Question 1

What size pipe do I need for a 24kW boiler?

Accepted Answer

A 24kW boiler on a standard 10K temperature differential produces a flow rate of approximately 35 L/min. This requires 28mm copper pipe on the primary circuit — 22mm would result in a velocity of around 1.9 m/s, which exceeds the CIBSE Guide C maximum of 1.5 m/s and will cause noise and erosion problems. For smaller zone circuits serving individual radiators or rooms, 15mm or 22mm is typically correct.

Question 2

22mm or 28mm pipe for a boiler primary circuit?

Accepted Answer

For most domestic boilers up to about 12–15kW, 22mm copper is adequate for the primary circuit at a 10K temperature differential. Above 15kW, 28mm is required to keep velocity below 1.5 m/s. A 24kW boiler needs 28mm; a 35kW boiler typically needs 35mm primary pipework. Always calculate based on actual flow rate, not just boiler output category.

Question 3

What is the maximum velocity for heating pipes in the UK?

Accepted Answer

CIBSE Guide C specifies a maximum flow velocity of 1.5 m/s for low pressure hot water (LPHW) heating systems in domestic buildings. The recommended design range is 0.5–1.0 m/s for comfort and noise control. Velocities above 1.5 m/s cause erosion corrosion in copper fittings and noise in the pipework. For heat pump systems, keep velocity below 1.0 m/s where possible as these systems run continuously.

Question 4

What size pipe for a shower in the UK?

Accepted Answer

A single shower at 8–12 L/min is correctly served by 15mm copper pipe, giving a velocity of 0.7–1.0 m/s — within the recommended range. A power shower at 15–20 L/min needs 22mm. If you are feeding two showers simultaneously or a thermostatic shower valve from the same branch, size for the combined flow — typically 22mm. The cold feed to a combi boiler serving multiple outlets should be at least 22mm.

Question 5

Why do heat pump systems need larger pipes than gas boilers?

Accepted Answer

Heat pumps operate at a much smaller temperature differential — typically 5 K (e.g. 45/40°C flow/return) compared to 10–20 K for a gas boiler. Since heat output equals flow rate × ΔT × specific heat capacity, a smaller ΔT requires proportionally higher flow rate for the same heat output. A 12kW heat pump at 5K ΔT needs around 34 L/min — the same flow as a 24kW gas boiler at 10K. This is why under-floor heating manifolds and larger distribution pipework are essential before a heat pump installation.

Question 6

How is pipe pressure drop calculated?

Accepted Answer

Pipe pressure drop is calculated using the Darcy-Weisbach equation: ΔP = f × (L/d) × (ρv²/2), where f is the Darcy friction factor (from the Colebrook-White equation), L is pipe length in metres, d is internal diameter in metres, ρ is fluid density in kg/m³, and v is flow velocity in m/s. The result in Pa is then divided by 1000 for kPa. For domestic systems, CIBSE recommends designing for 100–350 Pa/m pressure drop per metre of pipe. Higher pressure drops require a larger pump head.

Question 7

What is the flow capacity difference between 22mm and 28mm pipe?

Accepted Answer

At the same flow velocity, 28mm copper pipe (26mm internal diameter) carries approximately 1.7 times the flow of 22mm copper pipe (20mm internal diameter). This is because flow capacity scales with the square of internal diameter — (26/20)² ≈ 1.69. In practice, 22mm handles up to about 18 L/min at 1.0 m/s and 28mm handles up to about 32 L/min at the same velocity.

PIPE SIZE	ID	MAX FLOW (1.0 m/s)	TYPICAL USE
15 mm	13 mm	8 L/min	Radiator branches, single outlet, shower feed
22 mm	20 mm	19 L/min	Zone circuits, boiler primary up to ~12 kW
28 mm	26 mm	32 L/min	Boiler primary 12–30 kW, main distribution
35 mm	32 mm	48 L/min	Boiler primary 30–50 kW, large domestic systems
42 mm	40 mm	75 L/min	High-output boilers, commercial-domestic

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Homeowners researching heat pumpsare looking for someone to call.

Pipe sizing
calculator.

Homeowners researching heat pumps
are looking for someone to call.