Gas

Gas Pipe Sizing for Domestic Installations

Get the pipe size right and every appliance gets its working pressure; undersize it and you'll be chasing a boiler that under-fires no matter what you do at the gas valve.

Updated 20 Aug 20269 min read

Gas installation pipework has to carry the appliance load from the meter to each appliance without dropping the pressure too far. Undersized pipe is a common hidden cause of appliances that won't reach their rated input — they gas-rate low, and no adjustment at the appliance fixes a supply that can't deliver. Sizing it properly at first fix avoids the callback.

This guide covers the inputs to a sizing calculation — the appliance load, the effective pipe length including fittings, and the crucial 1 mbar permitted pressure drop across the installation — following the principles of IGEM/UP/1B. It describes the method rather than reproducing the full discharge tables; always size to the current IGEM procedure and manufacturer data. Gas work must be carried out by a Gas Safe registered engineer.

Key takeaways

The whole installation pipework (meter to appliance) is permitted a maximum pressure drop of 1 mbar for natural gas — that budget governs the sizing.
Size each section for its gas flow (appliance load in m3/h) and its effective length, then check the total drop from meter to the worst-case appliance stays within 1 mbar.
Effective length = measured pipe length plus an allowance for every fitting (elbows, tees, bends), which add resistance as extra 'equivalent length'.
Convert appliance heat input (kW) to gas flow (m3/h) using the calorific value, then read the pressure drop per metre for that flow and pipe size from the IGEM tables.
Work to IGEM/UP/1B principles and manufacturer data — undersized pipe under-fires appliances no adjustment at the valve can cure.

The 1 mbar pressure-drop budget

The governing rule for domestic natural gas is that the pressure drop across the whole installation pipework — from the outlet of the meter to each appliance inlet — must not exceed 1 mbar at full load. The meter delivers gas at around 21 mbar, and appliances need their stated inlet working pressure (commonly about 20 mbar at the appliance); that 1 mbar is the budget you have to lose in the pipework, and no more.

Everything else in sizing serves that limit. You choose pipe diameters so that, with every appliance running at full rate, the accumulated drop along the worst-case route stays inside 1 mbar. Blow the budget and the far appliance sees low working pressure, under-fires and gas-rates low — the classic undersized-pipe fault.

1 mbar is the whole installation

The 1 mbar is the total permitted drop from meter outlet to appliance for natural gas — not per section. Add up the drops along the route to the worst-placed appliance; that sum is what must stay under 1 mbar.

Appliance load and gas flow

Sizing starts from how much gas each section has to carry. Add up the gross heat input of the appliances a section feeds, then convert to a volumetric flow: gas flow (m3/h) = gross kW multiplied by 3.6, divided by the calorific value (about 39.3 MJ/m3 gross for natural gas). A 30 kW net boiler (about 33.3 kW gross) draws roughly 3.05 m3/h, for example.

A branch feeding several appliances carries the sum of their flows, so the pipe from the meter carries the whole installation's diversified load while a spur to a single appliance carries only that appliance. Size each section for the flow it actually carries — that's why runs typically step down in diameter as they branch out towards individual appliances.

Measured length plus fittings

The resistance of a pipe run depends on its length, and fittings add to that. Each elbow, tee, bend or fitting behaves like an extra length of straight pipe, so the figure you size against is the effective (equivalent) length: the measured run plus an equivalent-length allowance for every fitting. A run with lots of bends is effectively much longer than the tape says.

Measure the actual pipe length of the section, count the fittings, add their equivalent lengths from the IGEM data, and use that total as the length for the pressure-drop calculation. Skipping the fittings allowance is a common way installations end up marginally undersized on paper and low on pressure in reality.

Reading the tables and putting it together

With a section's gas flow and its effective length, the IGEM/UP/1B discharge tables give the pressure drop for a given pipe size and material (copper, steel, or the relevant medium-density polyethylene for buried/external runs). You pick a candidate diameter, read the drop per metre (or per the table's basis) for that flow, multiply by the effective length, and get that section's drop.

Then you sum the drops along the complete route to the worst-case appliance and check the total against the 1 mbar limit. If it exceeds 1 mbar, step up a diameter on the sections contributing most and recalculate. Working from the meter outward, sizing each section for its load and effective length and keeping the accumulated route drop under budget, is the whole method.

Size the worst-case route

It's the longest, most heavily loaded route — usually to the appliance furthest from the meter — that decides the sizing. Get that route under 1 mbar and the shorter, lighter branches look after themselves.

Testing and materials in practice

Sizing is proven on the job by the working pressure and gas rate: with all appliances running, the working pressure at the furthest appliance should still be within its stated range, and each appliance should gas-rate to its badge. If it under-fires and the appliance and valve check out, the pipework is the suspect — look back at whether the sizing budget was actually met.

Material matters too: copper and steel have different bores and resistances for the same nominal size, so size to the material used, and use the correct medium-density polyethylene with proper transition fittings for buried or external sections. Once the pipework is installed or altered, it must be tightness-tested to prove it's sound before commissioning — sizing and soundness are separate checks, and both have to pass.

Frequently asked questions

What's the maximum permitted pressure drop across the pipework?

For domestic natural gas, the total pressure drop from the meter outlet to each appliance inlet must not exceed 1 mbar at full load. That is the budget the whole sizing calculation works to — it applies across the entire route, not per section.

How do I turn appliance kW into gas flow?

Gas flow (m3/h) = gross kW multiplied by 3.6, divided by the calorific value (about 39.3 MJ/m3 gross for natural gas). So a roughly 33 kW gross boiler draws about 3 m3/h. Size each section for the total flow of the appliances it feeds.

Do I need to allow for elbows and tees?

Yes. Every fitting adds resistance equivalent to a length of straight pipe. Size against the effective length — the measured run plus an equivalent-length allowance for each fitting from the IGEM data. Ignoring fittings is a common way pipe ends up undersized and appliances end up low on pressure.

My boiler under-fires despite a good gas valve — could it be the pipe?

Very possibly. Undersized or over-long pipework drops too much pressure, so the appliance sees low working pressure and gas-rates low regardless of what you do at the valve. Check the working pressure at the appliance at full rate, and re-check the pipe sizing against the 1 mbar budget.

From guidance to action

Gas pipe sizing calculatorSize each section from load and effective length and check the total drop against 1 mbar.Gas rate calculatorProve the delivered rate at the appliance once the pipework is in.