View Full Version : Liebert Deluxe System 3 in computer room. low suction??

09-22-2011, 07:01 PM
Ok I got this Liebert that I keep going back for a low suction pressure alarm. I get there every time and put the gauges on and the pressure looks good before I even start it. And the low pressure switch is closed.(must be auto reset) I have to turn the main switch off and on and it starts up and the pressures look great, sight glass full. I've sat there for long periods waiting for something to happen and found no problem. In the alarm history it seems like it happens maybe 10-12 hours after I reset it sometimes longer. Any Ideas??? maybe liquid line solenoid getting weak and closing after running for several hours. I'm new with these units and could use a little guidance. Thanks, Bill

09-23-2011, 05:29 PM
if it's unable to reach a specific humidity level, it will cycle cooling well past it's cooling setpoint. Probably cutting out in the evening when trying to do this?

HJ Controls
09-23-2011, 10:27 PM
check heaters. when humidty is high and setpoint is reached, heaters should be energized to temper the air.

09-24-2011, 12:42 PM
What type of machine? DX? AG? AM? ICOM? Post the units model number and year of manufacture, with Liebert the model number will say exactly what the machine is & what it has.

If it was de-humidifying with out re-heats you would set off the low temp alarm before you would get low pressure. On AM or better processors you have an alarm history to check.

If it's a DX machine with a 2 or 4 fan condenser it's possible (though not likely) if all the fans come on in a low ambient condition (say 70 or lower) that you are pulling the pressures down to far. It's a 30 or 60 second window that the processor ignores the status of the low pressure switch when the compressor starts.

Whats the setting of the low pressure switch? Cut out & cut in? Is it factory of after market? Factory LP switches do NOT have the adjustment slot for a flat blade screwdriver. Air cooled machines usually run a 30psi cut out, although I have seen 15 psi switches from the factory before (in air cooled machines).

It's possible the solenoid is sticking, more probable other things are effecting it.

09-24-2011, 03:58 PM
If it's a DX machine with a 2 or 4 fan condenser it's possible (though not likely) if all the fans come on in a low ambient condition (say 70 or lower) that you are pulling the pressures down to far. It's a 30 or 60 second window that the processor ignores the status of the low pressure switch when the compressor starts.


Are the cfm contactors sticking? Bulb stats on the condenser set correctly and calibrated? Is the Penn 70 fan controller set up and calibrated properly? :cheers:

09-26-2011, 11:29 AM
What were your pressures if you don't mind?

Also, is this an air cooled unit or water cooled?

09-26-2011, 07:12 PM
post more info.

09-26-2011, 08:13 PM
Sorry Ive been away from the area. It is a air cooled unit. Suction pressure 65liquid pressure 200 I will get model number tomorrow, Im sure there will be another low suction pressure alarm and the compressor will be shut off and the other compressor will be trying to keep temp. Havent had time to troubleshoot it since my first post.They havent been complaining since theres three other units in the room. Thanks for the leads on what to look at, maybe I'll have time tomorrow.

09-28-2011, 12:55 PM
Sorry the suction line is 75psi and the liquid line 240psi
The model number is fh199a-c00 dated 1991 right now I know one thing, it's not humidifying something is causing the pan not to fill. Got to check it out.

09-28-2011, 03:27 PM
Those pressures seem to good to be true. Sounds like a faulty Low PSI Switch. They are Auto reset. Only HPS is Manual reset. You could test the LLSV by having guages on and cycle compressor off and see is Suction pressure rises after pump down. If that was the case, this would happen all the time.

09-28-2011, 08:02 PM
Yea I was thinking the low pressure switch but another tech just replaced it a month ago trying to resolve this same problem. Maybe ill swap the other compressors Lps see if that does anything. Cant see it being bad though. Check back soon. Thanks. btw the pressures are true.

09-28-2011, 08:35 PM
Do you have the ability to data log pressures, and temps? If so you will see exactly at is happening and when.

09-28-2011, 10:13 PM
Only one of the compressors is tripping, but both are on the same condenser. That would eliminate the fans not being set up right. Is there hot gas? If so is it working? Is the compressor that is tripping always the first one to come on? Are the starts rotated?

local 832s
10-01-2011, 08:01 AM
I would check LLSV. Start compressor (only the troubled one) let it run for awhile. Then raise set point and watch it. See if it pumps down correctly.

10-05-2011, 02:12 AM
filters clean and belts adjusted properly? set point set to low for the load you have ? you mentioned when unit is down there is sufficiant cooling in the room . maybe the lps is just doing its job. hmmm

10-05-2011, 07:32 PM
Ok i had more time to look at it, yes it had low suction pressure alarm. Seems like it pops up a couple days after i rest it. Yes filters are clean and belt is good. This Liebert has two comp's and when the one trips, it still satisfies the unit, set at 68 degrees 40% humidity. Theres four other units in the room. I want to change out the lps but Id have to recover 26 lbs of ref to change it out(no shrader) and it was just changed 2 months ago right before we changed the compressor out. (yes its a new comp).

Heres the weird thing that has me leaning toward the LLSV. Ok when I show up and find it with a new low suction pressure alarm, the said comp is off and the other comp is still running to satisfy the load. The comp in question wont come on till I reset it(breaker). So I turn it off at the screen to let the working comp pump down, then I turn the breaker off. When I turn the breaker back on, the comp in question comes on and pumps down.(like it didnt finish pumping down before the alarm) the other comp is off at this time.
Then I have to hit the "on" button at the screen. Both then come on and work great. If your understanding what I'm saying, could it be that when sometimes when it pumps down to shut off it takes to long and throws it into alarm??? ive sat there and watched it pump down several times and it worked great. Thanks.

10-05-2011, 07:59 PM
The compressor pumped down after power cycle due to it just completely shut off on a safety and did not pump down. Another concern is that the unit is making temp at 68F with one compressor, which tells me that there is very low load in this room or around this unit. You said that the SP is 75 and the HP is 240. Suction pressure at that psi is way to high for that return temp. That would be around a 75-80F return temp. This is starting to sound like the unit is overcharge. If it is, then it could be hard for the LLSV to close down and prolongs the pump down proccess. I would expect to see a 60-65F SP at that return temp. Is the Sight glass on the Liquid Line flashing (bubbling)? Shouldn't be or if it is with with that HP(240), you got non-condensables.

10-06-2011, 11:21 AM
Ok Im going with overcharged so far. Both site glass were full. The comp in question sp was about 74 and the lp was 240. On the other comp the sp is about 60 and the lp was around 200 I took about 5 lbs out of the comp in question. Now the sp is around 60 but the lp is still around 240 it didn't affect that much. Filters and condsr are clean and both comdsr fans are running. Another tech said that the 240 lp should be normal. Is it? Thanks

10-06-2011, 11:48 AM
What is your OAT? 240 would be ok if your OAT is 80+. Head pressure on an an air cooled unit will be affected by OAT. I like the numbers on your other compressor. 200 is more reasonable for current conditions. you may need to remove more juice and adjust TXV to get it right.

10-07-2011, 11:49 PM
What is the low pressure cut out set point set at? Make sure it is t set to high.

10-08-2011, 12:38 AM
If you are concerned with the charge pull it out and see what you have.

10-08-2011, 09:02 AM
The lps is factory set not adjustable. The site glass just started bubbling so I stopped removing ref. I think I'll pull it all and start fresh. But I did think I might need to adjust the txv. My OSA was about 76 that day.