Issue with high head pressure on Lennox unit

zepfan

We are having an issue with a Lennox three ton rooftop unit. The system is four years old,410a, straight ac with gas heat. The unit’s refrigerant charge is a little over 5 pounds, it has been recovered to verify that the unit was not overcharged. The unit comes on and over the course of ten minutes the high side goes up to a point that the high head pressure switch trips. The suction pressure stays around 130 psi the entire time, and there is next to no temperature drop across the filter drier. The system has one condenser fan, it is running in the proper direction, pulling normal amps, and it’s capacitor is good,, not showing a low reading. The unit has a discharge pressure tap and a liquid line pressure tap. There is about a five pound pressure drop between the high side tap, and the liquid line tap. The pressure measured at the liquid line tap starts out at around 280 psi, then steadily climbs up to 600, and then the unit shuts off. As I had previously mentioned the charge has been verified and the condenser coil has been cleaned twice. Any assistance would be much appreciated

EBEBRATT-Ed

Sounds like a head scratcher.
Does it have the correct condenser fan motor and blade with the right RPMs and the correct rotation?

Is the blade mounted at the correct height?

It's possible the condenser fins are no longer bonded to the tubing and not rejecting heat.

Is the liquid line leaving the condenser ripping hot or cool. Check the subcooling and superheat. They will probably point you in the right direction.

If you spray water on the condenser does it run with normal pressures?

Was it evacuated properly? Sounds like non condensebles

HVACNUT

Sounds like the TXV (or EEV?) shuts down. Does the vapor side drop as the liquid line rises? The bulb is in the right spot?

mattmia2

Did it ever work correctly?

pecmsg

Super heat and Sub Cooling Readings?

Does that have a split condenser coil?

Harvey Ramer

If you have a nested (multilayered) condensor coil, you may have to split it to get it cleaned properly.

SlamDunk

We recently had a similar issue on four newly installed units. Turns out, because the the building is empty with no load, when it goes into de-hum, it trips compressors on high pressure. The installers removed De-hum wire from tstats and problem went away. We'll re-connect when people return.

Ironman

Did it ever work right?

The fact that your low side pressure remains the same indicates that the problem is not there or in the TXV; it's in the high side.

I had a Comfortmaker back 25+ years ago that another tech had miss-diagnosed with a bad reversing valve that was only a couple of years old. He put in a new unit and I took that one and replaced the reversing valve and installed it in my house. It tuned out that when I fired it off the compressor was a no-pumper. I got a new one under warranty and installed it. When I fired it up, the head pressure went crazy like yours. Long story short, it turned out that two passes of the three pass parallel condenser coil were piped dead ended together from the factory rendering them useless and leaving only one pass working.

If you've done all the obvious things right to correct the situation, look for something unusual.

ChrisJ

Ironman said:

Did it ever work right?

The fact that your low side pressure remains the same indicates that the problem is not there or in the TXV; it's in the high side.

I had a Comfortmaker back 25+ years ago that another tech had miss-diagnosed with a bad reversing valve that was only a couple of years old. He put in a new unit and I took that one and replaced the reversing valve and installed it in my house. It tuned out that when I fired it off the compressor was a no-pumper. I got a new one under warranty and installed it. When I fired it up, the head pressure went crazy like yours. Long story short, it turned out that two passes of the three pass parallel condenser coil were piped dead ended together from the factory rendering them useless and leaving only one pass working.

If you've done all the obvious things right to correct the situation, look for something unusual.

It also sounds like the TXV isn't being starved, so doesn't that leave only one possibility, not getting rid of heat?

SuperTech

Harvey Ramer said:

If you have a nested (multilayered) condensor coil, you may have to split it to get it cleaned properly.

This was my first thought as well. I have come across several Carrier rooftop units with multilayered coils. They techs that were servicing them had a habit of doing "quick" condenser coil cleanings. When you split them open it would look like a blanket in between the layers.

EBEBRATT-Ed

You can't diagnose anything without superheat and subcooling readings.

HVACNUT

EBEBRATT-Ed said:

You can't diagnose anything without superheat and subcooling readings.

You can if it doesn't run long enough to get there.