Thursday, April 15, 2010

Sunboxes on Volcanic Ash day - Cold Sensor question

15 April : The Surya Sunboxes didn't come on much because it was a cloudy day with a cold north wind, blowing the Icelandic volcanic ash over the UK. It was densely clouded and one of our poorest days for Photovoltaic since March - and we had cold air temperatures all day, never getting above 12ºC, and more often below 10.
    I noticed that whilst the boxes remained off most of the day, they still came on correctly when the GSHP was putting out cold glycol, and we still pulled in 4 kWhours during the daytime heating period - all contributing to house heating.
    I reduced the trigger temperature difference back to 3.0 deg C to make it easier to come on, but am leaving the pump on at midspeed, because it seems to act more decisively - it downloads heat from the sunboxes and turns off quickly if the GHSP is satisfied.

Sensor fixed!
  A new problem became visible - the Cold sensor that triggers the Sunboxes is an oval rubber coated thermistor at the end of a black bellwire, and being blobby, is a bit slow to respond. (The sunbox sensor hangs in mid air, visible to the Sun, so has no such problem).
   David suggested connecting the cold sensor next to some copper pipe, not to the underground plastic pipe. As the oval blob makes only tangential touching contact with any pipe, the response is really very poor on any pipe - it is slow, and perhaps not accurate, even if surrounded by insulation.
  So, I got a small section of 28mm copper pipe, cut it to a 120º sweep segment, and flattened slightly so it perfectly makes touching contact to 28mm copper pipe. The Oval blob was polished with a file to give it one flat surface, and super-glued to the copper segment, with some sticky tape to maintain very tight contact. The resulting component (pictured) was then attached to the Copper outgoing pipe from the GSHP, with a large smear of Vaseline to close out any small air-pockets between the surfaces. This sensor is now surrounded with plenty of high quality insulation to ensure that it is only reading the copper temperature.

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