Heating & hot water
Instantaneous hot-water temperature rise
- where
- temperature rise (K)
- useful DHW output (W; any unit you pick is converted)
- fluid density (kg/m³)
- specific heat capacity (J/(kg·K); any unit you pick is converted)
- hot-water flow (m³/s; any unit you pick is converted)
Explain this equation
Works out how much a combi boiler or instantaneous heater can raise the temperature of water at a given flow. Use it to check whether an outlet will be hot enough at a given flow rate.
P is the useful heat output to hot water, not the gas input. ρ and c_p describe how much heat each litre of water absorbs per degree. q is the hot-water flow. ΔT is the rise above the incoming cold.
The heater adds heat at a fixed rate, so the faster the water flows, the less each litre is heated. It is the same heat-balance equation used for heating circuits.
Sense check: 30 kW at 12 L/min gives about 36 K, so 10 °C mains comes out near 46 °C. Double the flow and the rise halves. Combi outputs are often quoted at a 35 K rise.
Watch out: boiler modulation and temperature limits mean real outlets may differ.
Enter your values to see the result.
Good to know
Uses useful output, not gas input; actual boiler modulation and temperature limits apply.
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.