DIY solar water heating

[dan_aka_jack]

Hi!

I'm about to move into a new house in London and I'd love to make the house as eco-friendly as possible. However, we are on a tight budget. After doing a little research on the web, I've come to the conclusion that solar water heating is probably the most cost-effective way of using alternative energy.

Has anybody got any experience of building their own solar water heaters and then plumbing them in? Are there any reliable guides online?

Many thanks,
Jack

[Muddypause]

Hi Jack,

Heating water is a pretty energy-expensive exercise. This is because water has an unusually high heat capacity (ie, it takes a lot of energy to raise its temperature). This means that heating water by other means is the single thing most likely to have the greatest effect on how much energy you consume from the national grid.

Are you thinking of making your own solar collector? I've been toying with the idea for a while. I'm sure there would be plans on the 'net somewhere, but basically my idea involves a radiator (painted black) in a glass fronted box. Double glazed if possible, and obviously facing the sun. The water that flows through this will go to a coil inside a hot water tank, where it will give up its heat to the cold water there, in much the same way that boiler-heated water does.

Usually, people combine this with conventional water heating, so that in winter and times of no sun, they still have a hot water supply. This means having a control box that can sense which form of heating is most appropriate at any given time. Alternatively (the way I am thinking of doing it), have a separate tank for the solar heated water, which can be used for hand washing, food preparation, washing up, and pre-heating water that you cook and make hot drinks with. Two hot taps over the kitchen sink, basically.

I've also been thinking that some sort of heat scavenger, that extracts heat from water that you tip down the drain should be possible - you would need a holding tank for waste water, with a heat collecting coil in it.

Good luck - and keep us posted.

[dan_aka_jack]

Hi,

Thanks for the reply. Yes - right now I'm thinking of building and installing the whole system myself.

I was planning on plumbing my system in such a way that the sun's energy is used to pre-heat mains water before it goes into my normal boiler. That way I wouldn't have to build a control box and I'd always have hot water. Does that sound sensible?

Thanks,
Jack

[Muddypause]

I can see that this could work if you had a separate 'pre-heat' tank that was between the cold water supply and the boiler. TBH, I've not looked into how different systems work; the only one I saw had a control box and diverter valves, so that one hot water tank would switch between heat sources. But your idea sounds good to me.

[Wombat]

G'Day Guys,

I'm sure that I have hard copy plans/books on such things, let me know if you are seriously interested & I'll make copies and send over.

Nev

[hedgewizard]

That's pretty much how commercial passive solar heating works, although they usually use a convoluted length of black piping rather than a radiator. We had a 50m length of ordinary dark green hose left on the surface by Blue String Man and on sunny days the water coming out the bottom was too hot to put your hand into.

[Fisticuffs]

Hello people,

Just came across your site, I am currently (trying to) building a flat panel solar water heater, I would be interested to hear anyones stories good or bad on DIYing such a project.

I will post here when I have made decent progress if anyone is interested. Up until now I have got my prototype well on its way, I will put pictures up a bit later, got a big sheet of aluminium primed and painted matt black, chucked it outside in the sun and within a few minutes it was too hot to touch (so far so good).

Got my copper pipe in the post (25 meters of 10mm). And also a double glazed unit being built (its crazy a double glazed unit is going to cost me less than having a piece of glass cut to size!)

Pictures to come.

Kelvin

[dan_aka_jack]

Hi,

That's great - yes, please do keep us abreast of your progress... I'm very interested in building my own solar water heater although I wont be able to start work on mine for several months to come.

Thanks,
Jack

[Cheezy]

Hi,

Thanks for the reply. Yes - right now I'm thinking of building and installing the whole system myself.

I was planning on plumbing my system in such a way that the sun's energy is used to pre-heat mains water before it goes into my normal boiler. That way I wouldn't have to build a control box and I'd always have hot water. Does that sound sensible?

Thanks,
Jack

I came across this rather excellent web site while looking for advice on exactly doing this. Having just ripped out an old rad, and happening to have a spare double glazing unit (don't ask), I thought about preheating the water before going into my combi, as it's just off the mains so in theory it should shunt it through. My only concern was in winter when it can get bloody freezing up here, and it potentially could chill the water!.
Have you gone any further, as I'm itching to give it a go.


