First of all what is a thermal store and what does it do? Essentially it’s a large water tank (usually round) which is well insulated with a series of bosses to connect pipes to. (It should not be confused with a domestic hot water cylinder).

It can be constructed from mild steel or any other material; it is not pressurized to mains pressure, it is the same pressure as the heating system. The water in the store is not drinking water; it is the same water that circulates through the boiler and radiators.

The reason it is called a thermal store is that it has the ability to ‘store’ heat, essentially like a heat battery. This makes it so appropriate to use in a biomass installation, whether using logs, wood chip, or pellets.

Most biomass boilers are constructed more heavily than their fossil fuel equivalent, so they tend to take longer to heat up and longer to cool down. The residual heat energy needs to be absorbed safely within a heating system and this is where a thermal (or buffer) store comes into play.

The boiler flow and return is connected to the thermal store and then the heating system is plumbed into the thermal store. The buffer becomes an interface between the components of the heating system. This makes it possible to introduce several heat sources, which could include biomass, fossil fuel, solar thermal or indeed an electric immersion heater. By using a thermal store, the heating system doesn't usually need to be modified.

It is also possible to use the heat in the thermal store to provide clean hot water using a heat exchanger (either a coil within the tank, which when connected to a water supply is heated and provides hot water, or an external flat plate heat exchanger). Water from the store is pumped around one side of the heat exchanger, transferring heat to the clean water flowing around the other side. This is the same technology used by combi boilers, however with a thermal store and heat exchanger you can ensure high flow rates of hot water.

When sizing the store you need to consider a number of factors. A log boiler usually needs a larger store than a pellet boiler, and at TRECO we would not use a store of less than 1,500 litres. We recommend it has at least 20 litres capacity per kWh output of your boiler. For high flow rates of hot water we use a larger store.

Thermal stores help even out the peaks and troughs of heat demand throughout the day. Imagine a boiler running at a constant 10kWh, but heat demand is 15kWh at times and 5kwh at others. The thermal store releases additional stored heat into the heating system during peak demand and stores excess heat energy when there’s lower demand. This is achieved by changes in the water temperature of the store. If we take a theoretical store of 860 litres, then a 1 degree C change in temperature will release or store 1kWh of energy into the system.

Photo by Ordinary Guy

About the author: Gordon Traill is a founder director of TRECO

If you have a question about anything in the above blog, please ask it in the comments section below.