The core idea
Water heated in a boiler or heat pump does not travel on its own; it is pushed around a building by a circulator pump. The pump's job is to deliver a chosen flow rate, measured in litres per second (l/s), against the resistance of the circuit. That resistance is the head loss: every metre of pipe, every elbow and tee, every radiator and valve scrubs a little energy from the water. The total is called available pump head, quoted in metres of water column (m w.g.) or kilopascals (kPa).
Two laws govern the whole subject:
- Q = V × A — flow rate Q (m³/s) equals water velocity V (m/s) times the pipe's cross-
section A (m²). Pipe size sets how fast the water must move to carry the heat.
- Δp = R × L — pressure loss Δp grows with pipe length L and a specific pressure drop R
that rises steeply with velocity — roughly with its square: double the speed, nearly four times the loss. Bends and fittings add to it.
Hydronic design is mostly a compromise on velocity. Too slow, and air pockets and sludge settle in the pipes. Too fast, and losses climb with the square, while the rushing water wears the pipework and the noise travels through every wall. Designers typically hold velocities in small-diameter pipes to about 0.5–1.0 m/s.
Real-world example
A cottage needs a 10 kW radiator circuit with a 20 K drop across the loop. The flow rate required is 10 / (4.19 × 20) = 0.12 l/s. In a 15 mm copper pipe that flows at a brisk 0.68 m/s and loses pressure along roughly 40 m of pipe and fittings; a small circulator lifting that loss plus a few kPa through the boiler and radiators — a head of 2–3 m — does the job. This is exactly the wet central heating in most European homes: the pump is sized to the circuit, the radiators are fed at the design flow, and a bypass keeps flow present even when thermostatic valves close.
A different machine appears on bigger jobs: the variable-speed pump. Fitted with an electronic motor it watches its own pressure and slows down when valves close, so energy consumption falls dramatically. In a throttled (fixed-speed) circuit the pump simply burns the surplus head across a valve; in a variable-speed circuit the pump meets the demand and no more. In new buildings it is standard practice, and older fixed-speed circulators are being swapped for them for the electricity savings alone.
Common pitfall
Oversizing the pump "for safety". An oversized pump not only wastes electricity — it forces excessive flow, and the return water comes back so warm the boiler or heat pump cannot condense, cutting efficiency. It also produces velocity noise, erodes pipework and can starve other branches on shared circuits. The correct sequence is: compute the design flow, size the pipes for a sensible velocity, add up the losses along the index run, then select the smallest pump whose curve passes just above that point.