Gas Meter Sizing and Pipe Sizing for Gas Engineers: The Index Test Explained

Gas meter sizing and pipework design are areas where many gas engineers are technically competent at the installation level but less confident on the calculations behind it. Getting it wrong results in undersized meters that restrict flow during peak demand, or oversized installations that suppliers won't approve. Understanding the principles behind meter selection and the index test makes you a better engineer and a more credible advisor to clients looking to add appliances or extend existing systems.

This guide covers domestic and light commercial gas meter sizing, gas pipe sizing methodology, the index test procedure, and the relevant standards.

Why Gas Meter Sizing Matters

A gas meter is not just a measurement device — it also determines the maximum flow rate available to the installation. Every meter has a rated capacity in m³/hour. If the connected appliances demand more gas than the meter can pass, the meter will become a restriction in the system — causing pressure drop at the meter, appliance lockouts, and poor performance particularly during peak demand (cold mornings when the boiler, gas fire, and hob are all running simultaneously).

Conversely, an unnecessarily large meter takes up more space, costs more, and may not be approved by the gas transporter if the connected load doesn't justify it.

Standard Domestic Meter Types

In the UK, domestic gas meters are typically diaphragm meters. The standard domestic meter is the U6 (sometimes called a 6m³/hour meter, though the actual rating is slightly higher at approximately 6 m³/hr).

The U6 is suitable for most single domestic dwellings with a combi boiler (24–35kW) and a cooker. Adding a second boiler, a large range cooker, or a gas fire can push the demand close to or beyond the U6 capacity — at which point a U16 should be specified.

Calculating Connected Load

To size a meter, you need to calculate the maximum simultaneous gas demand of all connected appliances. The process:

Step 1: Identify All Appliances

List every gas appliance connected to the installation, including any planned future appliances if known. For each, find the maximum gas consumption from the appliance data plate or installation manual, expressed in kW (gross calorific value — GCV).

Step 2: Convert kW to m³/hour

Gas meters measure volume (m³), not energy. To convert from kW to m³/hour, use the calorific value of natural gas:

Natural gas (UK) gross calorific value: approximately 10.76 kWh/m³ (this varies slightly by region and season — use the value from your transporter if precision is required).

Gas consumption (m³/hr) = Appliance output (kW) ÷ Calorific value (kWh/m³)

Example: 35kW combi boiler
= 35 ÷ 10.76 = 3.25 m³/hr

Example: 12kW gas cooker
= 12 ÷ 10.76 = 1.11 m³/hr

Example: 5kW gas fire
= 5 ÷ 10.76 = 0.46 m³/hr

Total: 3.25 + 1.11 + 0.46 = 4.82 m³/hr → U6 meter is adequate

Step 3: Apply Diversity (if applicable)

For installations with multiple appliances, it is unlikely that all appliances will be running at full output simultaneously. However, for domestic installations, Gas Safe engineers typically design for no diversity — assuming all appliances could run simultaneously. This is conservative but safe, and is the standard approach for domestic meter sizing.

For light commercial installations with multiple appliances (kitchens with multiple burners, space heating and catering combined), your gas transporter's technical team can advise on diversity factors.

Step 4: Select Meter

If the total connected load exceeds 6 m³/hr, a U16 must be specified. Exceeding 16 m³/hr requires a U25 or above. Meter changes and upgrades require the network operator's agreement — this is not a decision the engineer makes unilaterally.

Gas Pipe Sizing: The Principles

Gas pipework must be sized to deliver the required flow rate at all appliances while maintaining the pressure drop within acceptable limits. The maximum allowable pressure drop in a domestic installation is defined in BS 6891:

• For installations with a U6 meter: Maximum pressure drop from meter outlet to any appliance burner of 1 mbar
• For installations with a U16 meter or above: Maximum pressure drop of 2 mbar from meter outlet to burner

The standard inlet pressure at a domestic meter is 21 mbar (natural gas). Appliances are designed to operate at a nominal burner pressure of 20 mbar — hence the 1 mbar maximum loss allowance for U6 systems.

Factors Affecting Pressure Drop

Pressure drop in a gas pipe is determined by:

• Flow rate — higher flow causes greater pressure drop
• Pipe bore (internal diameter) — smaller bore = higher resistance
• Pipe length — longer runs cause greater pressure drop
• Fittings — elbows, tees, and valves add resistance (expressed as equivalent length)
• Pipe material — steel, copper, and CSST (corrugated stainless steel) have different friction factors

Pipe Sizing Tables (BS 6891)

BS 6891 (Installation of low pressure gas pipework of up to 35mm in domestic premises) includes pipe sizing tables for copper and steel tube. These tabulate the maximum flow rate achievable for a given pipe size and run length within the 1 mbar pressure drop allowance.

Key practical sizes for domestic work:

These are approximate figures — always use the BS 6891 tables or a gas pipe sizing calculation for the actual installation, accounting for the specific pipe length and fittings.

