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 24 kW boiler and 7 kW cooker on the same upstream section give 31 kW total design load. At CV 39.5, 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 to take into the BS 6891 sizing check.
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.
It depends on the load, route, fittings and pressure-loss allowance. A short 22 mm run may be fine in one design and wrong in another, so the answer has to come from the standard-table lookup and commissioning checks.
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)
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.