Heating & hot water
Heat carried by flowing water
- where
- heat / power rate (W; the calculator shows kW)
- 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)
- temperature difference (K)
Explain this equation
Works out how much heat a flowing stream of water carries. Use it to find the output delivered by a circuit when you know the flow and the flow–return temperature difference.
ρ × q is the mass of water passing each second. c_p is the energy needed to warm one kilogram by one degree; about 4.18 kJ/(kg·K) for water. ΔT is how much the water cools between flow and return.
Heat equals mass × specific heat × temperature change. Doing that every second gives a power. Glycol mixes have lower c_p and different density, so the same flow carries less heat.
Sense check: 12 L/min with a 20 K drop is about 16.7 kW. A handy rule for water is kW ≈ L/min × ΔT ÷ 14.3. Double either the flow or ΔT and the heat doubles.
Watch out: this is a steady heat balance. It does not size pipes or pumps.
Enter your values to see the result.
Good to know
Steady sensible heat transfer. Use actual fluid properties for temperature and glycol concentration.
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.