I have a 4 ton water furnace with open canister/ non pressurized. Loop field is 4x 150 loop or 600 ft . I used to have a pressurized unit however in winter of 2015 my system would build extreme pressure blow the hose off the unit. I had the system flushed to among other things, remove any possible air in system. Same issue. I had an open canister installed. Fast forward a couple of winters/heating seasons and I don't have the pressure issue per se however water leaks over the top of my open canister but only when the system has been running in stage 2 heat for extended periods. Generally not an issue as I just add H2o once the levels settle back down however recently the water levels in my open canister dropped to a point where the pumps tripped off and I was on Aux heat all night. I should mention that we eliminated the possibility of the problem being a Freon or other issues with the geo unit as I literally had a new unit installed in 2015, had the same issue, and then had my old unit thoroughly inspected and reinstalled. Here's what I think is happening. My system is undersized- both the unit but perhaps more importantly the loop field. As such my unit is extracting every bit of heat possible, longer than normal run times etc. and the ground doesn't have time to recover resulting in loop contraction / shrinkage. It wouldn't take much to cause back pressure in that scenario. I've also heard that there are different types of pipe (some may be more prone to contraction than others). Really believe this has to be the issue as we've accounted for every other possible variable. Thoughts?