Heating & hot water
Hot and cold flow for a target mix
- where
- hot flow (L/min)
- total mixed flow (L/min)
- target mixed temperature (°C)
- cold supply temperature (°C)
- hot supply temperature (°C)
- cold flow (L/min)
Explain this equation
Works out how much hot and cold flow make up a given total mixed flow at a target temperature. Use it to estimate draw from the hot supply at an outlet.
q is the total mixed flow. T_h, T_c and T_m are the hot, cold and target temperatures. q_h is the hot share and q_c the cold remainder.
(T_m − T_c) / (T_h − T_c) is the fraction of the mix that must be hot water. It comes from the same heat balance as the mixed-temperature equation, solved for the hot flow. Whatever is not hot must be cold, so q_c = q − q_h.
Sense check: 12 L/min at 40 °C from 60 °C hot and 10 °C cold needs 7.2 L/min hot and 4.8 L/min cold. Raise the stored temperature and the hot share drops.
Watch out: the target must lie between the cold and hot temperatures. It is an ideal balance, not a TMV commissioning result.
Enter your values to see the result.
Good to know
Ideal same-fluid heat balance; not a TMV commissioning result.
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.