Heat Exchanger from the stores

23 April 2012: I have obtained a heat exchanger from the school stores and it is about the size of a large shoebox and about 5 kilos in weight - contains lots of metal plates to link the loops. There were a lot to choose from, but many of them had soldered on ports or seemed too small.  This one seemed to be about right, has 4x 22mm ports, (1 and 3 for the solar tubes and 2 and 4 for the ground loop).
    I will need to get a pump and another thermostat, or a solar controller to operate the link to the ground loop. I have plenty of spare Celotex with which to build a box to put it in - the skips at our department are bulging with off-cuts.
   The tubes system is working quite well (consistently), but I estimate it is only averaging 2-2.5 hours a day which is only about 2-3 kWh per day. It really needs direct sunshine to activate. But for a proper installation, it's clear from discussions with Jason (the plumber) that a circuit with a heat exchanger would work better. In another house in another place, I would try it with a directly south facing array, with the tubes vertical, and probably 4 times the number - 60 tubes, or 8 square metres for starters. This would cost more than my hand crafted Sunbox!