Pipe sizing

Gas loses pressure as it flows along a pipe and through its fittings. Undersized pipework leaves appliances short of gas, so they can't reach their heat input and may not burn properly. Pipe sizing makes sure every appliance gets enough pressure when everything is running.

When the limit applies

Design to the 1 mbar limit for new installations, when you modify or extend pipework, before fitting any new appliance, and when an appliance's heat input is increased. Existing pipework that isn't being changed should meet the standard that applied when it was installed.

What flow to size for

• Size every section for the total heat input of all the appliances it feeds.
• For a range-rated appliance, use its maximum heat input.
• For a variable-rated appliance, use the heat input the system actually needs.
• For a combi boiler with different heating and hot water inputs, use the higher of the two.

The method

BS 6891 Annex A gives a method, with tables of approximate pressure loss per metre for copper and steel pipe, and allowances for fittings. Get accurate data from the pipe manufacturer where you can, and always for CSST and press-fit fittings, which the tables don't cover.

1. Split the installation into sections. Each section between branches carries a different flow.
2. Find the adjusted length of each section: its actual length plus an equivalent length for each bend, elbow and tee (Table A.5).
3. Find the loss per metre for the section's flow and pipe size (Tables A.1 and A.2). If the exact heat input isn't listed, use the next one up.
4. Multiply the loss per metre by the adjusted length.
5. Add up the sections from the meter to each appliance.
6. If any appliance is over 1 mbar, increase the size of one or more sections feeding it, and work those sections out again. Repeat until every appliance is within the limit.

Worked example

A combi boiler (28 kW net) and a cooker (11 kW net) are supplied in copper. From the meter (A), 3 m of 22 mm with one elbow runs to a tee (B). The boiler is straight on through the tee: 2 m of 22 mm with one elbow. The cooker comes off the tee's branch: 4 m with two elbows.

• Boiler: 0.45 + 0.20 = 0.65 mbar. Within 1 mbar.
• Cooker: 0.45 + 0.69 = 1.14 mbar. Over 1 mbar, so this design fails.

Resize the cooker branch in 22 mm. Adjusted length: 4 + 1.80 + 2 × 0.60 = 7.0 m. At 0.0178 mbar/m, that's 0.12 mbar, so the cooker's total is 0.45 + 0.12 = 0.58 mbar. Now everything is within the limit.

The tee uses the "flow exiting into a branch" allowance for the largest connection on the fitting (22 mm here). In this example it's added to the cooker branch, where the gas leaves the tee. In practice, appliances with horizontal connections may need an extra elbow, so allow for it. Figures are from BS 6891 Tables A.1 and A.5 (copper, net heat input).

At commissioning

When the appliances are connected, confirm that the pipework lets every appliance run at its required heat input, as the manufacturer's instructions say. Measuring the actual loss between two points isn't required, because site gauges aren't accurate enough. If you suspect too much loss, check the meter outlet pressure first, then the heat input of each appliance, then all the appliances running together.

LPG: the LPG limit is 2 mbar, and LPG has its own sizing tables.

Sources: BS 6891:2015 5.2, 5.3.2design, 1 mbar BS 6891:2015 Annex Amethod, tables BS 6891:2015 10.4commissioning CCN1 Issue 16 §5assessed

▸Tutor notes

• Have learners rework the example with the boiler upgraded to 35 kW net. Which sections change?
• Ask why the tee allowance goes on the cooker branch and not on the straight-through run.
• CCN1 §5 knowledge item 15 includes a theoretical pipe-sizing exercise: set one in the same format using a copy of Tables A.1 and A.5.