Finishing the solar heating

I've been finishing the solar heating, by connecting up the heat exchanger for the pool to the solar panels. The idea is that surplus solar heat can be used to warm the water in the pool. I'm not expecting the pool to reach blood temperature - as the area of solar collectors isn't big enough - but it should make some difference, and maybe let us swim a little earlier, and later, in the year. But whether that will be months earlier, weeks, days or minutes I've no idea.

The pipe-work for the connection had to contain a two-way diverter valve, so that in the winter, and in high summer, the solar heating need not go into the pool heat exchanger. This valve - a manually operated one, as I am not a great believer in automated systems and electric valves always go wrong - had to be supplied from the US where I think it is normally used in very high pressure applications. Also in the new section of pipes is a stop valve (which will let me work on the solar panels without draining the rest of the circuit, a pressure release valve, and an expansion tank. The last two provide for very hot sunshine: the expansion tank supplements the similar one at the top end of the circuit (in the barn), and if despite the extra expansion capacity the pressure rises further, the steam can be vented off, I hope safely, close to the hottest part of the circuit (the collectors).

This was the pipe-work before the connections to the solar circuit were made. The loop of black insulated pipe is the temporary route of the existing circuit. The light blue tube at the bottom is the Bowman heat exchanger.



I thought it would be best to pressure test the new pipes before making the final connections, as I didn't want to have to drain down the circuit more than once. So I connected it up to a garden sprayer, and pumped it up to 2.5 bar (the most I could manage):



Using soapy water I was able to find a couple of leaky joints, and I fixed these. The pressure still dropped slightly overnight but I couldn't find where the leak was, so I decided that air might leak out where water wouldn't. Or at least that if I had a water leak, I'd be able to see where it was. In the end, there weren't any leaks in these pipes when the circuit was refilled.

It's been very sunny weather for the last few weeks, and I didn't wish to have the solar collectors empty and in the sun, as I didn't want them to overheat. So I covered the panels with a blind:



It's a section of the cover from the pool, the old cover which one of Jean-Louis cows walked on and split. I've found the bits of the spoilt cover extremely useful before, and for this it was almost perfect, just a few inches too narrow. And it kept the temperatures down quite nicely.

I then drained the circuit and made the connections. I also replaced two sections of copper pipe at the base of the collectors on the outside (where I hadn't had the proper fittings back in December), and it goes without saying that it was one of these soldered connections that leaked, so I had to drain it all down a second time, cut out that section of pipe, and replace the soldered joints. The second time it was all leak-free.

Here's the finished pipework inside the garden shed:



The blue conduit held on with orange tape contains the electrical connection to the temperature probe at the top of the collectors (it runs up to the barn and into the controller for the circuit pump). I haven't got the right size pipe clips for it, so that's a job to be done later.

Recommissioning the solar circuit was a nail-biting experience. The top of the solar collectors is only a little lower than the filling point in the barn, and as a result there was a substantial airlock in the circuit. I could tell it was there, as I knew how much fluid had come out, and it hadn't all gone back. I tried to push the air out with the garden spray, but the 2.5 bar testing pressure had burst a seal, and I couldn't get the fluid to displace the air. I thought that perhaps the electric pump for the solar circuit might shift the airlock, and to get that working I uncovered the panels (it relies on a temperature difference between the collectors and the hot water cylinder in the barn). Alarmingly it didn't, and the temperature of the fluid in the collectors rapidly got to over 100 degrees and the automatic air vent at the top of the panels started venting out steam.

Eventually by turning the pump on and off, and opening and shutting the valves in the barn I succeeded in getting the fluid circulating again, and the temperature went back down to something safer. I tell myself it was a useful test of what might happen if we have a power cut on a hot day. There's still quite a bit of air in the circuit, so I am gradually venting it off whenever I can, but it isn't enough to stop the circulation.

So far what I have found is that the pool heat exchanger does what it is supposed to do, and remarkably efficiently too: solar heated fluid goes in one end at 70 degrees or more and comes out the other at 20. I can't say the pool water is perceptibly hotter, but the heat must be going somewhere. We'll see what it's like after a week or so. The trouble is, fluid at 20 degrees doesn't heat the domestic hot water in the barn. I'm hoping that when the pool pump isn't circulating pool water through the heat exchanger (and it goes on and off on a timer) there'll be enough solar heat for the hot water in the barn. It might help if I insulated the heat exchanger too.

If none of that works, I can try putting the diverter valve into an intermediate position, so that the pool and the barn share the solar heat.