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I first did my solar hot water system in Feb 2008. The water in the solar panels was common to the central heating water and I had to shut off and drain the panels every winter.
My upgrade is to add a second coil to my hot water cylinder. The solar heating will then have it's own header tank and can be filled with antifreeze and it will be completely isolated from the central heating hot water.
 It will be able to run all year and will be more efficient as it will be heating the water lower in the tank and not competing with the gas boiler.
Luckily my hot water cylinder has an inspection plate near the bottom and with a lot of sound advice from the CR4 crew, (they dissuaded me from using cheapskate plastic fittings) I made myself a coil and installed it in a few hours (While Mrs Cat was out ).
Total cost about £40, whereas a new twin coil cylinder would have cost about £300.
I can complete the plumbing at my leisure now without much disturbance to the hot water supply.
When I removed the inspection plate I was surprised to recognise the gasket material, which was still sound. Last time I'd removed the plate, I'd made a new gasket from a piece of old Wellington boot!
The connectors are 10mm compression to 1/2" BSP fittings reamed out to allow the tube to pass right through. The plate is nice thick brass which allowed me to tap it to take the fittings. Rubber wellington boot washers and a smear of silicone sealant finished the job.
In the final shot the cylinder has been refilled and there are no leaks.
My top tip for bending the coil is to crimp one end, fill the pipe completely with fine, dry sand (sold as block paving sand) and then crimp the other end. It was then bent mostly by hand by wrapping it round a bit of plastic drain pipe. A pipe bender was used for the start and finish.
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Re: Solar Hot water Update
