Hi,

I'm looking at fitting solar thermal panels onto a detached garage roof. The cylinder (heat bank) is on the ground floor of the house. The pipework will run from the solar panels, down the garage wall, under the ground between the garage, into the house and up to 350L thermal store. The thermal store is already in existence as is attached to a gas boiler and wood stove, it has an unused solar coil fitted. The distance from end to end will be about 30m, which means a 60m feed and return circuit.

The questions I'm hoping someone can help answer are:

1. Is the circuit too long to be practical?
2. Will a standard pump would be able to work over that distance?
3. At night, in winter or on really cloudy days, a lot of water will be sat in the circuit at low temperature, will this act as a heatsink and cool down the water in the thermal store.
4. Following on from Q3, on cloudy days will intermittent operation of the pump cause the colder water in the circuit to also cool down the thermal store?

I guess what I'm really trying to find out is whether I should fit solar thermal (as I already have a solar coil in the Thermal Store) or just fit PV panels and use spare generation to power an immersion heater?

I am looking at fitting panels that are 3m long and 1100 wide (both PV and thermal are this size) and can fit 4 across the width of the roof. The PV panels each generate 500w. I could therefore either fit 4x pv to provide a max of 2KW or 2xPV and 2xsolar thermal to give 1kw electricity and hot water.

1 - No, that's about the length of my pipes. You will need to insulate them to a much higher standard than normal though.

2 - Yes.

3 - If you let a thermo-syphon build up, yes, otherwise not significantly.

4 - it will cool down the store slightly until all the circulating water has warmed up, but most controllers have a bypass option to just circulate the water until it's warm enough to contrinute.

If I were doing it now (and if there is enough space for the larger area needed) I would use PV, not solar thermal collectors. The cost for a given peak output is about the same and electrical energy is much more useful. They're easier to install as well.

Thanks for owlman's and billt's contributions. I think I knew the answer that it wasn't viable but just needed to hear it from someone else. The amount of effort to dig up the drive for the pipework across to the garage, routing the pipes around the house, etc. felt painful but I'd crack on with it if I felt it was a better option than PV. I've also looked into a PV-T panel, but the temperatures are too low without a heatpump attached.

I think the thought of a wasted solar coil in the thermal store is leading me down a path that isn't viable. Nice to have a couple of second opinions. Thanks guys. :)