How do I size a hot water storage cylinder?

Enter building type, fixture counts, or loading units. The calculator estimates daily demand and storage volume with recovery time for the selected heat source.

What recovery time should I allow?

Typical practice: 30–60 minutes for commercial calorifiers, longer for high-demand applications. Confirm with the equipment manufacturer and project brief.

How does loading unit demand relate to storage volume?

Loading units convert to peak diversified flow, which drives both pipe sizing and the storage/recovery balance for the system.

BuildFlo Design Guide · Water · CIBSE G / BS 8558

Hot water storage and calorifier sizing calculator

Size calorifier storage volume and recovery from building type or loading units. Free UK hot water demand calculator.

What this calculator does

Hot water storage helps UK building services engineers with size calorifier storage and recovery from fixtures, occupancy, loading units, or peak l/h. The calculation follows CIBSE G / BS 8558 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "hot water storage", "hot water storage calculator", or "how to hot water storage", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: litres.

Hot water storage calculator

Size calorifier storage and recovery from fixtures, occupancy, loading units, or peak L/h

Building type

Sets fixture demand rates (L/h), load factor, peak draw-off period, recovery time, and occupancy defaults

Input modeShowersWash hand basinsBathsSinksKitchen sinksDishwashersWashing machinesOccupancy / rooms / dwellings

People, guest rooms, dwellings or beds — depends on building type

Peak demand per unit (L/h)

Peak-hour litres per person / room / dwelling / bed

Total hot-water loading unitsDiversified peak hourly demand (L/h)Usage frequency

CIPHE LU table — mainly for Custom / LU mode

Peak draw-off period (hours)Recommended recovery time (hours)

Time to reheat storage from cold — industry default is often 1–2 hours

Load / diversity factorHeat source input (kW)

Optional — check recovery time for a given boiler or heat exchanger duty

Storage temperature (°C)Cold water inlet temperature (°C)Storage availability factor

Usable fraction of store due to stratification (typically 80%)

All calculators →Save results in BuildFlo →

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about hot water storage under UK standards.

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Frequently asked questions

How do I size a hot water storage cylinder?: Enter building type, fixture counts, or loading units. The calculator estimates daily demand and storage volume with recovery time for the selected heat source.
What recovery time should I allow?: Typical practice: 30–60 minutes for commercial calorifiers, longer for high-demand applications. Confirm with the equipment manufacturer and project brief.
How does loading unit demand relate to storage volume?: Loading units convert to peak diversified flow, which drives both pipe sizing and the storage/recovery balance for the system.