Header Image SPF Testing Speicherschichtung

SPF Testing – Storage Tank Stratification

The stratification efficiency of a storage tank has a decisive influence on the efficiency of the entire heating system. Especially when using heat pumps, there is a direct connection between the demand for electrical energy and the stratification efficiency of the storage tank. With a well stratified storage tank, the electrical consumption of a hot water system can be reduced by up to 40%

Testing and Certification

The certification of the stratification efficiency according to the SPF method enables the registration of the storage tank in the Heat Pump System Module (WPSM) of the Swiss Heat Pump Association FWS. For this, a stratification efficiency of more than 75 % must be proven


The method developed at SPF to determine stratification efficiency uses entropy in the storage system as a measure of mixing. For this purpose, the storage tank is installed including the hydraulics for loading and unloading. The loading of the storage tank via a heat pump and collectors, as well as the heat load for space heating and hot water production, are emulated. For this purpose, a reference profile is stored that contains weather data and the heat load of a single-family house.


Thermal storage tanks for space heating and domestic hot water, where no potable water is passed through the tank, may allow assessment based on submitted detailed tank design drawings and usage information. The fresh water module must also be approved. The conditions from the WPSM's specifications for heat storage tanks and fresh water modules must be met.


WPSM working documents and forms (German)