Heat Pump Tripping on High Head in Heating

[atcherservice]

Greetings,

I am working on a R22 Lennox Split Heat Pump 3 ½ ton. The unit will trip on the compressor relief valve in heating. I was unable to test the unit S/C and S/H accurately, due to the High Side pressure building up so fast with in a minuet will be hitting 400psi and blow the low side would sit at about 50psi. I tested the RVR from cooling to heating, and even tested the unit in cooling for a bit, pressures were ok. When in cooling my high side was 135 and low 50psi with an outdoor temp of 45degres and indoor temp of 75 degrees. I then switch back to heating and the same problem would occur. I have good air flow and in cooling the spilt seemed to be acceptable. The only other issue to mention, was when I arrived the customer stated that this summer they had a service company repair a pin hole leak somewhere in the system. I looked all over the coil, could not see any repair, or where they made the repair. I also did not see a new filter dryer installed, only the old one still in the system. I am puzzled the unit seems ok in cooling, the SH in cooling about 8 degrees, could the RV be not setting all the way in heating, or is there an issue with the TXV, with the possibility of recent sealed system work, I have a concern that there may be a restriction.

Any Ideas? Happy New Year

[SBKold]

You gotta pull some juice out and see what she runs like then.

You can see if the outdoor txv is working once you get it running steadily.

You may have a partial restriction or other charge issue leading to overcharge in cooling

Pretty common. IMO.

[Firebird A/C]

Sounds like a bad outdoor txv valve which is causing a restriction. Check the outdoor txv sensing bulb cap tube. Cap tube may have lost its charge. Look for crack or rubbed through spot somewhere on the small tube. Or just the txv is bad its self. You may have to dump some charge to keep it running to figure out where the restriction is.

[welling service]

When the head pressure goes up during heat mode but is okay in the cooling mode and it rises as fast as you say the problem will have to be a blocked liquid line dryer or a TXV problem. You should be able to see a frost line at some point unless it is a total blockage. Are you sure you have good air flow across the indoor coil? What about the strip heaters? Are they on the down stream side of the coil?

[Hvac216]

Maybe the leak repair was adding that leak stop stuff and that caused a restriction.

[LibertyTree]

When the head pressure goes up during heat mode but is okay in the cooling mode and it rises as fast as you say the problem will have to be a blocked liquid line dryer or a TXV problem. You should be able to see a frost line at some point unless it is a total blockage. Are you sure you have good air flow across the indoor coil? What about the strip heaters? Are they on the down stream side of the coil? If its a blocked line dryer wouldn't the suction side pump down in cooling?

[Huntinwalkerstyle]

Check the Discharge line temp outside the outdoor unit. If extremely hot like in the 200's the outdoor TXV is bad. I had a armstrong unit do the same thing. Run fine in cooling but in heat i had normal suction pressure head pressure pegged out and very very hot discharge. Changed TXV and unit running Ok

[SBKold]

This kind of shotgun diagnosis can get you in trouble.

[Milk man]

Was the indoor coil replaced? You said they repaired a pin hole leak but could not find where they did the work. Maybe they put in the wrong size of indoor coil.

Next thing I'd look at is the indoor TXV. If it's not bypassing you could see what you are seeing.

[Milk man]

Was the indoor coil replaced? You said they repaired a pin hole leak but could not find where they did the work. Maybe they put in the wrong size of indoor coil.

Next thing I'd look at is the indoor TXV. If it's not bypassing you could see what you are seeing.

[catmanacman]

You gotta pull some juice out and see what she runs like then.

You can see if the outdoor txv is working once you get it running steadily.

You may have a partial restriction or other charge issue leading to overcharge in cooling
Pretty common. IMO.

x2

[Huntinwalkerstyle]

The outdoor TXV is stuck open, you can try putting the bulb in ice water to see if TXV closes and also putting the bulb in hot water to see if it opens. Watch your suction pressure. You need to check airflow, you should be moving 1400 cfm. Put unit in emergency heat mode and check temp rise. You should have around 34 degree rise for 15 kw heat strip or 22 degree rise for 10 kw.

[Saturatedpsi]

You gotta pull some juice out and see what she runs like then.



I would agree with that. The 50 psi saturated temp is about right for 45˚ OD. The summer repair probably ended up with an overcharge.

