Ground Loop Reverse Return Opinions

[Comfrt1]

I am having a problem with a newly ( 1year old now ) installed 3 ton system. It was started up Feb 2015. All new duct work, vertical loop field and 2 stage water to air heat pump. The problem is it freezes up in the coaxial coil and the unit trips the pressure switch. A recent test after being down 4 days and only running 2 days prior to that is that it is pulling the temp of the loop field down quickly, from 48 deg to 37 in just over an hour and past experience showed that it had gone down to 18 after extended run time. Our deep earth temp is 52. My manual J shows it's the correct unit size
for the load and the loop design shows 44 deg for average loop temp and a balance point of 2 deg. The loop installers came out, dug up the header, cut out each circuit and tested with 100 psi on one end, blowing out a nice stream of H2o and glycol from each. then installed new headers and said there you go it's not our field. Still freezing! Manufacturer came to the site, took measurements ( no more than I did ) and determined it is still the loops having an issue. I am in the middle here with the home owners and they are running out of patience big time! and I can't blame them. Finally, here's my question. Looking at the attachment and knowing there are 3 vertical wells 12-15' apart, top of pic, middle and still buried at the bottom. Is this configuration correct?

[dorff]

I would start with a good loop flush. Really the only good way to ensure you don't have a large air bubble blocking one of the paths.


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[throrope]

I suggest measuring the water flow at the pump and insuring the piping isn't permitting a bypass of the field. A loss of flow or reduced flow could force the system to create a high delta T or high temperature difference between the water entering and returning from the field. Unless equipped with safeties, the heat pump will pull the heat out reguardless of temperature.

Consider installing low temperature cut-outs on both entering and leaving sider of the heat pump condenser coil.

Years ago a school experienced failure at elbows on the cpvc heat pump piping loop. They chemically tested the water and found trace amounts of refrigerant oil. Then I learned a few heat pumps had condenser failure. I immediately realized they froze and burst the condenser, which caused the high pressure Iiquid refrigerant to leak into the condenser water loop. This refrigerant flashed to vapor and froze the loop water. The ice was then driven by the flowing loop water down the smooth cpvc at high speed to the next plastic elbow and blew out the back side.

I suppose a low refrigerant charge may create a similar issue.

[crmont]

Easy. Check the temperature rise at each loop.

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[throrope]

Easy. Check the temperature rise at each loop.

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Temperature will only confirm the problem. Need the whole picture to determine the cause and configure a solution.

[Comfrt1]

Temperature will only confirm the problem. Need the whole picture to determine the cause and configure a solution.

I agree that taking each circuits temp drop is important and I did fail to that when we had the pit opened up, if nothing else presents itself as the problem, I will be doing some more digging. I had not considered a short circuit through the pump, I use B & D nonpressurized flow centers and don't really think that is the issue, BUT I will check into it.
I flush my own loops with a 2 HP flush cart and do reverse several times and valve off to check for air in the system.

[crmont]

I'm hoping the loop is connected like I've got it labeled. It's hard to tell. I didn't realize the header pit was covered.

Pressure and temperature readings at the coax will determine if the flow characteristics are within specs.

With only 3 vertical loops, each loop should be at least 150' deep. Unfortunately minimal piping loops don't leave much room for error.



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[eswin65]

can you verify that the wells were grouted correctly? In my area...limestone and shale formations are the regular. Limestone can be cavernous and if the grouters just mixed what they thought the grout should be...pumped the grout, covered the hole, packed up and left....its VERY possible that the grout went out into the formation. Same with shale, we have some holes that were 75+ gpm during drilling. Very hard to keep the grout from diluting and going into the formation. If the grout is not making contact with the loop and the ground for ???? many feet....you may have at best 1/2 of the heat exchanger that you need. You will saturate the loop temp real fast

[crmont]

can you verify that the wells were grouted correctly? In my area...limestone and shale formations are the regular. Limestone can be cavernous and if the grouters just mixed what they thought the grout should be...pumped the grout, covered the hole, packed up and left....its VERY possible that the grout went out into the formation. Same with shale, we have some holes that were 75+ gpm during drilling. Very hard to keep the grout from diluting and going into the formation. If the grout is not making contact with the loop and the ground for ???? many feet....you may have at best 1/2 of the heat exchanger that you need. You will saturate the loop temp real fast

Especially if the wells are minimal to begin with.

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[SkyHeating]

So you have not posted a few vital pieces of information
What did the load calc come out to. Heat loss and at what temperature.
What equipment(model number) are you using.
Do you have an outdoor temp sensor with heat strip lockout setup?
Depth of each loop, i assume its 3/4" piping or is it 1"
One pump system or two
Main lines are 1.25" I assume?

If your load calc came out to say 36,000 BTU's at a 5 degree outdoor temp and you sized your equipment for three tons, your equipment may be undersized, most geo systems will only provide around 29K BTU's at 20-30 degree entering water temps and if you have no outdoor temp sensor to allow the heat strips to energize you are going to run your loop temps down by never shutting off the equipment.

Without knowing load we can't confirm or deny if the loop is just to short overall either. at 150' of vertical drilling per ton with a low outdoor design temp you are probably short looped. In portland OR we have a 24 degree winter design temp and still drill 150-200' vertical into dense rock. However no two systems are the same

[crmont]

Any loop that can't sustain 100% call for heat is undersized or not working right in my opinion. I know some companies will design on the ragged edge but once that ground is a block of ice you're done. With ductless hyper heat systems capable of heating in these conditions it's important that your geo system be superior to what's otherwise comparable.

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