Solar Water Heating - a DIY guide

[David WE Roberts]

Just borrowed this from the library and thought I would have a grumble before I took it back.

It is basically a description of how to build a wooden box.

Tells you to put either a conventional steel radiator in the box, or make up a grid of copper pipes.

Then tells you if you want to mount it on the roof you should drill through the tiles and screw some angle iron on, or if the tiles are brittle then replace them with 'some other roofing material that you can drill through'. No advice on the alternative roofing material.

No advice on how to achieve this safely on the roof of a two storey house with a box full of steel radiator under one arm. [Certainly no inclusion of the cost of scaffolding in the overall pricing]

The pricing was very dubious as well - claimed to get your money back in about 5 years but didn't seem to account for the cost of the second (preheat) cylinder you would need to install and all the plumbing involved.

On the subject of the preheat cylinder - the design shows the solar panel heating what looks like a conventional hot water cylinder which then feeds water from the top into the bottom of the existing cylinder where the cold feed normally goes.

Now as far as I can see this means you only start to get the benefit from the solar heating when you use a tank full of hot water which has been heated by other means. If you want to run on solar hot water only in the summer you have a major problem - assuming your main hot water tank is full of cold water you start to draw off cold water and hot water from the preheat cylinder enters the main cylinder from the bottom where it starts to rise by convection and mix with the cold water.

Looks to me as though you would have to run off a lot of water before you got warm water out of the taps, let alone hot.

Why not just buy a single cylinder with two heat exchangers? As you are buying a second cylinder anyway this could replace the original hot water cylinder and benefit directly from any solar heating.

Gaaaahhhhh!

Told you not to get me started!!!!

Author is Paul Trimby Published by the Centre for Alternative Technology. Courtesy of Blue Peter.

[The Natural Philosopher]

Congratulations on working out that i this country at least, solar water heating is a complete waste of time and money for most people,

The better solution is to not have baths or showers so often.

I am already sewn into my winter underwear ;-)

[tony sayer]

Dickensian times complete with his candle on a tray;)...

[Andy Dingley]

CAT publications are utter rubbish. They're stuck in the 1970s and their "build a green roof from old carpet" mentality. Their own projects range from the hippy inept to the unrealistically over-funded glitz (their photovoltaic roof). Talk to them about something that's eminently practical (most solar based on current technology, anything grey water) and their response is, "You can't do that".

[Andy Dingley]

Bollocks. It means that one group alone can't find their arse with both hands and a GPS. Don't tar everyone with the same brush.

[The Natural Philosopher]

Thank god my wife doesn't read this NG. she would instantly knit one.

[The Natural Philosopher]

Do the sums. It makes no sense at all.

[pete]

If amazon sells this book, you could add a user review to this effect. Might stop a few people buying it if they can see it's useless (from an independent source)

[NT]

The idea of the preheat tank is to get 2 benefits rather than one:

1) When the preheat tank produces hot water in peak summer, this hot is fed into the main tank whenever HW is drawn off, resulting in little conventional heat used. 2) it prewarms the water all the time the system stays above freezing, feeding the main tank with lukewarm water instead of cold. The preheat tank stores lots of this prewarmed water, whereas just using the bottom of the main tank to do this would only store a little.

Its a fair way to make use of low performance panels.

Re costs, I think the assumption is generally that the preheat tank is an unwanted used tank costing little.

I'n not disagreeing with your overall take on it, but a couple of the points seem fair enough.

NT

[dom]

Those hippy books are the bane of anyone with more than an armchair interest in ecobuilding.

Particularly when it comes to receiving christmas presents.

Drop the author a note recommending the illustrations be redrawn to show more people wearing hippy clothes and HSS Hire delivering the access tower by horse and cart.

However if you're interested in a general book on alternative energy, have a look at

available as a free pdf download

[Andy Dingley]

at

> Also available as a free pdf download Bloody marvellous book.

[Andy Dingley]

What for ? "Domestic water heating by solar" and you've a point (Works, but capital payback is a problem) "Domestic space heating by low-temperature water systems from a solar input" is eminently practical and these days it's affordable. The trick is to do the useful proportion of the problem (i.e. not even trying to make hot bathwater), without spending quite so generously from the capital budget.