[Branderson0515]

Greetings,

I am working on a R22 Lennox Split Heat Pump 3 ½ ton. The unit will trip on the compressor relief valve in heating. I was unable to test the unit S/C and S/H accurately, due to the High Side pressure building up so fast with in a minuet will be hitting 400psi and blow the low side would sit at about 50psi. I tested the RVR from cooling to heating, and even tested the unit in cooling for a bit, pressures were ok. When in cooling my high side was 135 and low 50psi with an outdoor temp of 45degres and indoor temp of 75 degrees. I then switch back to heating and the same problem would occur. I have good air flow and in cooling the spilt seemed to be acceptable. The only other issue to mention, was when I arrived the customer stated that this summer they had a service company repair a pin hole leak somewhere in the system. I looked all over the coil, could not see any repair, or where they made the repair. I also did not see a new filter dryer installed, only the old one still in the system. I am puzzled the unit seems ok in cooling, the SH in cooling about 8 degrees, could the RV be not setting all the way in heating, or is there an issue with the TXV, with the possibility of recent sealed system work, I have a concern that there may be a restriction.

Any Ideas? Happy New Year

I would check 2 things. You mentioned a new filter drier. If it is a liquid line filter drier, is it a biflow?(being it is a heat pump the drier must be a biflow drier) if it is a suction line drier, is it installed between the suction line and the reversing valve?(this is the only place a suction line drier can work in a H.P). Usually on Lennox the liquid line drier is right before the txv and I have seen people add a liquid line drier on the outside of the service valves and leave the factory drier installed. I have seen that cause enough restriction to cause problems but doesn't really sound like the problem. If you have a biflow liquid line drier and it is pumping down and tripping high head pressure in heating but not A/C mode your ourside TXV is bad. The TXV is locked up and will not open to allow refrigerant to the outside coil resulting in low suction pressure and high head pressure causing the compressor overload to trip or a discharge stat to open. I would replace the TXV but at the same time take the drier out of the factory location(right before the TXV) put in piece of 3/8 copper pipe where the drier used to be and put a new biflow drier on the outside of the service valve of the unit so that in the future if the drier ever needs to be changed you can pump the refrigerant into the condenser and change the drier without reclaiming.

[terpfan_]

Correct me if I am wrong, but if txv on outdoor unit were bad. Would you not pump down the low side due the outdoor coil acting as evap coil. Since this is happening so quickly I would look at rvr, airflow at indoor ahu, indoor coil, indoor txv (make sure flowing thru in heat).

[Branderson0515]

Correct me if I am wrong, but if txv on outdoor unit were bad. Would you not pump down the low side due the outdoor coil acting as evap coil. Since this is happening so quickly I would look at rvr, airflow at indoor ahu, indoor coil, indoor txv (make sure flowing thru in heat).

If your txv is bad on the outdoor unit your suction pressure is in the outdoor coil and the txv starves the coil and your suction pressure will pump down just as if the indoor txv where to lock up in cooling. Your head pressure is not going to raise the levels that they are as quickly as they are with a mis-sized coil. If your reversing valve is bleeding your suction will normally raise and head with fall as you are dumping discharge gas into your suction line(will act like a discharge bypass). If your fan is not on it may jump that high but I assumed from the post the fan is running. If the system has an indoor txv for A/C you would not pass discharge gas through the indoor coil(the condensing coil in heating) in turn your head pressure would not be high if you are checking on the liquid line because you have not passed gas thru the condenser.

[terpfan_]

If your txv is bad on the outdoor unit your suction pressure is in the outdoor coil and the txv starves the coil and your suction pressure will pump down just as if the indoor txv where to lock up in cooling. Your head pressure is not going to raise the levels that they are as quickly as they are with a mis-sized coil. If your reversing valve is bleeding your suction will normally raise and head with fall as you are dumping discharge gas into your suction line(will act like a discharge bypass). If your fan is not on it may jump that high but I assumed from the post the fan is running. If the system has an indoor txv for A/C you would not pass discharge gas through the indoor coil(the condensing coil in heating) in turn your head pressure would not be high if you are checking on the liquid line because you have not passed gas thru the condenser.

I agree a drier could cause this problem. I am confused in your statement about indoor coil txv not allowing hot gas thru it. This is precisely what it does in heat pump mode, usually at about 80%. The poster never said where he had his gauges hooked up. If you hooked gauges up on cooling suction and liquid line ports, a pressure drop across indoor coil could be seen. This would help eliminate the indoor section. If outdoor txv was only allowing partial flow and the system were overcharged I could see high head.

