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Hot Water Storage Cylinder Calculator

Estimate hot water storage cylinder volume and recovery duty from demand, storage period and temperature rise.

Use this calculator for early-stage domestic hot water storage sizing where a project hot water demand is known or has been estimated separately. It can be used to estimate the required stored hot water volume together with an indicative reheat or recovery duty for a calorifier.

Inputs

This calculator converts mixed hot water demand into equivalent stored 60°C water volume using a simple mixing relationship, then estimates calorifier recovery duty from the stored volume and reheat period.

Result

Calorifier Size
-
Enter values and click calculate.

Method

Demand over the selected storage period is estimated as a proportion of daily hot water demand:

Demand over Storage Period = Daily Demand × (Storage Hours ÷ 24)

Mixed outlet demand is converted to equivalent stored hot water volume using:

Stored Volume = Mixed Demand × ((Tₘ - T𝒸) ÷ (Tₛ - T𝒸))

An allowance is then added:

Storage Volume = Stored Volume × (1 + Allowance ÷ 100)

Recovery duty is estimated from:

Duty = (Volume × 4.18 × ΔT) ÷ Reheat Time
  • Tₛ = stored hot water temperature (°C)
  • T𝒸 = incoming cold water temperature (°C)
  • Tₘ = mixed outlet temperature (°C)
  • ΔT = stored temperature rise above incoming cold water (K)
  • Storage Volume = required calorifier storage volume (L)
  • Duty = indicative calorifier recovery duty (kW)
  • Calculated result = -

Reference

  • This calculator provides an indicative calorifier storage and recovery estimate based on hot water demand, mixed outlet temperature and reheat period.
  • The conversion from mixed water to stored hot water volume is based on a simple blending relationship between stored hot water and incoming cold water.
  • Recovery duty is estimated using the specific heat capacity of water and assumes a simple sensible heat calculation without standing losses.
  • Relevant guidance may include CIBSE Guide G, BS EN 12897, HSE ACOP L8 guidance, and project-specific domestic hot water design requirements.
Design note: This calculator is an indicative estimating tool only. Final design should consider actual demand profiles, peak usage patterns, diversity, storage temperature strategy, pasteurisation requirements, recovery periods, primary flow and return temperatures, coil performance, standing losses and project-specific compliance requirements.

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