[Harry Bloomfield]

Because doing that the solar heated loop would _cool_ the water in the single tank. The idea would seem to be to just pre-heat the water from source, before it gets into the main hot tank - which sounds to have some merit. It cannot of course provide hot water by itself.

[David Hansen]

On Sat, 16 Jan 2010 12:04:48 -0000 someone who may be "David WE Roberts" wrote this:-

When was the book written?

Pre-heating cylinders date from the days when many people didn't really understand solar water heating and/or did not want to disturb the existing system. A single cylinder is a much better option, though it does push the price up.

[David Hansen]

On Sat, 16 Jan 2010 07:00:09 -0800 (PST) someone who may be " [e-mail]" wrote this:-

removed many of the earlier howlers.

Also it is worth pointing out that the book is not about DIY installations, which is what the discussion is about. It attempts, with varying degrees of success, to look at energy balances on a large scale.

The problem is that he is a scientist, and scientists usually make very bad engineers. The things he is discussing in the book are engineering, not science. The rather simplistic calculations he makes often suit small scale systems, but not large scale systems. For example he is spot on with regard to energy saving light bulbs. I'm amused that those on here who foam at the mouth about energy saving light bulbs tend to be the ones who recommend this book strongly. Perhaps they haven't read it, or they think that those who laugh at them will not read the book and note the inconsistency.

[Tim W]

[e-mail] wibbled on Saturday 16 January 2010 15:00

Oi - a real hippy would make his own bamboo scaffold.

And he's grow it first. And not harvest it until the pandas had had their fill of the finest shoots...

[David WE Roberts]

The idea of the preheat tank is to get 2 benefits rather than one:

1) When the preheat tank produces hot water in peak summer, this hot is fed into the main tank whenever HW is drawn off, resulting in little conventional heat used. ** This assumes that you start out with a tank full of hot water heated by some other means - you are saving on the reheating costs by warming the incoming cold water but you are still relying on another source of heat to provide the initial hot water ** 2) it prewarms the water all the time the system stays above freezing, feeding the main tank with lukewarm water instead of cold. The preheat tank stores lots of this prewarmed water, whereas just using the bottom of the main tank to do this would only store a little.

Its a fair way to make use of low performance panels.

** I think this must assume that you will not produce a high enough temperature in the panel to get useable hot water in a main tank (for most of the year?), and so with a low temperature difference between the panel and the tank contents the most effective way to store heat is a lot of slightly warmer water. Still requires another heat source all year round. I was expecting to be able to produce useable hot water by solar heating alone in the summer and not have any other heating running. The booklet, of course, explains none of this.**

Re costs, I think the assumption is generally that the preheat tank is an unwanted used tank costing little.

** Hmmm.....corroded and with the heat exchange furred up? **

I'n not disagreeing with your overall take on it, but a couple of the points seem fair enough.

** I suppose you could always fit a thermostatic (mixer?) valve on the preheat tank and draw hot water off directly if it is within a certain temperature range. This avoids feeding through the main tank if the water is warm enough and avoids scalding hazards if the midsummer sun manages to boil the preheat tank. The strategy for winter and summer are probably markedly different.** [Don't know what it is about your post but Windows Mail isn't putting chevrons '>' in.]

[Adrian C]

They do.

[The Natural Philosopher]

its on the second edition in print, now.

Sadly David, the physics that govern small scales systems also govern large scale ones.

You have a small point in that David is a physicist, and his book is only damning about alternative energy where it shows it to be completely impossible to make any difference whatsoever.

He never approaches the cost and environmental impact of any eco solutions at all.

If you do that, the answer are even bleaker. WE cant afford eco friendly power. We would be bankrupt long before we achieved carbon neutrality Unless we build nuclear power stations. The rest is meaningless ecobollox, with one exception. heat pumps.

All so called 'green' solutions are political solutions, not engineering solutions. Including those ghastly environment wrecking windmills that your livelihood depends on.

[tony sayer]

In article , [e-mail] scribeth thus

IMHO of course;)....