Thermal stores look complicated, but they are very simple. They are large
tanks of stored hot water.
Ok, now they get complicated...
They differ from normal hot water cylinders in that the hot water they
contain stays in the tank indefinitely, instead of being fed to the hot taps as
with a normal hot hot water cylinder. This permanently stored hot water is then
used as the heat source for heating mains pressure hot tap water and (sometimes)
water-filled central heating radiators.
The thermal store is heated by a normal gas boiler or by an immersion heater
running on low cost night-rate electricity.
Early thermal stores (such as the Gledhill Pulsacoil 3) have a coil of copper
pipe inside through which mains water flows on it's way to the hot taps, and the
water is heated through the wall of the pipe as it passes through. A simple
system with nothing much to go wrong eh? Nope! There is a major problem in hard
water areas.... Over a year or three, the copper coil gets blocked with water
scale, and need expensive chemical descaling to restore performance. Most
thermal stores using this principle also use a thermostatic blender valve to
prevent the system deliveing scalding hot water to the hot taps and this blender
valve also fails regularly due to water scale.
The answer to all this is to add a layer of extra technology. A plate heat
exchanger and pump (as featured by the Gledhill Pulsacoil 2000) supposedly fixed
this scaling problem. Plate heat exchangers are said to be far less susceptible
to scaling, but they have to be fitted externally and need some electronics and
a pump to to control them. In my experience plate heat exchangers also scale up,
so I'm not sure they are much of a step forward. They get rid of the blender
valve though, but introduce thermistor flow temperature sensors which also fail.
Thermal stores can also drive a conventional central heating radiator system.
Just add a pump and room thermost, and the stored hot water is circulated around
the rads to heat the house or flat. Bigger units are needed though, with more
electronics to control the pump and the re-heating. The Gledhill Sytemate is a
good example of a combined hot water and central heating thermal store.
As you can probably guess, I am getting more and more calls these days to
repair thermal stores. So many in fact that I am now a recommended repair
engineer for Gledhill 'out-of-warranty' repairs. Should you ring Gledhill and
ask them who can fix yours, they will give you my name if you live in Berkshire.
Thermal stores seem to spook many repair engineers (maybe because they are
full of electronics), but I find them quite easy to fix. The majority of
problems taking an hour or two of labour plus parts and VAT.
(I've written more about PulsaCoils, their common faults and repairs here: www.pulsacoil-repairs.co.uk)