Heating & hot water
Flow–return temperature difference
- where
- flow–return temperature difference (K)
- 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)
- volume flow (m³/s; any unit you pick is converted)
Explain this equation
Works out the temperature drop between flow and return for a given heat load and water flow. Use it to check whether a system is running at its design ΔT.
P is the heat being given out by the circuit. ρ and c_p describe how much heat the fluid carries per cubic metre per degree. q is the volume flow round the circuit.
Rearranged from heat = flow × ρ × c_p × ΔT. For a fixed load, a low flow means each litre has to give up more heat, so the temperature drop gets bigger.
Sense check: 24 kW at 12 L/min gives about 29 K. Double the flow and ΔT halves to roughly 14 K. A very large ΔT often points to too little flow.
Watch out: use the fluid properties that match the system, especially with glycol.
Enter your values to see the result.
Good to know
Steady heat balance with consistent fluid properties.
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.