Thursday, October 21, 2010

Cooling the Wall?

20 Oct: Someone on the Navitron forum asked if we are cooling our wall - and would this have the effect of cooling the house? (as usual, this question was asked with malicious intent)
    We are NOT intentionally cooling our wall. We are getting surplus heat from the wall in summer which gets up to 40degrees, and with that thermal capacity, continues to release some of that in the evenings. I would term 'cooling the wall' to be taking it below a comfort temperature like below 15.5, and if the temperature is this low the thermostat turns the box pump off.  The boxes need a temperature of over 18º to work, else, they need a delta-T of 5 degs.
    I have yet to see it work in deep winter, so imagine that when the thermostat is driven by delta-T and not by temperature, the heatpump might well be sending glycol up at 5 or 4, and the air in the boxes might be 10 or 11. In fact it was like that on some days this last week. 
  So Yes! with more thought, I recognise that it is possible that in Dec, Jan or Feb, the low air temp might cool the wall, even though the black collectors are a full 100mm distant from the wall. I will check that with my infrared thermometer, outside and inside, on cold nights.
Portable infrared thermometer
is a good way to test surfaces and
discover cold bridges
   However, most of the wall behind the boxes is in the LOFT Gable, so it is not taking heat from the inhabited part of the house even if it was. But the point is taken. Were the boxes to be fitted on the wall of a bedroom or living space, there could be a cooling risk.
   ON days like today, sunny but very cold air temperature, the air temp in the boxes would have been mid thirties up there, even when ambient air temp was about 5. So, overall, the wall probably benefits more than it loses.
   The next Sunbox that we are building is having a timber framed wall behind it, so no thermal mass to make use of. Side mirrors, not top and bottom. All part of the experiment. If if works satisfactorily without a wall backing it, then we could put insulation behind future panels. 
   It would still be substantially different from a standard solar thermal panel, but we are thinking for our next project (if there is one) is to use a standard solar thermal flat plate panel, but ask the manufacturer to supply one without the insulation backing panel.

Postscript: the people at Navitron Forum were alarmist idiots, as always. In the winter, the sunbox acts like a double skin facade over the wall, reducing the heat loss. Any heat taken out of the sunbox by the heatpump in winter is taken when the temperature in there is warmer than the air around. So even then, it is not losing heat from the house. In summer, it is cooling the walls, because they get up to 40º after a good day, but that is a good thing, and they have about 5 kWh worth of heat to release after sunset.

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