6 week update:
We have had a colder spell here, and the heatpump has been working for longer periods. After running constantly for about 10 hours in the morning a week ago, it tripped out on undertemperature, stayed off for 20mins, then back on again, tripped out again 30mins later - cycling like that. Prior to this, we left the house unoccupied and unheated for 3 days - this probably exasperated the freeze issue.
I suspect that once the heatexchanger ices up, just turning the system off doesn't properly de-ice it, so I changed the software fault condition so that on undertemperature it switched off the compressor and runs the pumps for 10mins - that seems to help a little. Ultimately though, the ground is getting colder, and the heatexchanger has been showing signs of freezing up and restricting the flow - which I expect is a horrible death spiral, so I decided to put glycol in the system.
I bought 15litres of "Coolflow DTX" glycol, to get a 25% glycol: water mix, and added it to the groundloop manually - while the system was off I blocked off the groundpump and put it in a header tank, draining away water, so all the glycol went into the groundloop, then released the pump and turned it back on.
It's definitely not as good with it in - I think I loose an extra 3C on the evaporator due to a combination of lower flow rate and poorer heat capacity.
The groundloop flow was 19litres/minute with tapwater, now it is generally only 12litres/minute. Originally I considered measuring entrence and exit brine temps - but these are difficult to measure as I have used flexi hoses, and I found if the heat ex freezes up, the out temp is irrelevent. There's a pic attached showing evaporator heat ex temp, also air temp , over the last 5 weeks or so.
I guess it is ultimately better now with glycol; I have dropped the trip temperature to -10C, and I doubt this will trip.
I measured the superheat of the heatexchanger, after it has been running and installed for a while. I calculated the evaporation point (from the suction line pressure) is 3degC below the measured gas suction line at the compressor, which is I understand about right.
So far total elec in of 304kWh used, and the calculated heat into the house is 1365kWh. I have ordered more accurate temperature sensors, to improve the accuracy of the heat output (DeltaT=5degC * flow rate = 7litre/min). I consistently measure ~2.4kW output, while the "coolselector" software predicts only 2.2kW. While it's nice appearing to have an efficient heatpump, I strive for accuracy:-)
|