Help with technical diagnosis of heat pump problem

I am trying to diagnose a problem with a York R22 heat pump. There are two units, each 3 years old, that sit side by side and are almost the same except that one is 3.5T the other 4T. They are both dual-fuel setups for a home. The 4T unit is the problem.

The measurements I got were:
OAT: 30 degrees (F)
IAT: 60 degrees -- kept cold on purpose
delta T across indoor coil: 18 degrees
discharge temp: 160 degrees
suction temp: 28
lo P : 25-30 psig
hi P : 190 psig
subcool : 30 degrees
superheat : 20-25 degrees

My concern is with the low suction pressure, high discharge temp and high superheat on this unit. Comparing with the 3.5T unit running next to it, the suction pressure is about 15 psig too low, the superheat is 10-15 degrees too high and the discharge temp is about 30 degrees too hot. Subcool is also higher than I would expect, but manufacturer only lists data on subcool for cooling mode, so I'm less certain on what value to expect.


I tried to adjust the charge to raise the suction pressure, but it didn't seem to have much effect other than raising the head pressure. I added enough gas to raise the head pressure up to 210-220 and decided this wasn't the problem and took the extra charge back out. Charge may still be part of the problem, but it doesn't seem to explain what's going on at the low pressure side of the system.


The outdoor coil seems to be OK, however there's a small band of ice on the bottom 2" that seems to remain even after the defrost cycle. The other unit next to it doesn't have this ice build up. I tried to see if I could remove it by forcing a few defrost cycles, but it seemed to stay there. The outdoor coil is large and the ice seems to only effect a small portion, so I'm thinking this is more likely to be an effect of the problem rather than the cause.

I noticed that after the defrost cycle completed, the high pressure jumped up to around 250 psig then quickly settled back down its normal value. Suction pressure was close to 45 psig right after defrost but steadily dropped from there.

I checked the temperature of the liquid line right after the filter/drier by hand and it seemed warm -- i.e. no apparent temperature drop before vs. after, so I'm thinking there's not an upstream restriction. The next thing in the line is the TXV followed by a distributor. I can see frost on the lines from the distributor to the evaporator.

I'm suspecting an underfeeding TXV but looking for advice on other things to check to confirm this or not.

Is the ice at the bottom of the outdoor coil a normal system of a bad TXV at below freezing temp.

If it is the TXV is there anything to do other than replace it? I haven't replaced a TXV so I'd appreciate any tips or advice.

thanks,

The TXV may have a plugged inlet screen.

Do they generally all have screens on the intake?

Not all... but most that I have worked on have inlet screens.

Thanks for your help.

One thing I didn't do was take the TXV bulb off the suction line and try warming it. Do you think that would have any effect on the suction pressure or superheat?

I guess I was assuming that with the high superheat the TXV is already as open as it can go and warming the bulb wouldn't make any difference, but I guess it doesn't hurt to try.

What do you think?

Does the system work ok in cooling mode?

Could be a plugged screen in the txv as others have stated, or the power element could have lost charge, once it begins to lose charge it doesn't push on the pin so much that meters the refrigerant through the valve, giving a lower evaporating temp and high superheat.

I didn't run it in cooling mode this time -- I could try it but it would be at a 30 degree OAT. It did provide cooling in the summer but it wasn't used often (cool climate) and I'm not certain if it was really working "right" or not.

I did observe a few defrost cycles -- and IIRC low side pressures were in the 55-60 psig range with the temp at the outdoor TXV bulb of around 130 degrees (I believe I was measuring discharge temp there during defrost). It looked reasonable so I didn't check more than that.

Absent a better idea, I think what I'm going to do is assume its the TXV and move to replace it along with the filter/drier. If when I get in there it looks to just be the screen I may just try cleaning it, otherwise I'll swap it out.

thanks for the help.

On a related question, I noticed that right after a defrost cycle the high side pressure peaked up to about 50-60 psig higher than normal. I'm assuming this is due to the high load on the indoor coil right after the defrost cycle terminates. The high load being due to the residual heat produced from the gas furnace that is activated during defrost. I've seen a few high pressure trips from time to time and I'm wondering if this is the cause.

I'm a bit surprised that there isn't some type of delay that turns off the furnace a few minutes before the compressor swithes back to heat mode. It seems like there should be some time w/ just the fan running to dissipate the heat. Is this normal and is some type of delay ever used to prevent the pressure spiking after defrost?

thanks,