I want a dual-coil water cylinder for use with a (potential) solar
heating system. However there are a few issues...

- I'd like to keep my existing airing cupboard un-cylindered. (We have a
Combi that can also be configured to heat a tank.)

- It would be nice to mount the cylinder in the loft

- (Most important) Have you seen how much unvented dual coil cylinders
cost!! £1200 when the the same size single coil cylinder from the same
supplier is £300!!

So.. what I was thinking, this being the DIY group and all, is of DIY'ing
it. But in an unusual way.

I was thinking of using several 3m long 100mm or so PVC pipes to store
the water, with screw on endcaps. These would be laid along the joists in
the loft to spread the weight (smallish house, so only 120l or so total
volume). I was thinking of making a frame or backplate they could be
fixed to so they could sort of zigzag at a shallow angle upwards to allow
the conventional 'hot draw from the top' to work.

I'd put a loop of copper pipe along the bottom pipe for solar preheat and
another loop along the next one up for boiler top up heating. That gives
over 5m pipe length in contact with the water, which AFAICS is roughy
equivalent to the standard coils in a conventional cylinder. Having it
done as a loop keeps the connections all at one end for simplicity.

I could also make a temporary cover to box it all in then use expanding
foam to superinsulate it - as much as needed.


So...

- Does anybody not think I'm mad yet?

- Is PVC pipe up to the temperatures and pressures you'd get in an
unvented system?

- Any other gotchas?

- If unvented is a no go for whatever reason, could I use this scheme and
go vented? It is mainly for central heating use, DHW would still be
combi, I think.


Cheers

Paul