Use of unvented cylinder as a heat store?!

21 Sep 2008
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United Kingdom
Please....can anyone point out the safety issues of using an unvented cylinder as a heat store (A sealed system closed circuit). It will be pressurised to 1 bar with a big expansion vessel and a 3 bar prv. Also there is the 8 bar temp / pressure relief also fitted on the cylinder. I can only think of one issue whereby there is greater risk than normal use with mains pressure DHW.
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I suppose you could...why not just use a thermal store as a thermal store?o
Because I don't have a thermal store but have two unvented cylinders available for use...!
Point is that usually a thermal store is open vented but i wanted to use the cylinder in a sealed system - the whole 250 litre volume as part of that sealed system (primary system water inside it).
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Thank you for your kind offer...!

So it runs at normal heating system primary pressure, same safety devices as normal mainsprpressure invented cylinder?

The only safety difference I can imagine is that in an overheat condition when the t/p valve fires there is no cold water rushing in to cool down the cylinder.
That was my worry at first when I ordered mine (now no longer in use) - along with a collapse of the cylinder. Not a problem with a gas boiler but if your doing Solar thermal then it is a proper concern.

You can get a tempering device that pushes mains water through a coil at the top of my store to cool it, dumping the heated water to the drain. Doubt you'll be able to retro fit it to a Megaflow though.

An anti vacuum device might help though, but it won't help cool the cylinder in an overheat situation and could make matters worse. However, I am multi tasking at the moment and not really thinking things through fully.
Does not the T&P valve have an anti-vac function built into it?

At the moment I have a solar unvented cylinder for the DHW and plan a secondary normal unvented cylinder as a heat store, whose coil is switched in by a motorised valve when the DHE cylinder is satisfied. The effect of the heat from the coil is no different from a boiler except in stagnation conditions, but in that instance there is a low fluid volume and the circulation is off. Plus there will be a heat dump fitted. The safety thermostat will cut off power to all heat sources, including solar pump, in the event of high cylinder temperature. As long as the stat doesn't fall off or fail. Then we would be relying on TP valve. Worst case scenario the solar pump stays on
against all orders, and the TP valve goes off at 90° could be bit of a steamy event...
I don't think Gledhill make sealed system thermal stores. Nu heat supply one as part of a package for UFH.

Sealed thermal stores are common in solid fuel systems. (Akvaterm)
You would be better off selling your unvented cylinders and buying a sealed system thermal store.
I don't see a great problem using yours Axel.

Also ask Mrs Om to dust off her saree in case there is a party in a couple of weeks.
I do. Plus the installation is outside the UK anyhow.

Brexit means " Get the **ck out of the UK before it implodes...
Maybe outside the UK it will be covered by even more EU regulations!
Why not use it open vented? No reason that you have to use it sealed just because that was it's original intention. It's just very over-engineered for open vent.

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