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

[MM]

I've made the first stage, i.e. the copper tube, about 3 metres of it in a "snake" pattern. This will be mounted on a south-facing wall. Can I feed the hot water (on thermo-syphon principle) into a black plastic cold water tank as used in the loft? Or will this plastic release toxins when containing hot water?

NB: The tank will be mounted initially on outside wall, above the copper tube "snake".

Or maybe you have suggestions for alternative tank suitable for hot water?

The cold water tank I'm looking at (on the B&Q web site) is the Titan Wizard 25/15 Rectangular Water Tank KM15 Black.

MM

[cynic]

For 3 metres of copper tube (15/22mm?) you are wasting your money and time

[meow2222]

Such tanks are unsuitable for hot water, they soften and collapse, possibly inflicting nasty burns. People have even died this way

I hope your description of your solar system is quite inaccurate, as its not going to achieve much as described.

NT

[Dave Liquorice]

Not long enough unless soldered along all its length (not spots here and there) to some other form of collector plate a couple of square metres in area, even then the spacing would be to high to have effcient transfer of collected energy into the water.

With your very poor collector it's not likely to get hot enough to deform, that would be the major risk. Deformation resulting in a split or collapse of the tank.

I can't find that Titan Wizard KM15 tank anywhere other than on the B&Q site. It's not a Titan Tanks product as far as I can tell. You need to find out its specified temperature range.

[newshound]

Such tanks are unsuitable for hot water, they soften and collapse, possibly inflicting nasty burns. People have even died this way

I hope your description of your solar system is quite inaccurate, as its not going to achieve much as described.

NT

Agreed. You might pick up more heat direct into the tank than from the coil. The tank won't release toxins, but it will become brittle because of the UV.

[meow2222]

yes :) This might be a start:

[MM]

What kind of (metal? plastic?) tank would you therefore recommend?

As long as my thermo-syphon domestic solar water heater can produce 15 litres hand-hot water for washing up and 5 litres for my daily ablutions, that's all I'll want from it! I know I can get half a bucketful of really hot water just from a garden hose in the sun. And even if I only got a kettleful for shaving from my contraption, that's one kettle for which I don't need to use electricity or burn oil to heat.

MM

[MM]

How about double the length?

MM

[MM]

As an experiment I took one black plastic builder's bucket (3 gall size) and filled it with 12 litres of cold water from the tap. I stuck in my (German) Tauchsieder (1200W) and switched it on for 15 minutes. I measured the amount of electricity consumed with my Tschibo meter at £0.035 (3½ pence) and the temperature of the water had increased to

*beyond* hand-hot. That is, I could just about dip my fingers in, but not for more than a second. The bucket did not deform at all. I doubt whether another 15 minutes (when the water would be VERY hot) would make any difference. Now I appreciate that a builder's bucket is not the same as a loft tank, and is probably far sturdier, especially in the rim. So what I'm looking for is either an even larger builder's bucket, or a tank made from the same kind of material. I could, of course, just use two (or three) builder's buckets connected together.

MM

[Dave Liquorice]

So probably not much above 60C or 70C a decent solar collector is perfectly capable of boil it's circulating water. Plastics do not soften in a linear manner.

water storage tanks are not designed to hold hot water.

[MM]

I don't need it to boil. Just to get hot. I've just popped out the back and run the tap after 2 hours in the (today, not very hot) sun (no collector tank yet!) and the water is already warm, despite today's strong winds. This is even before I construct the insulated box and paint the pipes black.

So this must exclude plastic (i.e. non-metal) as a material, yes? I've now stuck an advert in Freecycle for an old loft galvanised tank. (My old one in Bucks would have been ideal.)

MM

[Rod]

You might not need it to boil - but what are you doing to prevent it boiling?

Why are you persisting with trying to use a cold water tank? (I have never seen a loft galvanised HOT water tank.) Galvanised tanks are rather unusual these days - given that most new loft tanks are plastic. So you are likely to be offered a heap of junk.

Having a hot open tank in your loft (or pretty much anywhere else) is a bad idea as there will be lots and lots of water vapour leading to condensation and consequent problems.

The obvious thing to try would be a hot water cylinder.

[George (dicegeorge)]

why ae you using copper pipe? why not black plastic pipe?

I'd put a lot on my roof except the weight of the water micht collapse the roof

[Dave Liquorice]

You may not need or even want it to boil but if you have a decent collector how are you going to stop it on bright day?

Depends a cold water storage tank is for that, cold water. I'm sure there will be plastic tanks out there that can cope with 100C without trouble but I doubt you'll find them ina DIY shed or builders merchant.

[meow2222]

Right - 3m copper pipe in the sun isnt going to get you much though. Add it to a tank of colder water with no circulation and forget it. Sounds like you'd be better off with a hosepipe pancake collector. Make it a decent size and you wont need a tank at all. The hose can store several litres, and since you need lower temp you can dilute the hot at point of use, thus will require less than 15 litres in the collector.

The pancake collector would get hot enough to self sterilise, and will be flushed regularly. Your tank approach would be right in the bacterial breeding zone, and never get flushed out.

NT

[David Hansen]

On Sat, 21 Jun 2008 17:47:37 +0100 someone who may be MM wrote this:-

You can. Whether this is advisable is another matter. One question is what happens if the collector manages to make the water very hot, perhaps creates steam. Even a small collector can do this if it is only heating a small volume of water. Control of excessive temperatures is an integral part of good solar design.

There are some thermal stores made out of plastic. A (very expensive) range is at . The difference is that the plastic vessel is designed for this use.

The other important factor in solar heating is insulation of the cylinder/store to maintain temperature. As a minimum double thickness sprayed on foam is desirable, triple insulation is better. If I was making a small system for a few basins then I would consider having a special cylinder made by McDonald Engineers or one of their competitors.

For a home made collector plenty of designs are available and I would study several before deciding on a design.

[MM]

Add cold water.

I just need * a tank * ! I don't care what it's made from,or what its original use may have been. All it needs to do is hold hot water at a temperature of around hand-hot for a decent shower or for washing dishes.

But I might be offered a perfectly usable one! My old tank in Bucks wasn't leaking when I replaced it with a black plastic one. I only replaced it because it *may* have started leaking (it was 47 years old). So even if I get hold of a tank with a minor leak that I can easily repair, the tank will be good for years at what I want to use it for.

It's not going to be mounted inside the house. The tank will be painted black and mounted outside, against the house wall, above the collector. (This is only for summer use.)

Yes, I might put another advert in Freecycle.

MM

[Steve Firth]

The biggest thing to worry about is that the tank will, if the water gets hot enough, collapse and release hot water. You really need something that can take near-boiling water, and the tank needs to be insulated for best effect and closed (but vented) to prevent heat loss by evaporation.

My concern here is that you've shown a slight lack of clue from the outset and now it sounds like you're building up to kill someone, possibly yourself.

[Steve Firth]

FFS, why not use a hot water cylinder?

[MM]

Indeed. There are several YouTube videos that do exactly this.

It doesn't *have* to be the roof (mine is too steeply pitched anyway). A slanted south-facing collector-cum-frame in the garden is more than adequate. Just a garden hose left on the lawn for a few hours in the hot sun produces incredibly hot water, though not enough of it. That's the reason for the tank. The thermo-syphon principle will keep circulating the water throughout the late morning and early afternoon. Hot water should be available after only a few hours.

MM