DOMESTIC HOT WATER

Cylinder & recharge calculator.

A straightforward starting point for working out cylinder storage, recharge time, and the heat output needed to hit a chosen recovery window.

START HERE

Cylinder sizing

Household occupants (people) Bedrooms

Suggested minimum storage 135 L Occupant rule: 135 L · Bedroom rule: 135 L. Both rules are equal.

A quick domestic sizing guide: take the greater of 45 L per occupant or 45 L for every bedroom plus one. It is a practical starting point, not a substitute for checking the household’s actual demand profile, cylinder usable volume and system design.

SHARED HEATING FIGURES

Use the same cylinder in both recharge checks.

Cylinder volume (litres) Cold feed temperature (°C) Recharge start temperature (°C) Target cylinder temperature (°C)

Theoretical 40°C water at taps: 300 L

Heat required: 10.47 kWh

The tap-water figure assumes the full cylinder is at the target temperature and is mixed only with the stated cold feed. It excludes draw-off, pipe and standing losses.

KNOWN HEAT OUTPUT

Recharge time

Heat output delivered to the cylinder (kW) Ideal recovery time 3 hr 29 min Based on the shared cylinder values above

KNOWN RECHARGE WINDOW

Required heat output

Target recharge time (hours) Heat output needed 4.19 kW Delivered into the stored water