18 years ago
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. Yesterday it was
working very well - temperature half-way-up the tank (bottom of solar
coil) was about 75F.
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
This year I need to biuld & 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.
(I know that this isn't 'very hot' by normal standards - but if you
run the woodburner any hotter you have to open windows in the Winter
- so I was going to use the woodburner to 'pre-heat' the hot water
tank and reduce the amount of worh the electric immersion has to
Any comments on this - is my 'control logic' OK - or am I making it
too simple ?? I was planning on something like :-
IF the boiler water temperature is more than 4 degrees C higher than
the tank temperature
THEN flip the diverter valve to push boiler water through the tank
heat exchanger ofr a couple of minutes (preset timer)
ELSE leave the boiler water circulating through the radiators
Many thanks in advance - hope it's clear - sorry it's so long - but
difficult to explain more simply......