We have had a record breaking cold snap the last couple days, with night lows as low as -20F.
The plot below shows the performance of the $1000 solar domestic water heating system for today (Dec 15, 2008). The overnight low was -20F, and the high for the day was about -11F. There was good sun in the morning with only very wispy high clouds. By early afternoon, there was a thin overcast that coupled with the extreme low temps ended the collection period.
From top to bottom:
Red dash line is return temperature from collector F
Black dash dot line is tank temperature F
Blue solid line is supply temperature to collector F
Blue solid light line is sun watts/sq meter
Blue long dash line is ambient temperature F
Night time low was just under -20F (burr!)
Temperature during the collection period was about -11F -- wind nearly calm.
Sun intensity was good until about 1pm, peaking about 1000 w/m^2 around noon. A high thin overcast that gradually thickened ended the collection period a early.
The difference between supply and return temps was 6.2F, which is down only a bit from the Dec 5 test in spite of the very cold ambient temperature.
The 160 gallon tank temperature increased about 11.5 F over the collection period (somewhat over one days use for us).
There was no trouble at all with the drainback of the serpentine collector. The whole system stood up to the extreme temperatures with no problems.
The controller cycled several times at startup in the morning, and also at shutdown -- maybe this is normal, or maybe the differentials need adjusting?
Anyway, nice to see the system work at very low temperatures with no problems.
Gary December 15, 2008