Domestic Gas Pipe Sizing: Meter to Appliance Calculations

Summary

Gas pipe sizing is a calculation exercise, not a guessing game. Undersized gas pipework results in inadequate gas pressure at appliances, causing unreliable ignition, poor combustion, inefficient operation, and potentially dangerous incomplete combustion. Oversized pipework costs more and takes longer to purge. The goal is to size pipes to deliver maximum demand flow with a pressure drop not exceeding the allowable limit.

For most domestic installations (a house with a boiler and gas cooker), the existing pipework installed in the past 30 years is likely to be adequate. Where pipe sizing becomes critical is when adding appliances, extending runs, converting to a larger boiler, or working on older properties where original pipework may be undersized (some pre-1960s properties have 3/8" or 1/2" iron pipe that is almost always undersized for modern appliances).

The calculation methodology in BS 6891 uses pressure drop tables based on pipe diameter, flow rate (in m³/h or ft³/h), and pipe length. For more complex systems, IGEM/UP/2 provides a more detailed design methodology. In practice, most competent gas engineers use reference tables rather than calculating from first principles every time.

Key Facts

• Maximum allowable pressure drop (domestic NG) — 1 mbar from meter outlet to any appliance at peak demand; total pressure drop across the system
• Inlet pressure (NG) — typically 21 mbar at the emergency control valve; minimum 19 mbar; maximum 23 mbar
• Working pressure at appliance — typically 20 mbar for most domestic gas appliances
• Gas calorific value (NG) — approximately 38.4 MJ/m³ (standard conditions); used to convert appliance kW rating to gas flow rate
• Gas flow conversion — approximate: 1 kW = 0.097 m³/h NG; or divide kW by 10.3 to get m³/h
• Pipe material — copper to BS EN 1057 (most common domestic); CSST (corrugated stainless steel) for flexible runs within walls; steel to BS 1387; MDPE (yellow) for underground supply only
• Compression fittings vs solder fittings — either acceptable for domestic gas; WRAS-approved fittings required; do not use non-dezincification resistant (DZR) fittings in aggressive water areas [note: for gas, use standard copper compression fittings]
• BS 6891:2005+A2:2008 — specification for low-pressure gas installation pipework (inlet pressure not exceeding 35 mbar); standard for domestic natural gas
• IGEM/UP/2 — pipework design and installation for industrial and commercial premises; also referenced for complex domestic systems
• Purging — all new or modified gas pipework must be purged before use; IGEM/UP/1B provides the purging procedure; purge volumes depend on pipe diameter and length
• Tightness test — all new gas pipework must be tightness tested before commissioning; procedure in BS 6891; working pressure test with pressure gauge and allowed loss
• Maximum pipe length — no absolute maximum; determined by the pressure drop calculation; longer runs require larger diameter pipe
• Underground gas pipe — MDPE (medium density polyethylene, yellow) to BS EN 1555; minimum 375mm deep (450mm under roads); must have marker tape above

Quick Reference Table

Values are approximate for a 1 mbar pressure drop over 3m length. Longer runs require larger pipe. Always verify with pressure drop table.

Detailed Guidance

Calculating Gas Demand

Start by listing all gas appliances in the installation and their gas input ratings:

Gas flow (m³/h) = appliance input rating (kW) ÷ 10.3

Diversity: in a domestic installation, not all appliances operate simultaneously all the time. BS 6891 allows for diversity, but the conservative approach (and the one most commonly used for residential) is to size for simultaneous operation of all appliances.

Pressure Drop Method

The BS 6891 tables give the maximum permissible flow rate for each pipe diameter and length, for a total pressure drop of 1 mbar (the domestic allowable limit).

Method:

1. Identify each pipe section (from meter to junction, junction to appliance)
2. Calculate the gas flow required in each section at maximum demand
3. Measure (or estimate) the length of each section, including equivalent lengths for fittings
4. From the BS 6891 tables, determine the minimum pipe diameter for that flow and length

Equivalent lengths for fittings (approximate):

• 90° elbow: 0.5m
• Tee (straight through): 0.2m
• Tee (branch): 1.0m
• Ball valve (fully open): 0.3m

Example calculation:

• Total demand: 4.0 m³/h
• Main run from meter to boiler: 6m + 2 elbows + 1 tee = 6 + 1.0 + 1.0 = 8m equivalent length
• From BS 6891 table: at 4.0 m³/h, 8m equivalent length requires 28mm minimum
• Boiler spur only: 2.34 m³/h, 3m run → 22mm adequate
• Cooker spur: 1.17 m³/h, 2m run → 15mm adequate (just within limit — check table)

Tightness Testing Procedure (BS 6891)

All new pipework must be tightness tested before use. The procedure:

1. Ensure all appliances are disconnected or closed off at the appliance isolation valve

2. Pressurise the installation to working pressure (approximately 20 mbar for domestic NG) using the meter supply

3. Isolate the supply at the meter outlet cock

4. Allow 1 minute for stabilisation (temperature equalisation)

5. Record the pressure on a calibrated manometer (U-gauge or digital pressure gauge, 0.1 mbar resolution minimum)

6. After 2 minutes, re-check pressure

7. The allowable pressure drop depends on the system volume:

   • Small installation (≤0.035 m³ — typical domestic): 4 mbar allowable drop over 2 minutes
   • Medium installation (0.035–0.070 m³): 4 mbar over 2 minutes
   • Larger: BS 6891 Table 1

8. If the pressure drop exceeds the limit, there is a leak — find and repair before proceeding

Note: the tightness test does not locate leaks — it only confirms whether one exists. Use leak detection fluid (LDF) or electronic gas detector to locate leaks.

