Can I use a plastic loft-type cold water tank for my solar water heater?

[meow2222]

a low performance collector means your water's in the bacteria growth zone, not hot enough, so one thing you dont want to do is shower with it.

NT

[meow2222]

then why not use a proven design such as a hosepipe pancake. Lower cost, higher perfornance, simpler, and no lethal bacteria.

NT

[meow2222]

a tankless hose will all be steaming hot in half an hour, and that makes it a lot more useful.

NT

[meow2222]

If you must go with bare copper, at least put a reflector behind it or a poly cover over it.

NT

[Bob Mannix]

I believe such tanks are designed to withstand boiling water, for the very good reason that they may be subjected to it from a faulty immersion, although they will soften. The recent tragic accident resulting in the death of a baby was because the tank was not properly supported on its base and the tank *deformed* and split. The specification for withstanding boiling water requires adequate support.

As the tank is outside and will *probably* never reach boiling point (especially with 3m of copper pipe!), if it were adequately supported and not over a sitting area or door or something, I would say the risk was very low.

[MM]

Look, I can tell by now that you are a professional nitpicker, who so far has only been very negative, bombastic or patronising. If you have nothing to add except belligerent rudeness, then please, withdraw from this thread and offer your advice to others, okay?

MM

[MM]

Thank you, God! Sorry, I mean, Bob! At last a voice of reason in a tsunami of panic-mongering. If the darned water ever got anywhere near boiling, then obviously I would not want either myself or others scalded. I took that as a given. Also, while there have been continual warnings of superheated steam to the pressure of 7000 psi bursting out of every join (I exaggerate a tad), others or the same people have pooh-poohed the idea as a waste of time and money that will generate only lukewarm water! Some people seem to want to make it their life's work to be critical before being constructive, a very irritatating trait of us British, who seem to see it as a personal challenge to find fault. Maybe that's why we have such a terrible trade deficit, why manufacturing has been ruined, and why we whinge constantly about "Brussels".

MM

[MM]

3m of copper tube and a holding tank are far neater and more compact than 30 metres of garden hose. And I don't relish the thought of 30 metres of hose with polythene over it practially hiding the entire lawn.

It circulated well enough for the systems which relied on the thermo-syphon principle. An oft-demonstrated example of the KISS principle.

Why "fully"? Surely if you have a bowl of hot water and you pour cold into it, excess water spills over, then the temperature of the water - ALL the water - falls? Else, why did pouring cold water into overheated car radiators stop them from boiling over and allowing the journey to proceed? My "spill over" will be achieved by the simple expediency of turning on a tap and waiting for a few moments...

Green! Yellow! It HAS to be black this year! (I have to think of the garden's feng shui, too.)

MM

[MM]

Right. So all those tens of thousands of people around the world who use solar-powered showers like this

will be dead in a week, yes?

MM

[MM]

Because I think it's far neater to construct a system like this:

[MM]

I intend to paint the copper pipe matt black, then fit the matrix into a box lined with loft insulation and thick black plastic like DPC plastic. The box will be covered with either glass (if I can find a pane for free - our local window replacement supplier often has old customer windows stacked outside waiting to be collected) or a polycarbonate sheet.

MM

[<me9]

If the tank is outside there will be more solar gain from the tank itself than the measly 3m of pipe (unless it's 3m diameter!).

[Bruce]

In the Caribbean, black plastic water tanks are sold specifically to be used as solar water heaters. You see them on flat roofs or on top of a steel frame.

[Bob Mannix]

Possibly but I suspect history has a part to play (I thoroughly recommend the 3rd book (1776-2000 - The Fate of Empire) of Simon Schama's excellent history tv programme, what I have just re-read).

I should point out that the term "withstand boiling water" for these tanks represents a one off occurrence that they are deemed to withstand. It would be recommended that such a tank were replaced after exposure to boiling water. Like you I would be happy with the fact that boiling was exceedingly unlikely.

[Steve Firth]

Oh dear, refer to comment about lack of clue. You don't own any particular thread and you don't get to decide who may or may not contribute.

[Steve Firth]

You can beleive what you like. However the case of the mother scalded by boiling water when the cold water tank above the bed collapsed after being filled with overflow hot water from a faulty cylinder suggests that your beliefs are incorrect. It's not a unique case, a baby was killed in a separate incident by exactly the same sequence of events.

some newspapers referred to "a hot water tank exploding" the cause was the same in both cases. Hot water overflow from the DHW cylinder was vented into the plastic cold water tank. The heat of the overflow was sufficient to cause the tanks to deform.

You may think this is unlikely in a properly supported tank, but two incidents suggest that the potential for the tank to collapse is there and that the water does not have to be boiling for the tank to collapse.

Why design something that has a known, inherent flaw?

[Steve Firth]

It's a mistake to cling to the news that you want to hear because it is the news that you want to hear.

[Bob Mannix]

It is unlikely in a supported tank as their specification demands that they withstand the temperature, for the very good reason that they act as the receptacle for the vent pipe and the issuing of boiling water from this pipe is possible and therefore the standards demand it. I believe it was the result of the inquest on the baby that stated this and that this specification was only met if the tank were properly supported. In that case it was resting only on two joists, which caused the collapse.

As I reported to the OP, however, the fact that the tank had been subjected to boiling water would be reason to replace it and (by implication) it could not be regarded as "normal operation" but a failure. Were his system intended or likely to contain boiling water it would not be an appropriate soution. As it is extremely unlikely to ever boil, it's a non-issue.

Of more concern, if mounted outside, would be the effect of uv on the plastic of the tank. This might cause a drop in the tank's specification and might be grounds for changing it yearly or bi-yearly before it cracked. In terms of hazard and risk, I would say the risk here is the greater. Given that, I would keep such a tank in the shade myself and, of course, on a flat shelf.

One might choose to use a galvanised metal tank for longevity, sure, but I don't thing the risk of tank collapse from boiling is significant if you don't do this and use a plastic one. I probably would as I couldn't be *rs*d having to change it/move it and would leave it there permanently!

[Steve Firth]

Would you care to state what the critical temperature is for such use, because I'm as sure as hell that the water doesn't need to be boiling.

The maximum safe working temperature recommended for HDPE tanks is 37°C, that means that in almost any conceivable use of an HDPE cold water tank for hot water it will be used above its maximum rated oeprating temperature.

[Ed Sirett]

OK. A properly supported and correctly lidded cold water storage cistern will probably be able to take hot water up to around 65C. Given the trouble that some people have had with boiling water great care should be take to minimise the risk. - Can't you find a metal tank from somewhere, at least to use as a bund?