Hydraulics
Pipe velocity check and heat-carrying capacity
- where
- pipe bore area (m²)
- pipe internal diameter (mm)
- water velocity (m/s)
- volume flow rate (litres/min)
- velocity as % of limit
- design velocity limit (m/s)
- heat carried at the velocity limit (kW)
- specific heat capacity of water (kJ/kg·K)
- flow minus return temperature (K)
Explain this equation
Checks the water speed in a pipe for a given flow, and shows how much heat that pipe can carry at your velocity limit. Use it to pick primary pipe sizes for a heat pump.
A is the bore area with the diameter converted to metres. The flow in litres per minute is divided by 60,000 to give cubic metres per second (60 seconds, 1000 litres per m³); dividing by area gives speed. At the velocity limit the pipe passes v_l × A cubic metres per second; times 1000 gives kilograms per second, and times c_p ΔT gives kW.
Velocity is flow over area; heat capacity follows from the flow-rate equation at the maximum allowed flow.
Sense check: 23 litres a minute in 22 mm copper (20.2 mm bore) runs at about 1.2 m/s, over a 1 m/s limit. At 1 m/s and 5 K that pipe carries about 6.7 kW.
Watch out: too fast is noisy and wastes pump energy; too slow is fine for heat but larger pipe costs more. Use the true internal diameter.
Checks the water speed in a pipe at the design flow rate, and shows how much heat that pipe can carry at your velocity limit.
Enter your values to see the result.
Good to know
Internal diameters of BS EN 1057 Table X (R250) copper: 15 × 0.7 mm → 13.6 mm, 22 × 0.9 mm → 20.2 mm, 28 × 0.9 mm → 26.2 mm. For plastic pipe use the maker’s bore, and remember insert fittings narrow it further. Typical design limits are 0.5–1.5 m/s; around 1 m/s keeps noise and pressure drop reasonable in domestic copper. Water content per metre is about 0.15 l (15 mm), 0.32 l (22 mm) and 0.54 l (28 mm).
Common mistake
Using the outside diameter. A 22 mm tube has a 20.2 mm bore, which gives about 19% more velocity than 22 mm would suggest.
Limits
Average velocity in straight pipe. It does not give pressure drop: use the pipe sizing tables in CIBSE DHDG or the manufacturer’s data with the circulator curve.
Sources
- BS EN 1057 copper tube dimensions (R250 Table X: 15 × 0.7, 22 × 0.9, 28 × 0.9 mm), via Engineering ToolBox
- Engineering ToolBox: Copper tubes, maximum water velocities (hot water below 60 °C: 1.5 m/s)
- Heat Geek: Does my pipework need upgrading for a heat pump? (typical velocity limits 0.5–1.5 m/s; aim for about 0.9 m/s)
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
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.