Help Low Pressure Lockout

Discussion in 'Maintenance and Troubleshooting' started by ChrisF, Jan 7, 2012.

  1. ChrisF

    ChrisF New Member

    Here are the specs.
    FHP ES036 3 Ton
    4 -750' slinky loops
    4 year old system.
    2 Grundfos 3 speed pumps running at high speed
    EWT is about 38F ( this is from a temp gauge at the flow center and the only gauge on the system).
    Coils are clean filter changed monthly.
    DIY system.
    Here’s the story:
    I am located in White Salmon WA. temps have been averaging 30-40.
    The compressor locked out last week so I checked error code it was a Low Pressure Lockout. I checked the loop flow center where I have a pressure gauge and the pressure was about 2 lbs, really low. I have my water plumbed in so I added water and brought the pressure back up to 35lbs.
    We have never had a problem with the system until last October when we had a leak in one of the loops. I fixed it added antifreeze to the system and all has been fine until this lockout

    Last week I called a HVAC tech, not a geothermal guy but a good tech, to come out and check the refrigerant. The system had not been running for 2 days at this point and his pressures were 65/247 when he ran it, a little low but not to bad according to the manual. It ran for fifteen minutes then locked out again. He decided to add refrigerant but it ended up being a small amount. So it appeared not to be a refrigerant issue. After adding the refrigerant the system did the same lockout cycle it had been doing which is, run for about 1-2 minutes then lockout wait 5 min( probably short cycle timer) then run for 1-2 minutes and lockout.

    The Tech left to contact a “Geothermal Guru”. After four days he returned we hadn't run the system in that time. It started up and ran just fine for about an hour cycling normally but then it locked out again. It now is doing the same thing locking out after about 1-2 minutes.

    The “Guru” said it is almost always water flow issues and the refrigerant pressures he recorded were normal.

    So where do I go from here? I keep checking water pressures on the loop and they have been maintaining all week. I don't know why I had low pressure last week, I have an air bleed off valve in the flow center so I don't think it was air in the lines from the break in October.

    It appears that if the system is left for days then restarted it runs longer before lockout Why?

    Is there a low cost flow meter I can add to the system to verify flow?

    Any help is greatly appreciated.
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Low pressure

    As you've noted, low pressure lockout is a refrigeration pressure - ie, not related to water pressure except for a possible cause of the issue.

    But, from what you've described, I would look at the flow. Sounds like your loop fluid temperatures are dropping quickly (but they haven't been measured).

    You may have poor fluid flow and/or low temperatures in the loop fluid. Check pressure drops to determine flow. Monitor loop temperatures. Ensure air is out of the circulating pumps. And make sure you have adequate antifreeze protection.
  3. ChrisF

    ChrisF New Member

    I ran the system in cool mode and dropped the house temp 2 degrees and the compressor never locked out. I then put the system back into heat mode and ran the temp back up 3 degrees the compressor did not lock out in the 30 minutes it took to get there.
    I am thinking low antifreeze concentrations because this is the only thing that has changed in the system in the last 4 years.
    Any ideas?
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    As above

    This also points to low loop temperatures and inadequate antifreeze.

    Easiest thing to do is start measuring.
  5. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I hit the wrong button

    and lost a half hour of thinking and typing.

    You need the tools to measure if you are going to be hands on DIY.

  6. Howard Ek

    Howard Ek Member

    Squirt some water-antifreeze solution of the loop, and put it in the freezer to tell the freeze protection is doing.
  7. ChrisF

    ChrisF New Member

    Monday I brought the loop antifreeze up to a 20% concentration. System ran for 10 hours like normal then started tripping on low pressure again.
    Thursday I finally got PT ports in the system.
    EWT was 41 LWT was 38
    After 20 minutes
    EWT 38 LWT 34
    4 pound drop in pressure .
    16 degree rise in air temp.
    All the above in spec according to the manual.
    I had to jumper out the low pressure switch to get these readings because without it would trip on LP after 2 minutes.
    Now here s the weird thing after I pulled the jumper the system ran great all night.
    I had a Tech show up this morning to check the refrigerant his readings were 50psi suction and 300psi discharge. Manual gave a range of 90-111psi suction and 263-322 discharge.
    So the suction pressure is low and must drop low enough intermittently to trip the low pressure switch ( too bad it didn't happen when the tech was there to see if the switch is calibrated correctly).
    So I have what appears to be random changes on the suction side of the system.
    Anyone seen this or have an idea.
  8. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    -restricted air flow
    -damaged/kinked refrigeration lines
    -TXV valve
  9. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I would start

    by looking at the TXV . Make sure the sensor blub is in good contact with the copper tubing. The contact with the tube needs to be very tight. If you can move it with light pressure then it is not tight enough. It should be between 4 o'clock and 8 o'clock on the pipe and tight. It should also be VERY well insulated.

    If all of the above are true, then set up your test equipment and hold the bulb in your hand and see what changes.

    All of Chris' ideas for cause are correct, my vote is the TXV as it works as a variable in the system. Plugged filter dryer would shut do the system every time. The same would be true for refrigerant line kinks and poor air flow.


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