Hydraulics
Primary flow rate from heat output and temperature difference (Q = ṁ·cp·ΔT)
- where
- mass flow rate (kg/s)
- heat output to carry (kW)
- specific heat capacity of water (kJ/kg·K)
- flow minus return temperature (K)
- volume flow rate (litres/min)
- water density (kg/litre)
Explain this equation
Works out the water flow needed to carry a heat output at a chosen flow-return temperature difference. Use it to size pipes and check the pump and heat pump flow requirement.
Each kilogram of water carries c_p kJ for every degree it cools on its way round. Dividing the output in kW (kJ/s) by c_p × ΔT gives kilograms per second. Multiplying by 60 gives kilograms per minute, and dividing by density (1 kg per litre) gives litres per minute.
Heat carried equals mass flow times specific heat times temperature drop.
Sense check: 8 kW at a 5 K difference needs about 0.38 kg/s, about 23 litres a minute. At the 20 K difference typical of a boiler the flow would be a quarter of that.
Watch out: heat pumps run small ΔTs, so flows are much higher than with a boiler. Old 15 mm or microbore pipework may not carry it.
Finds the water flow rate the primary circuit must carry to move a given heat output at a chosen flow–return temperature difference.
Enter your values to see the result.
Good to know
Heat pumps are usually designed for a 5 K difference, so they need roughly four times the flow of an old boiler system at 20 K. At 5 K each kilowatt needs about 2.9 litres per minute. Check the result against the heat pump’s allowed flow range and the circulator’s curve.
Common mistake
Keeping the old boiler pipework and circulator setting. A 5 K design needs far more flow, and undersized pipes then give high velocities, noise and pressure drop.
Limits
Water only. Glycol mixes have a lower specific heat and higher density: use the glycol supplier’s cp and density. Density varies slightly with temperature (about 0.99 kg/litre at 40 °C).
Sources
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
- Ideal Heating Logic Air heat pump installation manual (design flow rates 22.9 l/min at 8 kW and 28.7 l/min at 10 kW, i.e. 5 K ΔT; minimum free system volume 24 L (4/5 kW) and 40 L (8/10 kW) for defrost)
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.