I have an 6 year old Waterfurnace Envision 4 ton open loop. When in cooling mode, works fine. When in heating mode, it is faulting out with Low Flow warning after running for just a few minutes. Checked temperature of line where thermistor is attached and found that it faults out just below 50 degrees. If I throw the dip switch to lower the freeze protection sensor from 30 degrees to 15 degrees, it does not fault out. Water flow checks out at the expected 6gpm and 49 degrees exiting temperature, and the line temperature where the thermistor attaches never appears to go below 44 degrees. So I assume the thermistor needs replaced, but I also note that the exposed line where the thermistor clips on is frosting up anywhere the insulation is peeled back. My question is, can frost on the line where the thermistor clips be considered normal? As mentioned above, I am not seeing temperature readings on this line going below 44 degrees or so and the exiting water temperature is around 49 degrees. I used both a non-contact automotive type thermometer and a standard probe thermometer to measure the temps.
Open Loop freeze protection is 30F refrigerant temperature. The frost confirms that it is reading correctly (you shouldn't be close to freezing on Open Loop). Something else is wrong.
Perform a HR / HE (heat rejected / heat rejected) calculation in both modes and check against extended performance tables for your operating conditions. If system is running short of spec, check charge. Frost near thermistor is strong evidence it is experiencing temps below 30*F. Non-contact thermometers are not useful for this situation. If charge proves low, investigate air coil for a refrigerant leak. Examine condensate drain tray for any traces of oil.
Do not leave dip switch setting at 15 or you may fatally damage equipment. Low entering air temperature, low gpm from a fouled coax or drain, low charge, low out put from well or plugged filter all might contribute to the fault. Your equipment is not accurate or as engineer points out there would not be frost while above reading temperatures. If you permit your system to freeze the water in the coax by changing settings you may very well be shopping for a new heat pump.
Good point; I agree and feel remiss at not pointing it out. DO NOT operate an open loop system with freeze protection set for 15*F.
Thanks to all for the replies. Looking through the attached troubleshooting guide that geoxne provided, I can rule out #1, #2, #3, and #4 since I have verified water flow and enter/exit temperatures are all ok and coax coil was just cleaned. So sounds like #5 low charge or clogged TXV valve. I will ask tech to come back and check again. Not real concerned about the coax freezing, the exit water hasn't gone below 50 degrees even after over an hour of running. I will keep checking it until tech can come out.
Do keep checking. A localized cold spot arising from low charge or malfunctioning TXV could cause ice to start to form, and if that formation blocked water flow it would snowball (almost literally). I'm not sure that could happen...it is an interesting case of competing heat transfer rates, but I wouldn't want to chance it. One might be tempted to scoff at the idea of 50*F water freezing, but the very same mechanism causes air (evaporator) coils to sequentially ice up even in the presence of 80+*F entering air.
Engineer has a valid point. In the coax coil, water flows through the central pipe surrounded by, in your case, subfreezing refrigerant. A layer of ice can form on the inner wall of the fluted water way even while flowing. This will quickly degrade the heat exchange process between water and refrigerant in the coil. Often, when operating that close to the edge, the ice layer can sluff off and dam the water flow. This would leave you seconds away from a frozen coil.
"Looking through the attached troubleshooting guide that geoxne provided, I can rule out #1, #2, #3, and #4 since I have verified water flow and enter/exit temperatures are all ok and coax coil was just cleaned." You have verified nothing. How do you suggest frost can form on a line that isn't at freezing temperatures? "Not real concerned about the coax freezing, the exit water hasn't gone below 50 degrees even after over an hour of running. I will keep checking it until tech can come out." You should be. Don't "keep checking". Stop running your heat pump without freeze protect or you will nullify your warranties and possibly total the thing. Ice can form very quickly. Thermistors respond more slowly.
Joe's right; I was trying to be gentle in conveying the risk of localized subfreezing temperatures since I'm distanced by the internet from the situation, but that sense of reserve was misplaced...STOP operating a water source heat pump using raw water with the freeze protection set at 15*F