[Milk man]

If your txv is bad on the outdoor unit your suction pressure is in the outdoor coil and the txv starves the coil and your suction pressure will pump down just as if the indoor txv where to lock up in cooling. Your head pressure is not going to raise the levels that they are as quickly as they are with a mis-sized coil. If your reversing valve is bleeding your suction will normally raise and head with fall as you are dumping discharge gas into your suction line(will act like a discharge bypass). If your fan is not on it may jump that high but I assumed from the post the fan is running. If the system has an indoor txv for A/C you would not pass discharge gas through the indoor coil(the condensing coil in heating) in turn your head pressure would not be high if you are checking on the liquid line because you have not passed gas thru the condenser.

The head pressure will sky rocket with an undersized coil on a Lennox heat pump. The only way to match an indoor coil with a Lennox heat pump is with the engineering data.

[Milk man]

An indoor txv not bypassing in heat mode will cause a high head pressure condition. It becomes a discharge gas restriction.

[BALloyd]

An indoor txv not bypassing in heat mode will cause a high head pressure condition. It becomes a discharge gas restriction.

This makes sense. OP should check TXV and make sure it is correct for a heat pump. Needs to have the reversing check valve or a bypass circuit with a seperate check valve.

Sent from my SGH-I747M using Tapatalk 2

[Saturatedpsi]

An indoor txv not bypassing in heat mode will cause a high head pressure condition. It becomes a discharge gas restriction.

I disagree. The indoor TXV is at the outlet of the coil in heat mode, so it would be a liquid restriction. But based on the given information,


low side would sit at about 50psi.

..there is no restriction, or very little, since the suction pressure is somewhere near normal for the OD temp.

[Milk man]

I disagree. The indoor TXV is at the outlet of the coil in heat mode, so it would be a liquid restriction. But based on the given information,



..there is no restriction, or very little, since the suction pressure is somewhere near normal for the OD temp.

I agree, but disagree. It the indoor TXV is not bypassing, partially or fully, it will back the liquid refrigerant into the indoor coil. As the liquid stacks in the coil there is less area for the discharge gas to condense. This will cause the head pressure to spike.

It takes less refrigerant in heat mode than cool mode. The excess refrigerant is usually stored in the accumulator in most units, or the properly sized indoor coil in a Lennox. Now the coil essentially smaller and hot gas is being continually add until the HPS opens.

[welling service]

Not necessarily. it is a bi-directional dryer and could be blocked in one direction.
If its a blocked line dryer wouldn't the suction side pump down in cooling?

[Saturatedpsi]

It the indoor TXV is not bypassing, partially or fully, it will back the liquid refrigerant into the indoor coil. As the liquid stacks in the coil there is less area for the discharge gas to condense.

This will cause the head pressure to spike.



That's what happens with any liquid line restriction, regardless the location of the restriction. But, the restriction alone, won't cause the head pressure to increase. That's due to something else.

[Milk man]

That's what happens with any liquid line restriction, regardless the location of the restriction. But, the restriction alone, won't cause the head pressure to increase. That's due to something else.

In straight cool units a liquid restriction, in most cases, will not cause high head pressure. The liquid will simply stack in the outdoor coil. Since it's a closed system the refrigerant is being pumped from the low side and not being replace. Run out of refrigerant on the low side and nothing will be added to the high side.

In a heat pump the liquid will stack in the indoor coil which is smaller than
the out door coil. The indoor coil will run out of room to store the liquid refrigerant. Plus if it's the indoor TXV causing the restriction, the liquid line will also not be able to store refrigerant like in the cooling mode.

Furthermore, the refrigerant that is being stored in the accumulator, the outdoor coil, and liquid line is being pumped as a hot gas into the vapor line and liquid filled indoor coil. This by all purposes now a discharge gas restriction. There is no place for the discharge gas to go. The head pressure will skyrocket.

I've seen this in the field before. Amana RTG36. Head pressure skyrockets. I replace the indoor TXV and solved the problem. I did nothing else to the system.

[Saturatedpsi]

I've seen this in the field before. Amana RTG36. Head pressure skyrockets. I replace the indoor TXV and solved the problem. I did nothing else to the system.

I've never seen it, but also never seen that particular issue. And I'm sure there's a lot of other things I've never seen...I certainly won't argue with someone who has. ;)

[Branderson0515]

The head pressure will sky rocket with an undersized coil on a Lennox heat pump. The only way to match an indoor coil with a Lennox heat pump is with the engineering data.
Correct to properly match you need to match the coil but the unit is tripping head pressure almost instantly. Mismatched coil would take longer to trip.

[Milk man]

Right. Unless it's overcharge to begin with. It's been a while since I've seen an undersized coil. I think it took about three minute of run time with the pressures slowly raising then only a minute or two to hit the high pressure switch.

First step has already been mentioned. Pull some gas so it will run and the diagnostic reading can be captured. Then the guessing can stop and the fixing can begin.