Copyright 2018, Michael Bryant
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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)