DIY unvented water 'cylinder'

[PCPaul]

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

[dennis]

All combis can heat a tank, there is nothing special about them.

Why do you need dual cylinder? I would take mains in into tank, heat with solar, then through combi. If the water is preheated the combi should just heat it less.

Space heating using solar needs a big heat store, say 1-5 cu meters of water for a small house.

Do you have a cellar you can flood?

[Fred]

Do you have space for 2 coil cyclinders? Connect top and bottom together and by siphon acton they should equalise in temperature at the same heights.

[Doctor Drivel]

I assume you do not have enough height in the loft for the cylidners. That so? You need a heat bank working at atmospheric pressure. Look at:

will make to order and are stainless steel.

[David Hansen]

On Sun, 13 Jul 2008 21:14:48 +0100 someone who may be "dennis@home" wrote this:-

Provided that the combination boiler can accept pre-heated water. Check with the instructions or manufacturer. Some say they cannot due to the presence of fragile plastic components in the cold part of the boiler.

Also ensure the cylinder has a solar coil. Boiler coils are somewhat different in construction.

[PCPaul]

Mine is a Vokera and they say max 20C on the inlet temperature.. not so good.

Aren't all dual-coil cylinders made for solar?

[PCPaul]

Interesting.. not sure if I can see the diff between them and normal dual coil cylinders at first glance, but I'll look again when I'm awake...

Any idea of price though? They look expensive...

[PCPaul]

Hmm.. not really. But anyway, are you saying that I could feed solar into one and boiler into the other and it would all just work out?

[Fred]

Essentially yes - it won't be perfect because the temperature gradient in one cylinder would be slightly different to the other.

Hot water would pass to the cold cylinder through the top connection and cold water from this (cold) cylinder would pass through the bottom connection until equilibrium.

[meow2222]

There are ways to make a a single coil cylinder double coiled. One is to make a coil that can be fed in through a hole for the electric element. Another is to wind microbore round the outside of the cylinder and solder it on. Microbore has high flow resistance, so 4 pipes in parallel.

For best output with solar you want the solar exchanger lowish in the tank for tubes, or down the bottom for flat plate.

Re insulation, expanding foam is unnecessarily expensive. Standard loft insulation's good, or assorted paper/plastic rubbish is super cheap.

NT

[dennis]

Not that it would make much difference.

[dennis]

So it un-useable in the UK then. You would have to refrigerate the input pipe work to ensure the input didn't exceed 20C. Make sure you set the thermostat to

[John Rumm]

it is very rare for ground water temperatures to go above 15 deg c in this country.

Don't think the limit applies to the hot water side of it.

[Doctor Drivel]

Ring them. Remember they are stainless, so not cheap but will last and last. They are cheap for what they are.

[dennis]

I have yet to see a boiler that is fitted at or below ground level. All had some pipe that got to room temp. Having a restriction of 20C is silly as it just can't be done.

It would be a good way to avoid warranty claims as it hasn't been fitted to spec. "You mean you put some of the inlet pipe in a warm room Sir? Oh that voids your warranty"

Who was talking about the hot side?

[PCPaul]

I do see your point. I was thinking maybe I remembered it wrong and it was "cold feed temp + 20C" that was the limit, so I looked up their original answer to me:

============== (From Vokera Technical Support)

There is no reason why solar technology cannot be connected to your boiler, provided, that adequate controls are installed. Inlet temperatures in excess of 20 degrees C may cause undesirable temperature fluctuations at the outlet, and in some cases premature component failure to non-metallic components. ==============

Ouch.

[PCPaul]

I guess the ideal two-cylinder configuration would be one above the other.. but that couod be trickier to arrange!

Has anybody got any comments on my idea of using PVC pipe as a storage/ transfer container? I calculate it would need 6 3m lengths to get approx

120l capacity, mounted above each other in a ply box giving support along the length and filled with expanding foam.

Can PVC take the temperature? How about the pressure if I run it unvented? ISTR something about unvented tanks having to be inspected, presumably there are some regs about that?

[Rod]

There was a thread recently on an outdoor solar system where several people expressed concern over using cold water things for (potentially) hot water. Although waste pipe has to be able to take normal hot water, not sure how well it would withstand higher temperatures. Couldn't find any manufacturer's data either (but there is probably something out there). I think you have to be very careful of putting a lot of hot water over your heads - however you do it.

Pressur? - I don't think you stand an earthly of getting something like that tested and approved. But that is gut reaction, not knowledge. And I don't know what approval would be required.

[John Rumm]

Running off the small amount of standing water in the pipe is unlikely to be an issue - especially as the boiler will still be getting its HX up to temperature. That is a different proposition from feeding a supply of pre-heated water that will stay above the recommended limit.

I have never heard of one trying that angle, and there are plenty of combis with maximum input temperature limits.

Your meaning was unclear...

[Ed Sirett]

Let me get this right: I think you are saying you are going to attemp to make an /unvented/ storage vessel from 110mm PVC pipe?

Some how I don't think so.