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Calorifier Mixing Valve / Usable Storage Calculator

Use this Calorifier Mixing Valve / Usable Storage Calculator to estimate the usable hot water available at a blended delivery temperature when stored hot water is mixed with cold water through a thermostatic mixing valve.

Input Parameters

Litres

°C

°C

°C

%

Calculation Results

Usable Multiplier — × stored
Effective Stored Volume — L
Usable Hot Water at Delivery Temp — L
Cold Water Blended In — L

Calculation Formulas
  • Effective Stored = Volume × (Draw-off / 100)
  • Blend Ratio = (Storage Temp − Cold Temp) ÷ (Delivery Temp − Cold Temp)
  • Usable Volume = Effective Stored × Blend Ratio
  • Cold Blended = Usable − Effective Stored

A thermostatic mixing valve blends stored hot water with cold water down to the delivery temperature, so the usable volume at that temperature is greater than the stored volume. The draw-off factor accounts for stratification and the fraction of the tank that is practically usable.

How to Use This Calculator

  1. Enter the calorifier storage volume.
  2. Enter the storage, cold water and blended delivery temperatures.
  3. Set the usable draw-off factor for stratification.
  4. Read the usable hot water available at the delivery temperature.
  5. Download a PDF report of the calculation.

Frequently Asked Questions

What is usable storage in a calorifier?
Because stored water is hotter than the delivery temperature, it can be blended with cold water. The usable volume at the delivery temperature is therefore larger than the stored volume.
How does a thermostatic mixing valve affect usable storage?
A mixing valve blends hot stored water with cold water to a safe delivery temperature, effectively increasing the volume of water available at that temperature.
How is the blend ratio calculated?
It is the temperature rise of the stored water divided by the temperature rise of the delivered water: (Storage − Cold) ÷ (Delivery − Cold).
What is the draw-off factor?
It accounts for stratification and dead volume, representing the fraction of the tank that can realistically be drawn off as useful hot water.
Why deliver hot water at a lower temperature?
Storing at a higher temperature helps control legionella risk, while blending down to a lower delivery temperature protects users from scalding.

Results are estimates based on the values entered and the calculation method shown. Actual system requirements may vary depending on operating conditions, demand patterns, equipment configuration and applicable project requirements.