PlumbReviseWork out the design load, gas volume and route length you need before a BS 6891 pipe-size table lookup.
Load, gas volume and route worksheet before the BS 6891 table lookup.
Method: BS 6891 (gas pipework design)
Gas pipe sizing starts with the load carried by each pipe section. A section feeding more than one appliance carries the combined rated heat input of all appliances downstream of that section.
The calculator converts total heat input into required gas volume using the same energy relationship as gas rating: required gas volume (m³/h) = heat input (kW) × 3.6 ÷ calorific value (MJ/m³).
Route length is not just the tape-measured straight run. Bends, tees, valves and other fittings add resistance. This worksheet lets you add a planning allowance so you can revise the process, but the real equivalent lengths and final diameter come from the current BS 6891 tables, manufacturer data and the actual route.
Do not use this page as a permission slip to install or alter gas pipework. It is a revision and pre-check tool: final pipe size, pressure-loss proof, tightness testing and commissioning belong to competent registered gas work.
A boiler with 24 kW gross heat input and a cooker with 7 kW gross heat input on the same upstream section give 31 kW total design load. At gross CV 39.5 MJ/m³, required gas volume = 31 × 3.6 ÷ 39.5 ≈ 2.83 m³/h. A 12 m straight route with a 20% planning allowance gives 14.4 m for a preliminary sizing exercise.
Imagine a meter feeding one shared section before a tee splits towards a boiler and cooker. With gross heat inputs of 30 kW and 7 kW, the shared section carries 37 kW: about 3.37 m³/h at gross CV 39.5 MJ/m³. After the tee, the boiler branch carries 30 kW (2.73 m³/h) and the cooker branch carries 7 kW (0.64 m³/h).
Label each section of the route and record its downstream appliances. Use the worksheet for each section, then assess the complete path from the meter to each appliance. Adding the cooker load again to the boiler-only branch would overstate that branch load; ignoring it on the shared section would understate the load.
A boiler’s advertised output is the useful heat it delivers, not the energy it takes from the gas. Find the rated heat input in the appliance data. With the default gross CV of 39.5 MJ/m³, enter gross heat inputs; do not mix a net input with a gross CV. Manufacturer gas-consumption figures can help check the required flow.
The default 20% is a worksheet assumption, not a BS 6891 fitting allowance. Two routes with the same straight length can have different resistance because their bends, tees and valves differ. Record the actual fittings and use the applicable sizing method and material-specific data before selecting a diameter.
Follow the BS 6891 pipe-sizing guide to connect load, route and pressure-loss reasoning. Then use the CCN1 practice questions to check whether you can select the relevant information before doing the arithmetic.
Use this as a revision checklist before the actual standard-table lookup. It records the inputs that decide the pipe section, not a copied pipe-size table.
| Worksheet line | Record | Why it matters |
|---|---|---|
| Appliances downstream | Every appliance fed by the section | Shared pipework carries combined load |
| Rated heat input | kW for each appliance | Sets the design gas flow |
| Fuel and CV | Natural gas, LPG or supplied CV | Converts kW into m³/h |
| Route length | Straight run plus real fittings | Pressure loss rises with length and resistance |
| Candidate material/diameter | Copper, steel, CSST or other approved system | Tables and manufacturer data are material-specific |
| Commissioning proof | Working pressure, tightness and appliance checks | A table result still needs live verification |
For live work, replace the planning allowance with the actual fitting equivalents and current standard/manufacturer data. This worksheet intentionally does not select a pipe diameter.
There is no single answer from kW alone. You need the total load on each pipe section, gas type, route length, fittings, pipe material, meter/regulator context and the current BS 6891 sizing method.
There is no universal maximum run for 22 mm gas pipe. Capacity depends on the load, route, fittings, material and permitted pressure loss. A boiler with a 22 mm gas connection may need a larger supply pipe along part of the route; the inlet connection size does not size the pipework back to the meter.
The appliance can be starved of gas under full demand, giving poor operation, lockouts, incomplete commissioning or unsafe results that need risk assessment and correction.
Oversizing is usually a design, cost, routing and support issue rather than the same immediate starvation problem as undersizing, but the installation still has to meet the standard, be supported and be proved sound.
Method: BS 6891 (gas pipework design)
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Put the system principles into practice with 25 free CCN1 gas safety questions, including answers and explanations.
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.