AC vs Heat refrigerant pressure difference?

[gregs]

I have a Trane water source heat pump (WSHP) operating on a well water open loop. This winter it started giving me some issues in the heat mode. Basically everything operates normally, but sometimes (usually in the middle of the night) the compressor doesnt come on when the thermostat calls for heat. The blower fan is operating normally but no compressor. I figured out if I power cycle it by using the disconnect at the unit I can get it to restart and it operates normally again. It usually takes two power cycles in succession. After that it may not happen again for another day or it may happen again in 3-4 hours. Here it what I checked.

First I looked over the wiring schematic to determine how everything is wired. Using my multi-meter I verified that when the compressor is not on, the contactor is not getting power from the control side. Next I verified that I am getting power from the thermostat. On the control side the power goes from the thermostat connection thru a lockout relay to the contactor. The lockout relay is controlled by 3 safety switches, (high pressure, low pressure, water temperature safety). I found a nos part for $20 on ebay and swapped it out but that didnt solve the problem. Saturday I bypassed the water temperature safety switch and thought that was the problem as it operated correctly all day and night until last night. Woke up to cold air in the middle of the night, power cycled it twice and went back to bed. The way the lockout relay works it takes a power cycle to reset it, but not sure why it takes 2 times? The blower motor is on its own relay and operates normally. So at this point I am guessing its either the high pressure or low pressure switch and am going to bypass each one to see if that is the issue as it really cant be anything else.

Now to the thread title. Would I expect to see different refrigerant pressures in AC vs Heat mode? I am wondering if maybe I have a situation with low refrigerant that causes an issue with either the low pressure switch or the high pressure switch. I would guess it would be more of a problem on the low side. And its showing itself during these cold temperatures in heat mode. I havent had any problems in the AC mode. The unit is about 20 years old. Looking inside I dont see any eveidence of a leak on the outside, but I guess it could be inside the heat exchanger. I havent put a set of gauges on it yet because I thought it was going to be an electrical problem, but I guess one of the switches could be bad.

Any thoughts?

 

[fitter30]

Have a strainer in the water lines check it. Have you even cleaned the water side coil? With a condenser coil cleaner? Do not put on gauges yet. Low water flow, dirty condenser coil could cause this problem. If you put gauges on it have to reclaim, evacuate then weigh in charge.

 

[gregs]

I do not have a strainer. Since its an open loop I did verify flow outside. I have not cleaned the heat exchanger. I did buy a gallon of cleaner years ago thats designed to clean any scale buildup but havent used it.

 

[PoorUB]

I agree with Filler, goos chance the water flow is too low, or the heat exchanger is dirty. That will cause the low side refrigerant pressure to drop too low, and get too cold and shut of the unit.

 

[gregs]

There seems to be normal flow outside where the water comes out. It operates on standard 40/60 well water pressure and it’s also filtered. The only issue I envisioned was lime scale build up in the heat exchanger. So a long time ago I bought a chemical that you pump thru and circulate until it changes color. I am guessing it’s some type of acid.

I am thinking about jumping the low pressure switch to diagnose it . Any potential issue with doing that? I don’t see how it would be that much different than when I power cycle it to get it going again.

 

[PoorUB]

I have no issue jumping the low pressure switch to test the unit, but leave it that way and you will be buying a compressor.

 

[gregs]

So I jumpered it last night and the heat worked normally. However, it wasnt as cold as it was the past 2 nights and its warming up this week. So basically it wont get run enough in the next week to see if thats where the problem is. So I will pull out the jumper tonight so I dont forget and leave it in there and try again when it gets cold again.

In the mean time I am going find a pump and rig up some kind of contraption to clean the heat exchanger as maintenance with the cleaner I bought years ago.