PS my first post at my new home...wow nutters like me

[Muddypause]

Hi, Cheezy.

I've been pondering this. Looking back over the thread, I realise that Jack may also have been refering to a combination boiler, too. I may have been mistaken in thinking he intends to install a pre-heat tank in series with a conventional hot tank that is heated by the boiler, with both tanks being supplied from a cold tank in the attic. Installing a solar collector or a pre-heat tank in series with a mains fed combi. boiler could raise several issues of both a legal and practical nature.

In the first place, my feeling is, that connecting the solar collector directly in series with the boiler is only going to have a limited benefit. The only water that will gain any solar heat will be that which sits in the radiator for long enough before you to run a hot tap. It will probably be OK for a bowlful of water at a time, but after the standing water has run through, the flowing water is not going to be in the rad. for long enough to pick up much solar heat.

One obstacle is that water regulations forbid connecting anything to the mains supply that could contaminate the public water supply in the unlikely event of a backsurge (for example if the main in the street ruptures, pressure drops, and water flows backwards from your property). They are pretty exacting about this, and I would have thought this may include an old radiator. Practically, you could prevent this by fitting a non-return valve before the radiator (preferably two, though this still may not be legally compliant).

Another thing to consider is the expanding water as it heats up in an 'unvented' (ie, closed to the atmosphere) system. Now, with just a small additional volume like a single rad., you may get away with it, but consider that a ruptured mains pipe can cause a lot of problems, and if the rupture is inside the boiler, even more so. I think a pressure relief valve (it's a spring loaded valve that will lift and allow some water to escape if the pressure gets too high) would be highly recommended.

My understanding of solar heating is that it works best in conjunction with a water storage tank. The water continuously circulates through the solar collector and back into the tank (through a heat exchanger), picking up a little bit more heat on every cycle until the water in the tank is significantly warmer. The idea of then using that tank as pre-heated water that feeds the main hot tank that the boiler then heats further sounds like it should work OK.

However, installing a pre-heat tank with a mains fed combi. boiler is not straight forward, as you will be storing a large volume of expanding water. Mains pressure hot water systems have only recently been legal in the UK, and Building Regulations require that only 'Registered Operatives' are allowed to install these, and you have to include an expansion vessel in the system. These are basically a small sealed container with a flexible membrane in them; on one side of the membrane is pressurised gas, and on the other side is a connection to the water system.

If the hot water tank is supplied from a cold tank in the roof, installing an intermediate pre-heat tank would be easy (the expanding water just pushes back up the pipe to the cold tank, as it does with a conventional hot tank).

I'm not meaning to dampen anyone's enthusiasm - just give you a heads-up about certain issues you may have to consider. The solar collector in line with a combi boiler will probably have some effect, but AIUI, not as much as using it in conjunction with a storage tank. You can buy the expansion vessels at plumber's merchants, and they're unlikely to ask if you are a registered operative, but I would advise you familiarise yourself with how unvented hot water systems work if you decide a bit of illicit plumbing is in order. And there may be other ways of doing it that I haven't thought about (eg. by using a 'thermal store' type of hot tank).

[Cheezy]

Thanks for the reply Stew,

Yes I'd already realised I'd need a non return valve on the system, and the pressure problem was something I was thinking about. I guess with it being a rad, there could be some way of of putting a vent on the bleed valve, you don't know how those automatic bleed valves work per-chance?, and would it work with water pressure rather than air pressure.(probably not as this is the whole point doh!), but something like it.

With a non return valve between the rad and mains water the pressure would only build between the rad and boiler, so your in line expansion vessel between the rad and then another non return valve then into the boiler might do it.

I was thinking of doing a test with the rad, connecting it up to a hose, and seeing how hot the exit water gets, and how much volume of preheated water (and how hot) it can produce, before going back to normal.

On volume, we don't bath, and only shower in the morning, so it will be realatively ineffective, however I'm thinking of all the times we run the hot tap to wash hands, and do the washing up, while waiting for the combi to kick in. (our combi actually has a heat store which already preheats the water, but it uses gas to do this.)

Also like you it bugs me that we put hot water down the drain and loose the heat.

Here what about running it through the rad before to get some of the heat out first!. Could get a bit sludgy in there!.

What about any other ideas for a spare rad, keep em coming and I might try them out!.

