Sizes a domestic hot-water cylinder by occupancy: recommended stored litres, the next standard cylinder size, and the heat-source recovery duty for your chosen reheat period.
Tender-stage only. Verify final design to project spec and CIBSE Guide G.
For design guidance only. Always verify with a qualified engineer.
About this DHW storage sizing calculator
This free DHW (domestic hot water) storage sizing calculator sizes a hot water cylinder by occupancy, returning the recommended stored volume, the next standard cylinder size and the heat-source recovery duty for your chosen reheat period. It is aimed at UK plumbing, heating and building-services engineers sizing cylinders and heat sources for dwellings, hotels, offices and schools at tender stage, using the CIBSE Guide G-lineage occupancy tables. Everything runs in your browser. Nothing is uploaded.
How DHW storage and recovery duty are calculated
Storage volume is occupancy × litres-per-occupant from the CIBSE Guide G-lineage table: 45 L per dwelling occupant, 45 L per hotel bedroom, 23 L per boarding-school pupil, and so on. The recovery (reheat) duty comes from sensible heat: ΔT = stored − cold-feed temperature, mass m = volume × 1 kg/L, and energy Q = m · cp · ΔT with cp = 4.186 kJ/kg·K, converted to kWh (÷3600). The heat-source rating is (Q ÷ recovery period) ÷ efficiency. The recommended cylinder is the next standard UK stocked size at or above the storage volume.
Frequently asked questions
Why is the stored temperature 60 °C by default?
60 °C is the usual design store temperature for Legionella control. The cold-feed default of 10 °C is a typical UK design value, giving a 50 K temperature rise. Both are editable if your project specifies otherwise.
What recovery period should I use?
CIBSE Guide G design practice is roughly 1–2 hours. A shorter recovery period reheats the cylinder faster but needs a larger heat source; the calculator shows the recovery duty in kW so you can trade the two off.
What if the volume exceeds the largest cylinder?
Beyond the largest standard size the calculator flags that multiple cylinders or specialist plant are required. The recovery duty figure is still meaningful for sizing the heat source. Verify the final design to project spec and CIBSE Guide G.