Purging New Pipework

All new gas pipework must be purged of air before commissioning. Air/gas mixtures are explosive at concentrations of approximately 5–15% gas in air (LEL/UEL). Purging clears the mixture to safe levels before ignition.

Purging procedure (IGEM/UP/1B):

• At the appliance test point or disconnected appliance, allow gas to flow to displace air
• Volume to purge: minimum 5× the volume of the new pipework
• In a domestic installation with new pipework of 15–28mm diameter, a few minutes of flow at a range point (gas hob burner) is typically adequate
• Do not purge into an enclosed space with an ignition source — purge to outside or through a range burner outdoors in a well-ventilated space
• Confirm purging complete with a gas detector or by lighting the appliance and observing a stable flame

Pipe Installation Standards

Pipe penetrations through walls: use sleeve with fire-stop sealant (see fire stopping); never fix gas pipe with ferrous metal clips (corrosion risk); use plastic-coated clips or isolating bushes.

Concealed pipe: gas pipe in a concealed position (in a wall, under a floor) must be run in a sleeve to allow replacement without opening the structure, or the pipe must be in a duct that is ventilated at both ends. Never joint concealed pipe in compression fittings — only soldered end-feed or capillary fittings are acceptable for concealed pipework (fewer potential leak points).

Minimum pipe support centres:

• 15mm copper: 1.0m horizontal, 1.5m vertical
• 22mm copper: 1.5m horizontal, 2.0m vertical
• 28mm copper: 2.0m horizontal, 2.5m vertical

Corrosion protection: gas pipes in concrete or plaster must be protected from corrosion; wrap with self-amalgamating tape or use factory-coated pipe; MDPE underground pipe is inherently corrosion-resistant.

LPG Pipe Sizing Differences

LPG (butane or propane) has a different calorific value from natural gas:

• Propane: approximately 95 MJ/m³ vs NG 38.4 MJ/m³
• Flow rate for same kW output: much lower for LPG
• LPG operates at different pressures (37 mbar for propane domestic)
• Pipe sizing tables are different — use IGEM/UP/11 for LPG

See lpg installations for full LPG-specific guidance.

Frequently Asked Questions

How do I know if the existing gas pipework is adequate for a new larger boiler?

Check the input rating of the new boiler and measure/estimate the existing pipe diameter and run length. Calculate the pressure drop using BS 6891 tables. If the pressure drop at the new boiler's gas demand exceeds 1 mbar, the pipe is undersized. In practice: a 24kW boiler (2.34 m³/h) on 22mm copper for a run under 8m is generally fine. A 35kW boiler (3.4 m³/h) at the end of a long run may need 28mm pipe.

Can I use flexible hose for connecting gas appliances?

Yes — flexible corrugated metal hose (BS EN 14800 compliant) is acceptable for connecting cookers and some fires to a gas point. Maximum length is typically 1.5–2.0m depending on the standard. The hose must not pass through walls or be concealed. Replace flexible hoses every 5–10 years or per manufacturer recommendation — they degrade and can develop pinhole leaks.

What pressure should I measure at the meter inlet?

Natural gas domestic supply: typically 21 mbar at the emergency control valve upstream of the meter. Below 19 mbar at minimum load indicates a network pressure problem — contact the gas transporter (Cadent, Northern Gas Networks, etc.). Above 25 mbar indicates a potential regulator problem on the network — also contact the gas transporter.

Is plastic pipe acceptable for gas?

MDPE (yellow medium-density polyethylene) is acceptable for underground gas supply pipes only. Above ground inside a building, only metallic pipe (copper, steel, CSST) is acceptable. Do not use plastic water pipe (blue MDPE or grey CPVC) for gas — it is not rated for this application.

Regulations & Standards

• BS 6891:2005+A2:2008 — specification for installation of low-pressure gas pipework in domestic premises (not exceeding 35 mbar inlet pressure)

• IGEM/UP/1B — strength, tightness, and purging of domestic and small commercial meter installations

• IGEM/UP/2 — installation of above-ground pipework for industrial and commercial purposes

• Gas Safety (Installation and Use) Regulations 1998 — all gas installation work

• BS EN 1057 — copper and copper alloys; seamless round copper tubes for water and gas

• BS EN 14800 — flexible hose assemblies for domestic gas appliances

• IGEM — Technical Publications — UP/1B, UP/2 available to members

• Gas Safe Register — Technical Bulletins

• HSE — Gas Safety Guidance

• Corgi Homeplanning — Gas Pipe Sizing [verify current resource]

• gas safe requirements — Gas Safe registration requirements

• boiler installation — boiler installation commissioning checklist

• lpg installations — LPG pipe sizing differences

• pipe sizing — water pipe sizing comparison