When can I step down from 22mm to 15mm pipe?

You can step down to 15mm pipe when the cumulative load drops below approximately 8,000-10,000 BTU (2.3-2.9 kW) at an 11°C ΔT, which gives a flow rate the 15mm pipe can handle without excessive velocity or noise. The exact point depends on pipe velocity limits (typically 1.5 m/s max).

How do I calculate cumulative load along a pipe run?

Starting from the boiler, add the BTU output of each radiator along the pipe run. The section of pipe between the boiler and the first tee carries the total load of all radiators it serves. After each tee, subtract the BTU of the radiator branching off. The pipe can reduce in size as the load decreases.

What velocity should I design for?

CIBSE recommends a maximum velocity of 1.0-1.5 m/s for domestic heating pipework. Higher velocities cause noise (water rush), erosion of copper, and air entrainment issues. For microbore (10mm), limit velocity to 1.0 m/s due to increased friction losses.

Can I use microbore (10mm) pipe for radiators?

Microbore (8 or 10mm) can be used for individual radiator connections (from a manifold or tee) where each radiator is up to about 3,000-4,000 BTU. It should not be used for main runs carrying multiple radiators. Microbore is commonly used with manifold distribution systems.

What is the maximum load for 15mm pipe?

At 1.5 m/s maximum velocity, 15mm copper pipe (13mm internal diameter) can carry approximately 11.5 L/min or about 12,000-14,000 BTU at 11°C ΔT. At a more conservative 1.0 m/s, the limit is about 8,000-9,000 BTU. Use the lower figure for quieter installations.

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Radiator Pipe Sizing Calculator — Cumulative Load

Size pipe sections along a radiator run based on cumulative BTU load. Determine where pipe can step down from 22mm to 15mm.

Radiator 1 (nearest boiler)

First radiator on run

Radiator 2

Second radiator

Radiator 3

Third radiator

Radiator 4

Fourth radiator (0 if none)

Radiator 5

Fifth radiator (0 if none)

How We Calculate This

This calculator sizes pipe sections along a radiator run based on the cumulative BTU load at each point. As radiators branch off and the remaining load decreases, the pipe diameter can step down.

Method

Cumulative load: Sum of all downstream radiator BTU outputs
Flow rate (L/min): (kW × 1000) ÷ (4.186 × ΔT × 60)
Minimum pipe area: Flow (L/s) ÷ max velocity (m/s)
Pipe diameter: √(4 × Area ÷ π)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.