C

[gug]

hi,

before you go building any flat plate collectors, whilst cheap, they arent much good unless you get a lot of direct sunlight.

I built one out of an old copper water cylinder (but not plumbed it in) a while ago. I can confirm that (the plates) get bl**dy hot in direct sunlight ! - although I've havent run water through it yet (i'll post pics if anyones interested). How it will perform on an overcast october afternoon is a different matter unfortunately.

Anyway, this chap... http://www.navitron.org.uk does evacuted tube systems at VERY good prices.
These systems work throughout the year and are many times better than flat plate systems.
This is the method that I intend to use in the future. Many companies charge an arm and a leg for decent solar thermal systems, but basically many of them are, well, ripping customers off.


If nothing else, just check out the link above, this guy seems to do amazing prices on all sorts of green power.


Just my 0.02eu.

cheers
Gug.

[Muddypause]

Anyway, this chap... http://www.navitron.org.uk does evacuted tube systems at VERY good prices.

That's a darned interesting site, Gug.

I take your point about needing sun to make the solar collector work, but my (limited) understanding says that any amount of sun will provide at least some heat, even on a cold day. I've yet to do any real research on it, but my feeling is that insulation of the collector, to prevent it re-radiating the heat it collects, and, perhaps more importantly, the amount of time that the system can spend collecting the heat, are significant, too.

To answer Cheezy's question about bleed valves: You are right that an air-bleed valve is not the same as a pressure relief valve. The air-bleed valves that I've come across work in one of two ways - a little float that rises in a small chamber to block the bleed hole (when there is air, it drops to the bottom, and the air escapes, when the water comes along, it floats again and blocks the hole). I've usually found these inside boilers. Simpler than this are ones that have a plug made from the same stuff that fibre washers are made from - when it gets wet it expands and plugs the bleed hole, so water doesn't get out; when air rises to the valve, the plug dries out and shrinks enough to let the air out. Both need to be mounted on a high point of the system. They are usually only needed in a closed loop system, like central heating or an indirect hot water circuit.

A pressure relief valve is usually a simple spring loaded device that can be fitted into a pipeline.

[Cheezy]

Cheers gug and stew.

I'm rapidly going off the idea, ganna cause too many probs with my current system. However its all good fodder for the future!.

Still want to think about utilising this rad. Came up with an idea while in the car driving home.

I've got raised vege beds against a East facing wall, so get morning and afternoon sun. What if I place my black rad facing west/south filled with water, and run a hose pipe around my bed, thus the water will heat and then travel out around the bed, then come back into the rad (ie heat exchange), and keep my bed warm, and make cropping earlier. Put closh system over it in the winter and you've got a mini heated green house for salad crops etc.

n

[gug]

I'm rapidly going off the idea, ganna cause too many probs with my current system. However its all good fodder for the future!.

ahh, dont be put off. Flat plate collectors DO work - but my personal ideal is to get the *vast* majority of my hot water via a system year round (hence my interest in evacuated tube models) + without the need for electric or solar pumps (it kills it for me when you consider that many solar heat systems will still require electricity !).
my idea (dont know whether its entirely feasable) is to install a fortic tank in the loft (in place of the standard cold water tank) and preheat the mains water before it gets to my hot water tank (below in an airing cupboard on the first floor). To do this without a pump requires the panel lower than the "preheater" tank so that gravity and natural heat convection (not sure if thats the correct term) take care of moving the heated water to the tank and the cold to the panel(via the usual tank heat exchanger). The panel will be mounted on a rear extension and thus my hope is to mount it sufficiently below the fortic tank in the loft to make it work.
at least I *think* this should work

If you intend to heat the ground around your veggies then your idea, whilsts feasable needs to take into account that your panel will need to be lower than your pipes (unless you intend to use a pump), or your heated water will simply stay in the panel and not circulate - I'm pretty sure I've seen this kind of setup on a website somewhere - it may have been on downsizer.net or somesuch.

I've just noticed you mentioned raised beds, so I'm guessing having a panel lower wont actually be a problem (next time i'm read properly !)

As far as heating a greenhouse goes, I quite like the idea of a solar powered fan driving air into a heat store for a greenhouse as seen on bbc 2's "Its not easy being green" program thats currently running.