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Solar hot water in the UK - advice please - long!

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Posted by Adrian Brentnall on August 22, 2004, 4:14 pm
Hi All

I have a solar hot water system installed out here on the East Coast
of England (Suffolk).

It consists of
Thermomax evacuated collectors on the roof
A tall, narrow hot water tank (1.5m x 450cm) - with two coiled heat
exchangers - bottom one is for the solar circuit - top one it going to
link into the woodburner.
Thermomax controller for the solar circuit
Standard 240V mains immersion heater / thermostat fitted at the bottom
of the cylinder, with timeswitch.

All works really well - even on not-so-sunny days. Today it's working
very well - temperature half-way-up the tank (bottom of solar coil) is
about 75F.

Anyway - why am I writing this ??

Two things. Because of the tall, thin tank, I suspect that I'm getting
reasonable stratification of the hot water. However - the thermostat
on the electric immersion heater, which is set to come on in the
evening to top-up the hot water as necessary, is sensing the
temperature of the colder water at the bottom of the tank - and still
runs, even when the water in the tank is already quite warm enough....
There's only 2 of us in the house - so we don't use vast amounts of
hot water.
On reflection, it would have been better to have the immersion boss &
stat on the tope of the tank... but it's a bit late for that <g>
I was thinking of fitting a strap-on thermostat half-way up the tank -
and feeding the immersion through that - any comments / caveats ?

2nd thing ....
We live in an area which has lots of woodland - which is regularly
harvested by the owners. They don't mind us peasants collecting
firewood for a nominal charge - so we have three nice big woodsheds
full of firewood for the winter. The woodburner/multifuel stove feeds
a wet radiator central heating system, and works quite nicely (can be
supplemented by solid fuel if necessary). Last year I fitted a
diverter valve to the wet heating circuit - so that I could push the
hot water from the woodburner either round the radiators, or through
the upper heat exchanger on the hot water cylinder. This year I need
to fit a simple differential controller to control the valve. The hot
water from the boiler is between 40 - 60 Centigrade - so I was
planning on a very simple circuit (maybe using a couple of silicon
diodes as temperature sensors) just to look at the temperature of the
circulating water and the temperature of the water tank about the top
of the heat exchanger - and push boiler water throught the heat
exchanger when the tank was cooler than the circulating water. Any
comments on this - is my 'control logic' OK - or am I making it too
simple ??

Many thanks in advance - hope it's clear - sorry it's so long - but
difficult to explain more simply......

Suffolk UK

Posted by Cosmopolite on August 22, 2004, 11:44 pm

Adrian Brentnall wrote:

You should also post your last question on   sci.engr.control  , if
your ISP has that NG. They are very knowledgeable about
control systems, simple or complex.

Posted by pete on August 23, 2004, 12:01 am
 You require a 'differential temperature controller'. This is a common type
of controller, and can be constructed from a simple op-amp. Alterantively,
you can buy a commercial unit - for example, mine costs 65 including VAT
and two temperature sensor probes. If you need one, please fee free to
contact me (sales at navitron dot org dot uk)


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Posted by Adrian Brentnall on August 23, 2004, 7:34 am
 Hi Pete

Ah that's good - I'm on the right track then <g>
Thanks for the offer of a ready-built unit - but I'm pretty sure that
I have the bits & pieces I need to make one.

I was considering using a couple of those temperature-measuring ICs -
but, on thinking about it, I really want to measure 'differences'
rather than absolute temperature - so I'm leaning towards the silicon
diode approach - possibly with a 555 timer on the output to save the
thing from 'hunting' due to fast variations in temperature.

The 'logic' would look like this :-
IF boiler temperature is greater than tank temperature by (say) 4
degrees C then divert boiler water though tank for a couple of minutes
ELSE leave boiler water circulating through radiators.

Sound reasonable ?

The Solar controller I've got actually has 3 sensors - one on the
tank, one on the collector and another in the collector circuit on the
return water to the collector - but I think that's probably overkill
for boiler / hot water situation.

Thanks again
Suffolk UK

Posted by nicksanspam on August 23, 2004, 5:37 pm

You might look at


I like the one on the left.


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