The Index Test

The index test (also called a standing and working pressure test) is carried out on an existing installation to verify that the existing pipework and meter can adequately supply additional appliances, or as part of commissioning a new installation. It is a requirement of Gas Safe practice and is referenced in IGEM/UP/2.

Why It's Called the Index Test

The "index" refers to the index circuit — the most hydraulically resistive path from the meter to the furthest or most demanding appliance. If the pressure at the end of the index circuit is adequate, the rest of the system (which has shorter or less loaded paths) will be satisfactory.

Equipment Required

• Digital manometer (U-gauge or electronic) — accurate to 0.1 mbar or better
• Test nipple adaptors for the appliance test points (typically located at the gas valve or burner inlet)
• Appropriate test point connections for the meter outlet (usually a 1/4" BSP adaptor)

Procedure: Domestic Installation (U6 Meter)

1. Connect the manometer at the meter outlet test point (or as close to the meter outlet as possible)
2. Standing pressure reading — with all appliances off, record the static pressure. This should be 19–23 mbar for natural gas (typical domestic). A standing pressure below 19 mbar may indicate a network pressure issue — report to the transporter.
3. Light all appliances — bring all gas appliances to full rate simultaneously. This is the maximum demand condition.
4. Working pressure reading — with all appliances at full rate, record the pressure at the meter outlet. The pressure drop from standing to working should not exceed 1 mbar for a U6 installation (i.e., working pressure should be ≥20 mbar if standing pressure is 21 mbar).
5. If pressure drop exceeds 1 mbar: The system is undersized. Investigate whether the cause is in the meter (request an upgrade) or the pipework (pipe bore too small, run too long, or excessive fittings). A meter upgrade alone will not solve a pipework sizing problem.

Pressure Drop at the Appliance

For a more precise assessment, measure the pressure at the appliance burner inlet (the test point on the gas valve) rather than at the meter outlet. This captures pressure losses in all the pipework between meter and appliance. The pressure at the burner inlet should be within the appliance manufacturer's specified operating range — typically 17–25 mbar for natural gas appliances.

When to Request a Meter Upgrade

A meter upgrade should be requested from the network operator (Cadent, SGN, Northern Gas Networks, or Wales & West Utilities, depending on region) when:

• The calculated connected load exceeds the existing meter's rated capacity
• The index test shows a pressure drop exceeding 1 mbar (U6) or 2 mbar (U16) at peak demand, and the pipework is confirmed correctly sized
• A new large appliance is being installed that will push demand over the limit
• Meter upgrades are typically free of charge for domestic customers — the network operator arranges and pays for the work

Document the connected load calculation and index test results clearly in your commissioning paperwork. If you request a meter upgrade without supporting evidence, the transporter may decline or delay.

CSST (Corrugated Stainless Steel Tube)

CSST is increasingly used for gas distribution within buildings — particularly in new build and extension work. It is flexible, quick to install, and available in sizes from 12mm to 54mm equivalent bore.

Key considerations for CSST:

• CSST has a higher resistance per metre than equivalent copper tube — use manufacturer's sizing tables, not copper tube tables
• Bonding requirements — CSST must be bonded to earth at the consumer unit end and at the meter, and at any point where it passes through a room that contains a gas appliance (requirements vary by system; follow manufacturer's guidance and current IET Wiring Regulations)
• Installation must be by a competent Gas Safe registered engineer following the manufacturer's training
• CSST is not suitable for direct burial without appropriate protection

Relevant Standards

• BS 6891:2015+A2:2021 — Installation and maintenance of low pressure gas pipework up to DN 35 (32mm) in domestic premises
• IGEM/UP/2 — Installation pipework on industrial and commercial premises
• Gas Safety (Installation and Use) Regulations 1998 — legal framework for all gas work
• IGE/UP/1B — Tightness testing and direct purging of small low pressure gas installations

Summary

Gas meter sizing and pipework design are not complex once you understand the underlying principles:

• Calculate total connected load in m³/hr by dividing appliance kW by the calorific value (10.76 kWh/m³)
• A U6 meter handles up to ~6 m³/hr — a standard domestic installation is typically well within this
• Adding large appliances (range cookers, second boilers, gas fires) can push demand toward U16 territory
• Maximum pressure drop is 1 mbar for U6 systems, 2 mbar for U16 — verify with the index test on existing systems
• Pipe sizing uses BS 6891 tables — 22mm copper is adequate for most domestic boiler supplies; 28mm or 35mm for higher flows
• Meter upgrades are arranged with the network transporter and are typically free for domestic customers
• Document everything — connected load calculation, index test results, and commissioning data

If you're fitting gas appliances for APM customers or specifying products for a gas installation, our heating collection includes a range of gas-related fittings and accessories for qualified Gas Safe engineers.