So I’m having an issue with my geothermal loops. I did a 4 ton slinky loop system, 4 loops 600’ long each, and a non pressurized 2 pump flow center. Initial startup of the unit proved good. I was getting 12 GPM of flow, and 3-4° of delta-T. I had good heat and everything worked fine for about a week and a half. I came back one day to find that the geothermal unit had a low pressure lockout. I proceeded to check everything. The pump was moving water, refrigerant pressure was good. So I started it up again. It ran for several hours and then cut out again. This time upon further investigation, I found that the loop pumps had stopped moving water. The pumps were still turning but no water was moving. This was causing the low pressure lockout. I disconnected the pumps from the unit and ran them separately. The pumps almost immediately picked right back up and started moving water again. (Just FYI it’s a 15% glycol mix) water temps were running consistently 34° in and 31° out, a little cold but I had backfilled part of my trench on a cold day and the ground is still settling. The important note is that it hasn’t fallen to the glycol freezing temps. I’ve tried restarting the unit 2 other times with the same result. I am stumped as to why the pumps keep doing this. Any thoughts?
It sounds like the HX might be freezing up. A 15% glycol solution only gives you freeze protection to about 26 degrees. Most GSHPs have a freeze protection sensor set to about 15 deg, so it would be better to get a freezing point closer to that. If the LWT is 31, the inside of the HX would be substantially colder than that, so ice could be forming on the inside. You might need more glycol for a lower freezing point, something like 22% or 25%. EWT of 34 at this point in the heating season isn't a great sign. Are you saying you have 2,400 feet of pipe in the ground? You could need twice that in PA.
Yes it’s approximately 2400 ft probably closer to 2300. Around here the rule is 6-8 ft underground for horizontal loops, and 500-600 ft per loop. For horizontal loops it’s 400 ft per loop. I made the mistake of backfilling a small portion when there was snow on the ground and in the hole where the pipes were. On the flip side the EWT fluctuates between from 41-32° Probably because the ground has not completely settled on a small portion that was backfilled last against the house. I have some more glycol and will put it in and test to see what happens. The good news is that the last time I tested the system (thinking it was the low pressure sensor that had gone bad) the EWT stayed consistent at 34-35° for the 1 1/2 hrs or so it ran. This means it was picking up enough heat from the field loops to meet the demand - hope that makes sense. And just FYI the reason it ran so long is because the house was 50° inside when I started it up.
That rule of thumb might work for straight pipe out and back in its own trench. However, the overlapping coils in a slinky mean you will need a shorter trench but vastly more pipe--about 3X as much pipe. You EWT should not fluctuate that much unless its over a couple months. I have a 4T system with 4,200 feet of pipe in the ground. It will run all day with only a degree or two change in the EWT. The graph below shows my EWT and LWT on a day the system ran for 24 hours. You can use LoopLinkRLC to estimate how much pipe you should havee for your house's heating load and location.
