved 7 August 2015, original source: https://www.carbontrust.com/media/147197/j7980_ctl136_minimise_pressure_aw.pdf
John Tomczyk, "The Professor: Air Conditioning System Troubleshooting", August 2008, op.cit. - original source: www.achrnews.com/articles/107725-the-professor-air-conditioning-system-troubleshooting
Reader Question: My service tech replaced the condensing coil as a cure for high head pressure. Does this sound right?
6 Aug 2015 Tim nordyke said:
I had a lennox system that had a very weird set of symptoms. It was an r22 3ton that was around 20 yrs old.
In the course of 10 days the system kept popping the high head pressure switch (8 times total) yet the temp difference between intake and output (inside) was only 6 degrees.
The AC tech from the home warranty co. blamed it all on a 20% damage area to the condenser from corrosion, and his test set was a clamp on amp meter.
He never once connected gauges to check high/low pressures.
10 days after first incident, the compressor locked up and gave up the ghost.
When the new system was being brought in, the installer noted the pressure in the system (both sides) was only at 100# and the ambient temp was 95 degrees (per the gauges), the installers claim that the old system did not have enough refrigerant, and that the TEV was clogged/jammed.
Does this sound right?? Too little freon, clogged TEV, 6 degree drop from intake to output, and still pop high head pressure?
Maybe, Tim. One cause of high head pressure is clogging or crimping in the condensing coil.
Causes of high head pressure in an air conditioner or heat pump system
I guess the tech is seeing sometihng we don't or she was not an English major and is not a great communicator who didn't explain in more detail how the problem was traced to the coil. .
Perhaps the thought was that internal corrosion was causing the condensing coil, check valve, or a refrigerant metering device to clog, risking blowing a compressor?
I'm suspecting a clogging TEV or a clogging valve on the compressor itself.
danjoefriedman... Well that's why I'm coming here.
The clueless tech only used an ammeter to check current draw, said that 6 degree drop from intake to out wasn't the best, but was not a solid indicator of a problem.
I learned last night that the system had to be recharged 7 years ago and all they did was add freon.
So I'm thinking that the prior problem contaminated the system, the freon was leaking out, the TEV was clogged so it couldn't shut off flow to the evap coil, and since the return line wasn't cold enough to cool the compressor, it overheated and locked up.
Had tech 1 hooked up his gauges when the system was still running he would have caught the pressure issue and saved my system.
Maybe so Tim. I like to assume just one snafu at a time:
Certainly if they added freon and didn't fix the leak you'd be low on freon again.
Low freon can ice the coil reducing or stopping actual cooling and it might ice the TEV causing it to block all refrigerant flow.
That MIGHT give a high head pressure.
The problem would continue until so much refrigerant has been lost that the TEV no longer ices (and the coil no longer cools).
What does your service manager say about the repair?
Tim Nordyke said:
The tech offers nothing......
He is sticking to his claim it is all the condensers fault.
When the system ran just before it gave up the ghost, there was no ice or even condensation on or around the TXV (found correct name).
What has me wondering is how did I get a high head pressure (faulty switch?) when the pressures were at 100 lbs. just before they evacuated the system, and the amb. temp. was 95 degrees.
If there was debris or corrosion-produced crud in the system it may have been stirred or moved by the refrigerant evacuation and re-charge system, then causing a clog at a check valve, condensing coil, TXV or filter/drier. If the tech was convinced that the blockage was at the condensing coil it would be reasonable to replace it.
Question: short cycling A/C heat pump at 10 second interval
(Jan 30, 2015) ed said:
have a 4yr old american standerd a/c heat pump unit.cycles every 10sec.tech serviced 12/06/2014 had to shut unit down01/20/2015.live on a island only way to get here in winter is by plane. expensive! put gauge on high side,unit off temp out side 20deg f read50#psi
Ed you are right to shut off the system; it may be that at low outside temp the unit wants to switch to backup heat anyway.
Short cycling could be a shot or frozen compressor, a burned out compressor base heater, a failed start/run capacitor, a failed relay or control board, a frozen expansion valve that's causing a high pressure safety switch to shut off the system, or something else. With just your note I'm doubtful one could actually diagnose the system.
Is your backup heat running?
Start by checking the control relays and check for a clogged or frozen TEV
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"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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