Gas Pipe Sizing Guide for UK Homes

Getting Gas Pipe Sizing Right

Undersized gas pipes mean appliances don't get enough gas. The boiler short-cycles, the hob flames are weak, and the system doesn't perform as it should. In the worst case, an undersized supply can cause incomplete combustion — a genuine safety hazard.

Gas pipe sizing isn't guesswork. It's governed by IGEM/UP/2 Edition 3 and every Gas Safe registered engineer should understand the principles. This guide covers the practical side of domestic pipe sizing.

Important: Only Gas Safe registered engineers should work on gas installations. This guide is a reference — always verify pipe sizes against the current edition of IGEM/UP/2 and manufacturer specifications.

The Basics: What Determines Pipe Size?

Three factors drive pipe size:

1. Total gas demand — how much gas (in kW or m³/hr) all the appliances need
2. Pipe run length — the total length of pipe from the meter to the furthest appliance
3. Acceptable pressure drop — the maximum pressure loss allowed across the installation (1 mbar for domestic, from a 21 mbar supply)

The longer the pipe run and the higher the demand, the bigger the pipe needs to be.

Step 1: Work Out Appliance Demand

Every gas appliance has a rated input (gross) in kW. You'll find this on the data plate or in the installation manual. Common domestic appliances:

| Appliance | Typical Input (kW) | Gas Rate (m³/hr) | |---|---|---| | Combi boiler | 24–40 kW | 2.5–4.2 | | System/regular boiler | 15–30 kW | 1.6–3.1 | | Gas hob (4 burner) | 8–10 kW | 0.8–1.0 | | Gas fire (decorative) | 3–7 kW | 0.3–0.7 | | Gas fire (DFE/open flue) | 5–9 kW | 0.5–0.9 |

To convert kW to m³/hr for natural gas:

Gas rate (m³/hr) = Input (kW) ÷ 10.76 (calorific value)

10.76 is the average gross calorific value of natural gas in the UK in kWh/m³.

Total Demand

Add up the gas rates of all appliances connected to the pipework section you're sizing. For a typical domestic installation:

30 kW combi + 9 kW hob = 39 kW total

39 ÷ 10.76 = 3.62 m³/hr

This is your maximum simultaneous demand — assume everything runs at full rate at the same time (diversity factors don't apply in domestic installations under IGEM/UP/2).

Step 2: Measure the Pipe Run

Measure the total pipe length from the gas meter outlet to the furthest appliance. Include:

• Actual pipe length along the route
• Equivalent length for fittings — each elbow, tee, and coupling adds resistance. As a rule of thumb, add 0.5m per elbow and 0.3m per tee to the run length

For a boiler in the kitchen with the meter outside the front of the house, a typical run might be:

8m straight pipe + 4 elbows (2m) + 2 tees (0.6m) = 10.6m effective length

Round up to the next whole metre for safety.

Step 3: Look Up the Pipe Size

The standard pipe sizing tables from IGEM/UP/2 give maximum gas rates for each pipe diameter at various run lengths. Here are simplified figures for copper pipe (the most common domestic material):

Maximum Gas Rate for Copper Pipe (1 mbar pressure drop)

| Effective Pipe Length | 15mm | 22mm | 28mm | 35mm | |---|---|---|---|---| | 3m | 1.2 m³/hr | 4.6 m³/hr | 10.1 m³/hr | 20.3 m³/hr | | 6m | 0.8 m³/hr | 3.2 m³/hr | 7.1 m³/hr | 14.3 m³/hr | | 9m | 0.7 m³/hr | 2.6 m³/hr | 5.8 m³/hr | 11.6 m³/hr | | 12m | 0.6 m³/hr | 2.3 m³/hr | 5.0 m³/hr | 10.1 m³/hr | | 15m | 0.5 m³/hr | 2.0 m³/hr | 4.5 m³/hr | 9.0 m³/hr | | 20m | 0.45 m³/hr | 1.8 m³/hr | 3.9 m³/hr | 7.8 m³/hr |

These are approximate figures for guidance. Always refer to the current IGEM/UP/2 tables for exact values.

