Thermal Stores


British by Design


These are a british concept, and come in two forms, direct and indirect. The direct thermal stores work on the idea of keeping a tank of water hot full of central heating water, and then passing a coil through it with cold water, which comes out hot the other end. These tend to suffer with water problems such as sludge and oxidisation. They are also more expensive to run than standard cylinders, you are in effect running two central heating systems as you heat up at least 140 litres of water in the tank, turn on the heating, this water goes round the radiators, and is then re-heated again.They are a hybrid of standard cylinder because they are vented (open to atmosphere), but they give you mains pressure hot water.

Gledhill Torrent Thermal Store


We also have indirect thermal stores, with two coils inside, one to heat the bulk of the water, and one for the hot water delivery. These are much better than direct, as any work on the central heatiing system does not require you to drain the whole cylinder down. You can also powerflush these systems, whereas with direct you are hindered by their design. You need an extra tank for these, one for the cylinder, and one for the heating circuit. You can have an immersion heater for thermal stores, as a backup or as a main heat source instead of connecting it to the heating system. With these cylinders you normally have two pumps, one from the boiler to the cylinder, and one from the cylinder to the radiators. With both set-ups your hot water temperature is fixed by the blending valve pre-set temperature. This may hinder some people who like to vary their water temperature if they have children visiting for instance. These take up a lot of space in your airing cupboard and are very heavy when full.

Please note that some boilers are not fully compatible with these cylinders in the way they heat up. Some boilers may get too hot too quick, and then take some time to come back on again. For example. your boiler may take 30 seconds to get hot, cut out, then be off for 5 minutes before it comes back on again. So its off for longer than it's on, resulting in poor performance. These are rare, but be aware there are boilers out there that won't work with thermal stores.


Advantages Disadvantages
Mains pressure hot water Expensive to run
Quick radiator heat up time Direct; problems with system water/flushing
Immersion as back up Expensive to replace
No permission from building control required Direct; work on system means full drain down
No strict safety rules needed as in unvented Some boilers won't work with this set-up