Air Conditioning Pressure Switches


It has finally gotten hot and it seems we are fully into air conditioning season (let’s hope it stays fo a while).  With air conditioning season comes a whole other set of diagnostics that we probably all have forgotten over the long winter.  One of those diagnostics is units cycling or locking out on pressure switches.

Pressure switches are safeties installed on units to protect the unit, compressor, and anyone working on the unit.  Pressure switch trip points vary from unit to unit and with the type of refrigerant in the unit.  Usually, there are 2 types of pressure switches  — HIGH PRESSURE switches and LOW PRESSURE switches.  It is important to remember that there are multiple causes for safeties tripping.  A complete diagnostic of the refrigerant side of the system is necessary to determine the cause if the safety tripping.  Pressures are one part of the diagnostic but so is super heat, subcooling, and amp draw of the compressor along with electrical diagnostics.

Let’s look at the diagnostics of the HIGH PRESSURE switch. The high pressure switch is there to keep components from failing and releasing the refrigerant either to the atmosphere if the leak occurs outside, or into the conditioned space if the leak occurs inside.

When a unit cycles or locks out on the high pressure switch, what should you look for?   What causes the high pressure switch to trip?

  • Most common cause is a dirty condenser coil.
    • just because the outside looks clean, don’t forget that some units have splitcondenser coils and the inner coil could be dirty.
      • you need to verify is the unit has a split condenser coil and clean as necessary.
  • The next most common cause is a malfunctioning condenser fan motor.
    • Take an amperage reading on the motor and compare to the motor name plate.
      • High amperage could mean the motor is possibly going bad.
  • Check the motor run capacitor
    • a capacitor out of the microfarad range can cause a motor to draw high amperage or over heat causing it to fail.
  • Of course an over chargeof refrigerant can cause the high pressure switch to open
    • this is especially true with microchannel coils.
  • Non-condensables in the system can cause high head pressure.
    • moisture, air, mixed refrigerants.
  • a heavy load  in the occupied space adds more super heat to the coil and can cause high pressure readings.
  • Unit “short cycling” on a thermostat or because of a “loose connection” in the control circuit.

These are the most common causes of a unit cycling or tripping on the high pressure switch.  Now, lets look at the low pressure switch and what causes that to trip.

When a unit cycles  or locks out on the low pressure switch, what should you look for?

  • the most common cause is low air flowacross the evaporator.
    • look for dirty filters, dirty evaporator coil, dirty blower wheel.
    • Take an external static pressure reading before the coil and after the filter and compare to the unit specification for proper CFM.
    • Check the blower motor amperage and compare to the name plate.
    • Check the motor capacitor, if it has one, and make sure it is within specification.
    • Check the belt, if it has one, and make sure it has proper tension.
  • The unit could be undercharged.
  • There could be a  high side restriction
    • check the filter drierfor a restriction
      • a 2 degree drop across the drier means it is “plugged” and need to be replaced.
  • there could be a metering device, either TXV or orifice, that is under-feeding the coil.
  • A very light load in the occupied space can cause low pressure.
    • An economizer on a commercial unit that is bringing in too much outside air cooling the coil down too much.

Of course, you could have a defective pressure switch in the unit.  This can be determined by comparing where the pressure switch is opening and what the actual pressure is  in the unit.  To that end, I have attached a list of pressure switch open and close points for our various split systems and packaged units.  The information  in the spread sheet is based on current part numbers and models.  It will get you close but, if there is any doubt, you can call with the serial number of the unit and we can get you the set points for that specific year of manufacturer.

Remember, a complete diagnostic of the system should always be performed before replacing any part.  There is nothing worse than replacing a part and still having the unit malfunction.  Hope this helps!

Pressure Switch settings

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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29 Responses to Air Conditioning Pressure Switches

  1. Emir says:

    Great job explaining all possible cuases and on diagnosing such problems.

  2. trachsel says:

    Hey Travis, the pressure switch chart is great. Are you thinking about a Coleman nomenclature chart to go with this? THX! D.

  3. Great blog here! Also your site loads up very fast! What web host are you using?

    Can I get your affiliate link to your host? I wish my
    website loaded up as fast as yours lol

  4. Jennie says:

    Wonderful article! That is the type of information that should be shared around the net.
    Shame on the search engines for not positioning this put up higher!
    Come on over and discuss with my website . Thanks =)

  5. Hi! I am working for different types of air conditioning unit servicing and installations ,I have low and high pressure sensors problem in one of York air conditioner,its sumptoms was ,jurk or vibrate the compressor and leak the refregerant.due to this i bay pass this switches and it works for some times,but now i want to replace both switches because of ice forming in evaporator coil and that means no cooling. The big issue is I can not get such parts her in Ethiopia and currently i am surching some one who can send this part asap.

  6. Jeff says:

    My pressure eventually gets high enough to trip the switch, but only after eradically going up and down. What would cause the pressure to be eradic and not steadily climb until the switch is tripped? Thanks! Jeff

    • could be a malfunctioning TXV. Could be non-condensables. Could be an over-charge. could be the condenser coil/fan is not moving enough air through the coil. All of these need to be checked.

      • Peter kam says:

        What is TXV? My Ac keeps cutting out when I heat with outside temp around 5 centigrade.ls it due to fault dual pressure control? It is intermittent. When I switch off the mains I can hear a pressure release

      • A txv is a metering device (thermostatic expansion valve). If your unit is a heat pump, the pressure release you hear is when the reversing valve moves to the “ready” position when it is shut off.
        As far as shuuing off, it could be as simple as where the “cut out” temperature is set on the control. It could be some other issue. You probably should contact your HVAC contractor to check it out.

