BS 6891 gas pipe sizing explained: load, length and pressure drop

What BS 6891 covers

BS 6891 is the UK standard used for low-pressure gas installation pipework on premises up to 35 mm, covering second-family gas such as natural gas and third-family gas such as LPG. It deals with design, installation, commissioning and maintenance of the installation pipework, not the gas main in the street or the pipework inside an appliance.

For revision, the important point is scope. If the job is outside domestic low-pressure pipework, uses larger pipework, involves commercial plant or uses a manufacturer-specific pipe system, the design route may move to other standards or manufacturer data.

What else BS 6891 sets out besides sizing

Sizing is only one part of the job. Installation pipework also has to be supported, jointed, protected and routed correctly, then proved tight and purged before the gas is used. The current edition, BS 6891:2015+A1:2019, sets out how, alongside the Gas Safety (Installation and Use) Regulations and their approved code of practice.

• Support: secure both ends of every run, then clip it at maximum spacings that are wider on vertical runs and on larger pipe.
• Joints: seal a threaded joint with a gas-rated jointing compound or PTFE tape to BS EN 751. A compression joint seals at the olive, so it is never taped.
• Walls and floors: cross by the shortest practicable route inside a sleeve, seal between pipe and sleeve at one end only, and keep joints out of the sleeve.
• Cavities, ducts and voids: cross a cavity wall rather than running along it, and ventilate a duct or void the pipe runs through unless the pipe has its own ventilated duct.
• Testing and purging: prove the installation tight and purge it before use, following IGEM/UP/1B for domestic installations.

Why there is no single 22 mm answer

A common search is "how far can you run 22 mm gas pipe?". The honest answer is that 22 mm is not a distance by itself. It is only a candidate size once you know the load, route and pressure-loss context.

• Total load: an upstream section may feed a boiler, cooker and fire together, not just one appliance.
• Route length: longer pipe creates more pressure loss.
• Fittings and changes of direction: elbows, tees, valves and reducers add resistance.
• Material and system: copper, steel and approved flexible pipe systems are not interchangeable for table use.
• Fuel and calorific value: natural gas and LPG do not use the same assumptions.
• Meter, regulator and appliance data: the installation still has to satisfy working-pressure and manufacturer checks.

The revision workflow

1. Split the installation into pipe sections. Each section carries the appliances downstream of it.
2. List the rated heat input for every appliance fed by that section, with its gross or net basis. Do not substitute heat output.
3. Add the downstream heat inputs to get the section design load.
4. Convert kW to gas volume if the method or table route needs it: gas volume in m3/h = kW x 3.6 / calorific value. Use gross heat inputs with a gross calorific value, or net inputs with a net calorific value.
5. Measure the route and include the correct fitting equivalents from the current standard or manufacturer data.
6. Use the current BS 6891 sizing method for the chosen material and candidate diameter.
7. Commission and prove the result with tightness, working pressure and appliance checks before leaving the installation in service.

Load and gas-volume example

Suppose one pipe section feeds a boiler and cooker with gross heat inputs of 24 kW and 7 kW. That section load is 31 kW gross. Using a natural-gas gross revision CV of 39.5 MJ/m3, the required gas volume is 31 x 3.6 / 39.5 = about 2.83 m3/h.

That 2.83 m3/h figure still is not a pipe size. It is one input for the sizing check. You still need the route, fittings, material, pressure-loss allowance and final commissioning proof.

Label the common section A, the boiler branch B and the cooker branch C. A carries both loads up to the tee; B and C each carry only their own appliance. The boiler route is A then B, and the cooker route is A then C. No appliance is supplied through A, B and C in series.

The pressure-drop trap

For domestic natural gas, the design allowance for installation pipework is no more than 1.0 mbar from the meter outlet to each appliance inlet at full connected demand. The meter installation has its own pressure loss; the pipework allowance starts downstream of it. A satisfactory standing pressure cannot establish the loss while gas is flowing.

Add the losses along each complete meter-to-appliance path. Using the section labels above, suppose A loses 0.30 mbar, B loses 0.55 mbar and C loses 0.80 mbar at the design flows. These are supplied exercise losses, not figures calculated from the kW loads alone. The boiler path A + B is 0.85 mbar, leaving 0.15 mbar of the allowance. The cooker path A + C is 1.10 mbar, exceeding it by 0.10 mbar even though each individual section loses less than 1.0 mbar.

The cooker path therefore needs its sizing or route revised and checked again. Passing the boiler path does not establish that the cooker path passes. If a change affects the common section, recheck both paths. Do not add B and C together, or give each section a fresh 1.0 mbar allowance.

Use the same demand when comparing pressure readings and identify the test points. An internal gas-valve test point can include losses inside the appliance as well as its supply pipe. Follow the manufacturer’s test-point guidance before deciding where a loss occurs; appliance-specific internal allowances do not increase the installation pipework budget.

How to use the calculator

The gas pipe sizing calculator is a worksheet. It totals appliance loads, converts the load into gas volume and gives a planning route length so you can practise the decision chain. It deliberately does not reproduce BS 6891 tables or certify a pipe diameter.

For live work, the final answer comes from the current standard, the actual route and fittings, the pipe system data, manufacturer instructions and competent gas commissioning.

Replacing a boiler: recheck the shared section

If the example boiler is replaced by one needing 30 kW gross input, the cooker still needs 7 kW. Section A now carries 37 kW, about 3.37 m3/h at the same gross CV. Section B rises to about 2.73 m3/h, while C stays at about 0.64 m3/h. The old connection diameter cannot establish that A and B can carry their increased loads.

Recalculate the affected section losses and check both complete paths. The cooker branch load is unchanged, but its supply still passes through A, whose flow has increased. Do not carry forward the old 0.30 mbar common-section loss without checking it.

Keep design demand separate from gas rating a boiler. Sizing starts with the rated input needed by the connected appliances; gas rating measures actual consumption from timed meter readings under the specified test conditions. A low measured rate on an under-supplied appliance is not a reason to design for a smaller load.