Heat stores, thermal stores, tank in tank, sounds complicated, not sure which tank to use?
The basics of any heat store (also known as a thermal store, thermal storage cylinder or buffer tank) are that the cylinder accepts hot water directly from a boiler and stores it until the heating system calls for it. Because the heat store should be insulated, the water will stay at optimal temperature for a period of time. Once it dips below the temperature set, the tank should call for more heat from the boiler. A quality thermal store with a good integrated insulation jacket can help reduce the amount of fuel used for on-going heat applications.
If this basic type of tank is used and domestic hot water (DHW) is also required, then a separate heating tank must be installed for the heating of mains potable (drinking) water.
A tank in tank cylinder also stores water for the heating system but it also has an internal tank or coil that heats domestic hot water (DHW) using the heat from the water used for the heating system.
Both of the above can have one or two heating coils as extra components (we always recommend two but it customer choice as it depends upon customer need as to which system is required). Rather than feed the hot water straight from the boiler to the tank, the heat would be issued to one of the coils and this would heat the water stored in the tank. As this would be utilised through the top coil, you would use less energy to reach the optimal temperature for heating of the house as you are not attempting to heat the whole tank of water. The top of the cylinder would also be the area that the DHW tank would be in so this would also benefit from this scenario.
The second (bottom) coil would be used for an extra heat source such as solar thermal. Having access to solar thermal essentially gives free energy that can used to warm the mass of water from the bottom up so that the boiler does not have to work so hard in maintaining the temperature. This, in turn, saves you money against which ever method of fuel you are using to heat via the tope coil as you are using less of it to heat your house.
One thing to remember about heat stores. Make sure that you can fit them into your house. They are large and bulky items, especially with an integrated insulation jacket. Ours come with a jacket that can be zipped off for access purposes and then replaced but it may be that others do not. It can be a costly business taking out windows, doors and even walls to fit something like this in a house so buyer beware!
Written by Andy Holburn 16/03/12