Friday, May 21, 2010

Short term Cooling effect from Sunbox

21 May : Since the diverter valve was fitted, I noticed that there is sometimes a short term cooling effect, only after a GSHP heating cycle. Blaise has noticed this in the datalogging record. I have noticed it at exactly the times he sees it on the datalogger – after the morning warm up of hot water 9-11 and in the evening when a bit of heating or hot water is needed again. He suggests raising the Delta-T or the operating temperature that triggers the thermostat.
    When the GSHP ceases a heating cycle, there is a short time (about 10 mins) when the loft pump has to flush the cold glycol that was still in the pipes through the panels. The slow pump is working at about 360 litres per hour, so as the panels and pipes are equiv to about 60 litres, that takes 10 mins or so. In the meanwhile, it is using warmer glycol that was coming up from the ground to push the glycol through. If there is enough delta-T it continues to pull slightly warmer glycol up from the ground loop until the temperatures balance out. If after ten mins, the warmer glycol reduces the delta-T, the loft pump goes to sleep. If there is a valid Delta-T, the glycol going round is still being warmed so this short 10 min flush-out it is not really cooling the ground. 
   This process doesn’t take longer than 10 mins, I let it happen – as it balances out fully later in the day when more real heat is put down. Overall, it is not a loss.I really do trust the thermostat, it has shown itself to be pretty reliable in the two months since mid March.
   I can avoid this by having a higher delta-T than five degs, or by having a higher SB temperature (more than 20ÂșC). I think in winter this cooling is an overhead that is not too serious because I haven't seen it happen until the weather got warmer – it seems more of a summer problem. Or perhaps, more likely, it is after I put in the diverter valve that moves All the glycol through the boxes when the Thermostat says Yes.

During the summer, I might stop Night time circulation (by raising both those numbers as per Blaise's Suggestion) to reduce pump consumption when sun is not shining and no PV power for the pump. I no longer want the pump to run through the night based on ambient night time warm air, as the 30W pump energy consumption would eliminate the benefit of the energy gained. During the sunny time this morning, the Sunboxes were at air temp of 40 degrees, even with cooling glycol running through them, this is good heat!
So for tomorow, I will try a delta-T of 6 and a Sunbox T of 21.... and do some weekend 'watching', to see how it goes.

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