Using a simple mixing balance to work out how much hot and cold water gives a safe delivered temperature.
When hot and cold water mix, the delivered temperature is a weighted average of the two flows. This example shows the arithmetic so you can see how a safe outlet temperature is achieved. The numbers are chosen for the worked example.
The situation. Stored hot water arrives at 60 °C and cold water at 10 °C. We want a basin to deliver water at 40 °C.
Step 1 — find the hot fraction. The fraction of hot water needed is (target minus cold) divided by (hot minus cold): (40 - 10) / (60 - 10) = 30 / 50 = 0.6.
Step 2 — read the proportions. A hot fraction of 0.6 means 60 percent hot and 40 percent cold.
Step 3 — apply to a flow. If the outlet runs at 6 litres per minute, that is 0.6 x 6 = 3.6 litres per minute of hot and 2.4 litres per minute of cold.
Check the application. The worked example uses a 40 °C basin target; it does not prescribe one setting for every user. For a new dwelling in England the fixed-bath supply has a 48 °C ceiling, while vulnerable bathers may need a lower limit. A suitable thermostatic mixing valve regulates the blend and provides its specified cold-supply-failure response when properly installed, commissioned and maintained.
Takeaway. Blend fraction = (target - cold) / (hot - cold). The calculation predicts the blend for the stated temperatures and flows. Confirm the actual delivered temperature during commissioning. Temperature blending controls scald risk; it does not by itself establish drinking-water quality or the hygiene of the whole system.