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.

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.

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. In a revision example, a common section loses 0.30 mbar and the boiler branch loses 0.55 mbar: that route totals 0.85 mbar and has 0.15 mbar left. Check the cooker route separately using the same common section and its own branch. The 1.0 mbar allowance is for each complete path, not for every section independently.

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.