DHW storage cylinder sizing

The calculator sizes DHW storage cylinders for residential buildings, hotels and dormitories according to DIN 4708. It first determines the required demand number N from apartments, occupants and fit-out standard, or from rooms, beds and object category. It then compares that demand with the catalog performance number N_L of a cylinder. The result is not a simple conversion from N to litres. DIN 4708 requires selecting a cylinder or cylinder set for which the manufacturer declares N_L ≥ N under defined temperatures, source power and heat-exchanger conditions.

How DIN 4708 Is Calculated

DIN 4708 reduces building demand to the dimensionless demand number N. One N corresponds to a reference apartment with a defined bath draw and DHW use. For residential buildings the calculator sums apartments, occupants and equipment standard. For hotels or dormitories it accounts for rooms, beds, category and occupancy. After N is calculated, the selected catalog unit must have a performance number N_L not lower than the required demand. N_L is a property of a specific cylinder, depending not only on volume but also on exchanger area, loading power, water temperatures and test conditions. For that reason the calculator shows a catalog example, not an invented litres-per-N conversion.

Catalog Selection N_L ≥ N

Capacity in litres is the outcome of catalog selection, not the direct result of DIN 4708. The same cylinder can have a different N_L at a different source power, supply temperature or coil area. Treat the result as an example device meeting N_L ≥ N and verify it against the current manufacturer data sheet. For multiple cylinders the calculator sums catalog N_L as an approximation for preliminary sizing. The final design must confirm the hydraulic arrangement, loading method, source power and manufacturer data for parallel operation.

Heat pump compatibility

At 55/45°C supply the LMTD is ~4× lower than a boiler 80/60°C, so the coil needs proportionally more area. Rule of thumb: ≥0.3 m²/kW of HP output. The calculator warns when below this — use HP-dedicated cylinders or buffered systems.

Legionella protection

Periodic 70°C shock heating (mandatory in Polish and EU regulations) requires extra power. The calculator computes it separately from the working coil duty — shock is typically scheduled at night when the source is not loaded by draw.