Product claims sometimes place maximum pressure, maximum flow and maximum temperature beside each other even though they may not be achievable at the same time. A simple heat calculation helps assess whether a hot-water specification is realistic.
The basic heat relationship
For an engineering estimate, water heating demand can be approximated as Q ≈ 0.07 × flow in L/min × temperature rise in °C, with Q in kW. Add margin for burner, coil, pipework and ambient losses. More flow or more temperature rise needs more heat.
A quick example
At 15 L/min with a 60°C temperature rise, theoretical water heating demand is about 63 kW. Real heat-source capacity needs to be higher once losses are considered. If a machine claims the same continuous rise at that flow with a much smaller heater, ask for the actual test conditions.
- Ask for inlet and outlet water temperature
- Ask for actual measured flow
- Confirm continuous value versus short peak
- Confirm fuel value, efficiency and ambient conditions
- Check pump hydraulic duty and motor margin together
How to state it in a technical agreement
Do not write only a maximum temperature. State inlet temperature, rated flow, stable outlet temperature, continuous running time and allowed variation at one acceptance condition. This prevents peak values from separate tests being combined into an imaginary working point.
How to Decide
Any hot-water claim should be checked at one continuous operating point: flow, inlet temperature, outlet temperature and heating power. Maximum pressure, flow and temperature are comparable only when their test conditions match.