Heating & hot water
Required heating-water flow
- where
- volume flow (m³/s; the calculator shows L/min)
- heat / power rate (W; any unit you pick is converted)
- fluid density (kg/m³)
- specific heat capacity (J/(kg·K); any unit you pick is converted)
- temperature difference (K)
Explain this equation
Works out the water flow needed to carry a given heat load at a chosen flow–return temperature difference. Use it when setting pump flow or balancing a heating or heat-pump circuit.
P is the heat to be delivered. ρ is the density of the fluid and c_p its specific heat, together telling you how much heat each litre carries per degree. ΔT is the design drop between flow and return.
It is the heat-power equation turned around. Each cubic metre per second carries ρ × c_p × ΔT watts, so dividing the load by that gives the flow. The calculator also shows mass flow in kg/s.
Sense check: 24 kW at 20 K needs about 17 L/min. Halve ΔT to 10 K, as a heat pump might run, and you need double the flow, about 34 L/min.
Watch out: this gives the flow needed, not the pipe size or pump selection.
Enter your values to see the result.
Good to know
This is a heat-balance flow requirement, not pipe or pump selection.
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.