From 2-16 Feb, the Peveril Sunboxes captured 90 kWh, and the East Leake ones only 46 kWh. There is a difference in the plumbing layout, but I don't think that should be the reason. We are trying a number of things:
1. Reduce system losses. Need to fully insulate the pipes from the sunboxes to the heat pump now that we have checked for leaks (and there are none).
2. Check that there isn't a lot of cold air getting into the box somehow, eg the back of the box is against a slightly uneven wall and there is a case for some duct tape to do some sealing.
3. Check that the two sensors are correctly positioned. I am not sure if the Cold sensor against the ground loop is strongly enough taped with thermally conducting paste. Perhaps the warm sensor needs to be positioned higher in the box, so it detects a rise in temperature more quickly (eg when the sun comes out from a cloud) On my system, the sensor is at the highest point of the box where heat rises first, but has a foil hood to stop it being triggered by direct sunlight.
4. Reduce the pump speed. If it is slower there is a more distinct temperature difference between the flow and return, if it is too fast, then the energy flowmeter isn't precise enough to measure the temperature difference. In my case, it can run at middle speed, as the pipe length is longer and the volume is about 60 litres. In E Leake's case, with the compact radiators and small pipes, the volume is smaller.
As if that wasn't enough we also had the Energy flowmeter stopping recording for a few days, it was showing an error earlier in the week and with a bit of tapping and shaking, it worked again.... new technology, huh!?
Postscript 27 Feb '11: I visited the site and the main problem seems to be a large gap at the top letting warm air escape. There is a smaller gap at the bottom, and rain penetrating from the top - needs a flashing.