Gas pipework sizing: maximum allowable pressure drop

The sizing rule

For domestic natural gas, installation pipework is sized so that at design flow the pressure lost between the meter outlet and the inlet of each appliance is no more than 1.0 mbar. The meter installation has its own separate allowance, so the 1.0 mbar starts at the meter outlet, not the regulator.

This is resistance while gas flows, not a tightness-test allowance: never borrow a leakage figure such as 4 mbar. Undersized pipe loses more, so the appliance gets less than its design pressure and can be under-gassed.

The method

1. Work out each appliance's flow. Flow (m³/h) = gross heat input (kW) × 3.6 ÷ gross calorific value (MJ/m³). The 3.6 converts kW to MJ/h. Example: 27.5 × 3.6 ÷ 39.6 = 2.5 m³/h. Keep both figures gross, and never use useful heat output.
2. Add the loads on each section. A section carries every appliance downstream of it that can run at the same time. A 2.7 m³/h boiler and a 0.9 m³/h cooker put 3.6 m³/h through their shared section; the boiler-only branch carries 2.7 m³/h.
3. Find the equivalent length. Straight pipe plus the allowance for each fitting: 7.2 m + (3 elbows × 0.4 m) + one tee at 0.8 m = 9.2 m.
4. Read the table at the next length up, without interpolating. For 8.6 m, use the 10 m row. If that row gives 3.4 m³/h and you need 3.6 m³/h, the size is 0.2 m³/h short and fails.
5. Pick the smallest size that carries the load. For 3.5 m³/h, with sizes carrying 2.3, 3.1, 4.6 and 6.0 m³/h, choose the 4.6 m³/h size. The 6.0 also works but is not the smallest.
6. Add the losses along each complete route. 0.24 + 0.43 + 0.18 = 0.85 mbar, leaving 0.15 mbar of the 1.00 mbar.
7. Check every route on its own. After a 0.35 mbar common section, a 0.72 mbar boiler branch makes 1.07 mbar (fails) and a 0.40 mbar cooker branch makes 0.75 mbar (passes). Never add both branches into one route.

Those capacities and fitting allowances are exercise figures; a real design uses the tables for the actual pipe and fittings.

When an appliance is replaced

Swapping a 2.4 m³/h boiler for a 3.1 m³/h one, beside a 0.8 m³/h cooker, takes the shared section from 3.2 to 3.9 m³/h, up 0.7 m³/h. Recheck the shared section as well as the new branch: the old connection size proves nothing.

A sound installation

Pipework must be sized, supported, protected, sound and tested tight.

• Supports stop the pipe sagging and keep strain off the joints; repeated flexing fatigues joints until they leak. For 22 mm copper run horizontally, supports are no more than 2.0 m apart.
• Threaded joints take a gas-suitable non-hardening compound or PTFE tape to BS EN 751-3, never bare threads. Hemp is used only with compound on existing long-screw back-nut seals.
• Through a wall, sleeve the pipe: the sleeve protects it and lets a small escape vent instead of collecting in the structure.
• Wall cavities, ducts and voids: never run pipe within a wall cavity; it may only cross one by the shortest route, sleeved. A duct or void it runs through must be ventilated, because escaping gas could build up there.
• Buried or embedded steel is wrapped or coated against corrosion from damp. Buried natural-gas pipe needs at least 375 mm of cover where no vehicles cross.
• Keep clear of electrics: unless insulating material separates them, at least 150 mm from electricity meters, consumer units, distribution boards, switches and sockets, and 25 mm from cables.
• Main bonding, where required with an internal meter, connects on the consumer's side, near the meter outlet and before the first branch, never to the supplier's incoming pipe.

Supports, joints, sleeves, cavities, burial depth and clearances are set out in full in Installing gas pipework: supports, joints, sleeves and routes.