  7. Hi!
    I have problem with ROCA YORK heatpump AVO 104BG. It is working in heatpump mode only.
    When i plug mains on, it starts normally, reaches preset temperature and switches out. When temperature drops, its starts again, but outdor ventilator does not work, evaporator freezes and unit switches to defrost cycle ( at this moment LED is showing 24 code) . Than he stops and does not starts again. BUT, indoor thermostat DPC-1 shows that heatpump is still working, and no errors ish shown.
    Control board led is blinking, 2/5 times thats mean a incident code 25.
    I found in service manual, that is PRESSOSTAT HP2, ON. HP2 is HIPress switch (24bar).
    What does it means? The gas is leaking/ pressure is too low?
    With regards,
    Peter.

  8. Thank you!
    I was testing the HP switch and found that it is always open, pressure is within limits.
    Now i must to replace it.
    With regards,
    Peter.

  9. David Campbell says:

    Good morning. I have an engineering question. How much higher, than normal operation of a refrigerating machine should, the high pressure control be? Just wondering. Any help would be nice. Thanks

  10. Younis Bhat says:

    HI!
    I am currently doing the testing and commissioning for Samsung split units. I recently encountered a problem with a 3 ton 3 phase unit not passing the current to indoor unit n consequently the thermostat.Almost everything being OK but not able to determine whether it is pressure switches or the jacks that are malfunctioning. Any help will be appreciated.
    p.s compressor Contactor being OK.
    thanks

    • this is a mini-split unit? are there ANY diagnostic flash codes on the control boards? You could try ohming out the pressure switches or do a voltage check across the switch to seee if it is open or closed. I would suggest first looking at your diagnostic codes first to save a lot of time.

  11. Casey says:

    I have a Goodman 2.5 ton Model No. CHPF2430B6 with a serial No. 1004172732. Does my outside unit or compressor have pressure switch? We removed all refrigerant, installed a new inside evaporator coil, performed nitrogen test (no leaks), but can’t re-load refrigerant beyond outside unit. I would imagine that the low pressure switch disengaged or malfunctioned; if there is one. I’m being told there isn’t by one party, and that there is by another party. Please advise and also email reply to caseycecil@gmail.com

    • You need to check with Goodman as to your specific model to see what safeties your unit has. When you say that you can’t “reload refrigerant” , does the outside unit run at all? Did you evacuate the system below 500 microns?. Did you add refrigerant while the system was still in a vacuum?

  12. Parker says:

    The condenser of our brand new York YJCF48 is cycling VERY quickly, about 10 second off, 1/2 second on, over and over, nonstop. The installers tried to fix it, but suddenly it started working properly. Two days later, same thing, then it was OK again. The next day, it was condenser fan blowing but no compressor, suction & high pressure lines both ambient temperature. That night, back to rapid cycling again. Would a bad HPS or LPS cause this? Any other thoughts? Of course, it “fixes” itself often, so the installers can’t diagnose the problem. Thanks in advance!

    • Your unit has a high pressure switch standard. Low pressure switch would be a field installed option. Your contractor could put a device called a “tattle tale” on the switch. This would tell him if that is the problem.
      While he is working on the unit he could bypass the switch and see what happens but he cannot leave the safety bypassed when he leaves. The tattle tale will tell him if the switch is the problem.
      He should also check the contactor and make sure he has proper voltage from the indoor unit to the outdoor unit.

  13. TexasHeat says:

    I have a York (Model: MC48C3XH1;Serial: WON9423359) and recently when the air conditioner turns on I can hear the unit outside turn on then turn off, then quickly turn back on. It keeps doing this for 3-4 times and then remains on until the thermostat reaches the set temperature and stops. I’ve replaced the capacitor and connector but to no avail. Same symptoms. A previous tech came out and said the electrical readings were normal and refrigerant levels normal. Any thoughts? Have scheduled a different tech to come out but I wanted to get your feedback. I’ve cleaned the outside unit and replaced the air filter.

    • Your tech needs to see if he main. Contactor is opening. If it is, it is either a problem with the thermostat, safeties, or the contactor itself. If it is staying closed, then you might need a start assist kit to get the compressor started. You could have low line voltage or the compressor is drawing high Amps at start up causing an internal overload to trip

  14. Dru says:

    I have a Older York Rooftop Package, runs great up until a week ago the AC stopped. The unit turns on Blower runs,the compressor starts up fan runs, then about 5-7 minutes later the compressor fan shuts off then the compressor shuts off. I try to restart but the compressor starts but not the compressor fan and then about 30 seconds the compressor shuts off again. once unit has cooled down for a while the compressor fan will start with the compressor and start all over again for about 5-7 minutes. Has good pressure on High 180 and Low 70 side. any thoughts ?

  15. Brandon says:

    Have an older York roof top unit that was reading a low pressure lockout when the service man came and put gauges on and said it was a little low so he went ahead and recharged it. The next morning came into work and the unit wasn’t working again so checked it and this time it has a 3 code which is high pressure lockout. Thought it could be the condenser fan motor so replaced with a new I had on hand, but still get the same code. Do you have any suggestions? Could have the service man over charged the unit?

    • Did you change the fan capacitor? Is the condenser coil clean? Depending on the age and size, some units had “split” condenser coils with a “front and rear” coil arrangement. Outer coil could be clean but the inner coil is dirty. Adding refrigerant with dirty coils could cause the pressure switch to trip. It could be an overcharge. Flash code are there for assisting in the diagnostic process but the cause still need to be determined by doing a thorough checking of the possible causes.

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    good Day? Please subscribe me

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