Heating & hot water
Usable mixed-water volume
- where
- usable mixed-water volume (L)
- stored hot volume (L)
- stored temperature (°C)
- cold temperature (°C)
- delivery temperature (°C)
Explain this equation
Works out how much water at a usable delivery temperature you get from a store of hotter water once it is mixed with cold. Use it to compare cylinder sizes and storage temperatures.
V_h is the stored hot volume. T_h is the storage temperature, T_c the incoming cold and T_m the temperature delivered at the outlet. V_m is the total mixed volume.
Each litre of stored water holds T_h − T_c degrees of heat above cold. Each litre delivered needs only T_m − T_c. The ratio of the two tells you how many delivered litres each stored litre makes.
Sense check: 150 L at 60 °C, 10 °C cold, delivered at 40 °C gives 250 L. Store at 65 °C instead and you get 275 L.
Watch out: this assumes the whole store is usable at full temperature. Stratification, mixing in the cylinder and pipe losses reduce the real figure.
Enter your values to see the result.
Good to know
Ideal mixing; stratification, unusable storage, recovery and distribution losses are excluded.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.