Calculating the correct buffer tank size

Calculating the buffer tank size for wood log boilers

Stratification buffer tanks make the application of solid fuel heating systems almost as easy and comfortable as oil and gas boilers. Our wood log boilers produce significantly more energy with one boiler filling during the burn-off period than the heating system needs at that moment. The surplus of energy provided by the heat generator is stored in the buffer tank. After the fire in the wood log boiler is burned down the heat of the buffer tank is at the building’s and maybe at the hot water generation’s disposal.
The system can be fed with heat without permanently fueling the boiler. When dimensioning the wood log boiler system it is one’s goal to generally heat up the wood log boiler only once or twice per day in order to supply the building with heat twenty-fourseven. At the same time the heating comfort is increased by that.
The size of the buffer tank should be chosen in such a way that the amount of energy resulting from one boiler filling can be stored completely in the buffer tank. For an excellent operation of the system it is necessary to use a stratification buffer tank with smart layering and efficient energy utilization.

Rule of thumb for calculating the buffer tank capacity for wood log boiler systems:

• a minimum of 55 liter per kW nominal boiler output is legal obligation
• even better: 80 liter per kW nominal boiler output
• alternative calculation: 12 ltr heating storage volume each ltr volume of the boiler fuel chamber
• with standard buffer tanks you should not use more than 100 liter per kW nominal boiler output unlike the high performance Solarbayer buffer tanks because of the exact heat layering you can apply tanks with an even larger capacity

The buffer tank capacity should also perfectly match the application of other renewable energy sources. In practice a total tank capacity of 50-70 liter per m² collector surface area has proven as useful. If a bigger tank capacity should be necessary due to wood log boiler size (see top) as for the calculated optimal dimension of the solar system, than you have to take care that the hydraulic integration of the solar system in order to load the tank has to take place in row, e.g. SLS with 2 heat exchangers resp. several buffer in a row. It is important that idleness is guaranteed when applying the collector and system technology.

Calculating the buffer tank size for solar systems

With solar thermal systems the solar heat does not occur at the time as the heat requirements. Most heat is needed in the morning and in the evening - either for room heating or for the preparation of hot water. This always requires storage of the solar energy in a buffer or DHW tank.
The size of the solar tank has to be aligned to the size of the collector area. If the tank’s capacity is too big no useful temperatures will be achieved in the tank, if the tank is tank’s capacity is too small the available solar energy is not used efficiently. Our technicians will be glad to advise you.

Rule of thumb for calculating tank systems for solar thermal use:

energetically perfect: per m² aperture surface approx. 50 liter buffer tank capacity.
Bigger tank capacities can store more solar energy and are economically reasonable.
The correct hydraulic integration is important in that case.