CPSU and Thermal Stores.
A CPSU or Combined Primary Storage Unit is a boiler with an integral thermal store. The thermal store must be greater that 70 litres, otherwise the system is simply a ‘storage combi’ and in RdSAP would be entered as a combi.
A thermal store is a vessel for holding hot water, but unlike the hot water cylinder that provides domestic hot water with a regular boiler, the hot water within the store provides the hot water to the radiator system.
The boiler heats the water in the thermal store directly and within the store is a heat exchanger (coil of pipe) that provides the hot water.
Sometimes a CPSU will have the thermal store separated from the boiler unit. In these cases the RdSAP convention is to enter the heating system as a regular boiler with cylinder. The volume and insulation of the thermal store should be recorded against the hot water cylinder.
The diagram below gives a schematic view of how the CPSU differs from conventional boilers. The red lines indicate water heated directly by the boiler and the black lines indicate water from the cylinder or store that is heated indirectly.
A more in-depth look at CPSUs and thermal stores can be found in Appendix B of SAP2012 available from https://www.bre.co.uk/sap2012