Reading the Table

For our example (3.62 m³/hr demand, 11m effective length), look along the 12m row and find the first column that exceeds 3.62 m³/hr:

• 15mm: 0.6 — too small
• 22mm: 2.3 — too small
• 28mm: 5.0 — this works

So the main supply pipe from the meter to the first branch needs to be 28mm copper.

After the branch to the hob (which takes roughly 1.0 m³/hr off the demand), the remaining run to the boiler carries 2.62 m³/hr. At that lower demand and shorter remaining length, you may be able to drop to 22mm.

Use the Gas Pipe Sizing Guide tool for a quick lookup — enter your appliance demands and pipe run length and it shows you the minimum diameters.

Branch Sizing

Each branch from the main run only needs to carry the demand for the appliances it serves:

| Branch | Demand | Typical Pipe Size | |---|---|---| | Combi boiler (30 kW) | 2.8 m³/hr | 22mm (short run) or 28mm (long run) | | Gas hob | 0.9 m³/hr | 15mm (up to ~3m) | | Gas fire | 0.5 m³/hr | 15mm (up to ~5m) |

Key rule: Never reduce a pipe size and then increase it again downstream. The reduced section becomes the bottleneck.

Pipe Material Options

| Material | Typical Use | Notes | |---|---|---| | Copper (to BS EN 1057) | Most domestic runs | Soldered or compression fittings | | CSST (corrugated stainless steel) | Longer runs, re-routes | Flexible, quicker to install, needs bonding | | Steel (mild steel, to BS 1387) | Older installations, exposed runs | Threaded fittings, more labour |

CSST has become increasingly popular for domestic work. It's faster to install than copper on long runs and easier to route through floor voids. However, it requires proper earth bonding to the gas installation and the fitting manufacturer's specific connectors. Don't mix CSST fittings between manufacturers.

Pressure Testing

Every new gas installation or alteration must be tightness tested before commissioning:

1. Let-by test — check the meter and ECV for leaks
2. Tightness test — pressurise the installation to 20 mbar and monitor for 2 minutes. Acceptable pressure drop: less than 4 mbar (for installations under 0.035 m³ internal volume — covers most domestic work)

If it fails, find the leak. The most common culprits are compression fittings that haven't been tightened properly, or solder joints that didn't flow fully.

Record all test results and keep them with the installation records.

Common Mistakes

• Running everything in 22mm because it "should be enough." It often isn't, especially on longer runs to combi boilers. Always calculate.
• Forgetting to add equivalent lengths for fittings. Four elbows in a pipe run adds 2m of equivalent length. On a borderline calculation, that's enough to bump you up a pipe size.
• Not checking the meter. The gas meter itself has a maximum output rate. A standard U6 domestic meter delivers about 6 m³/hr. If your total demand exceeds this, you need a meter upgrade from the gas transporter.
• Ignoring existing pipework. When adding a new appliance to an existing system, check that the existing pipe from the meter is big enough for the combined demand. This is the most common sizing error on boiler upgrades.
• Not purging properly. After any new pipework, purge each appliance individually until you smell gas at the test point, then tighten up and re-test.

Gas Safety Checks

Once the installation is complete and tightness-tested, carry out a full gas safety check on every appliance:

• Operating pressure
• Burner pressure
• Heat input check
• Flue gas analysis (CO and CO₂ readings)
• Ventilation adequacy
• Visual flue condition check

Record everything on the appropriate Gas Safe notification form and issue the landlord/homeowner with their documentation.

Quick Summary

1. Total up all appliance gas rates in m³/hr
2. Measure the pipe run length and add equivalent lengths for fittings
3. Look up the minimum pipe diameter from the sizing tables
4. Size each branch individually based on the appliances it serves
5. Never reduce and re-increase pipe sizes on the same run
6. Tightness-test every installation before commissioning

The Gas Pipe Sizing Guide tool gives you minimum pipe diameters based on demand and run length — useful for quick checks on site or when quoting a job.