Gas Appliance Commissioning: What to Check and Record

Why Proper Commissioning Matters

Commissioning isn't just the final tick-box exercise before you hand over the keys. It's the process that confirms an appliance is safe, performing to manufacturer specifications, and operating efficiently. Get it wrong, and you're looking at callbacks, complaints, or worse — a dangerous installation that puts lives at risk.

Under BS 6891 and the Gas Safety (Installation and Use) Regulations 1998, every gas appliance must be properly commissioned before being left in service. The manufacturer's installation instructions are a legal document — not a suggestion. Gas Safe Register audits routinely pick up commissioning failures, and they remain one of the most common reasons engineers receive "Not to Current Standards" or "At Risk" classifications.

Before You Start: Pre-Commissioning Checks

Before you fire up any appliance, work through these fundamentals. Skipping them causes the majority of commissioning headaches.

Installation Pipework

• Tightness test — carry out a let-by test followed by a tightness test to BS 6891. The installation must hold at 20 mbar with no perceptible drop over 2 minutes (for installations up to 35 mm pipework).
• Purge — purge all pipework to remove air before attempting ignition. Vent to a safe location, never into a confined space.
• Pipe sizing — confirm the pipework is adequately sized for the total gas rate of all connected appliances. An undersized supply will show up as a low burner pressure reading.

Flue and Ventilation

• Visual flue check — inspect the full flue run for correct joints, support, and termination position. Check clearances to openable windows, air bricks, and boundaries per BS 5440-1.
• Flue flow/spillage test — for open-flued appliances, carry out a spillage test with all extract fans running and doors closed. The flue must establish draught within 2 minutes.
• Ventilation — confirm any required air vents are fitted, correctly sized to BS 5440-2, and not obstructed.

Core Commissioning Checks

Once pre-checks are complete, work through the commissioning sequence. Always follow the manufacturer's specific instructions — they override generic guidance.

1. Gas Rate Check

The gas rate confirms the appliance is burning the correct amount of fuel. You need a gas meter reading and a stopwatch (or the test facility on a digital meter).

1. Turn off all other gas appliances on the supply.
2. Fire the appliance on maximum load (for combi boilers, run a hot tap to bring on the burner at full rate).
3. Time how long the meter takes to consume a known volume — typically 0.01 m³ or one revolution of the test dial.
4. Calculate the gas rate using: Gas rate (kW) = (seconds in 1 hour ÷ time in seconds) × volume × calorific value ÷ 3.6
5. For natural gas, the calorific value is typically 38.76 MJ/m³ (check your local value on the National Grid website if needed).

The calculated gas rate should be within ±5% of the manufacturer's stated input. If it's outside this tolerance, check the gas valve setting, inlet pressure, and burner pressure before adjusting anything.

2. Pressure Readings

You need to record three key pressures:

• Standing pressure — the gas supply pressure with no appliances running. Should be approximately 21 mbar for natural gas. If it's below 19 mbar or above 23 mbar, investigate before proceeding.
• Working pressure (inlet) — the pressure at the meter or appliance inlet with the appliance firing at full rate. A drop of more than 1 mbar from standing to working suggests a restriction in the supply pipework.
• Burner pressure (operating) — measured at the appliance's test point with the unit firing. This must match the manufacturer's data plate value. For most domestic natural gas boilers, expect somewhere around 10–15 mbar, but always check the specific appliance data.

Use a calibrated digital manometer. Analogue gauges are acceptable but harder to read accurately at low pressures.

3. Combustion Analysis

For boilers and warm air units, a flue gas analysis (FGA) is essential. Using a calibrated flue gas analyser:

• Measure CO₂ or O₂ levels in the flue products — these confirm correct air/fuel ratio.
• Measure CO (carbon monoxide) — for room-sealed boilers, CO in the flue should typically be below 100 ppm on an air-free reading, but always compare against manufacturer limits.
• Record the flue temperature — useful for confirming heat exchanger condition.
• Calculate the CO/CO₂ ratio — should be below 0.004 (0.4%). A high ratio indicates incomplete combustion regardless of the raw CO reading.

4. Appliance-Specific Checks

Boilers

• Check the system pressure on sealed systems (typically 1.0–1.5 bar cold).
• Bleed radiators and confirm circulation on heating mode.
• Verify hot water temperature at the tap — should reach 50–55°C within a reasonable draw-off time.
• Test all safety controls: overheat stat, frost stat (if fitted), and programmer/timer operation.
• Check condensate drain runs freely with correct fall.

Gas Fires

• Confirm the fire is suitable for the fireplace opening and flue type.
• Check the fireplace recess dimensions against manufacturer minimums.
• Test the oxygen depletion system (ODS) if fitted — follow manufacturer test procedure.
• Verify any flame-effect settings and thermostat operation.

Cookers

• Check all burners ignite correctly and flame pattern is even.
• Test the flame supervision device (FSD) on each burner — the gas should cut off within 60 seconds of the flame being extinguished.
• Confirm the stability bracket is fitted (BS 6172 requirement for all freestanding cookers).
• Check the grill and oven light correctly, and door seals are intact.

What to Record and Document

Commissioning records are your proof of a competent installation. If it's not written down, it didn't happen — at least as far as Gas Safe and any future investigation are concerned.

Minimum Documentation

For every gas appliance commissioned, you should record:

1. Appliance details — make, model, serial number, GC number.
2. Gas type and supply — natural gas or LPG, meter location.
3. Standing pressure (mbar).
4. Working/inlet pressure (mbar).
5. Burner/operating pressure (mbar).
6. Gas rate (kW) and how it compares to the data plate.
7. Flue gas readings — CO, CO₂, flue temp (where applicable).
8. Ventilation provision — size and location of vents.
9. Flue type and termination — confirmed correct.
10. Safety devices tested — FSD, overheat stat, spillage test result.

Notification Requirements

Under the Building Regulations (Part J in England and Wales, Section 3 of the Building Standards in Scotland), certain gas work must be notified. As a Gas Safe registered engineer, you self-certify through the Gas Safe notification system. This includes:

• Installation of any gas appliance.
• Any work on a flue serving a gas appliance.

Notification must be completed within 30 days of the work. Failure to notify is a legal breach and can result in enforcement action.

Common Commissioning Pitfalls

• Not reading the manufacturer's instructions — every appliance has specific commissioning steps. Generic knowledge isn't enough.
• Skipping the tightness test — "I only changed the boiler, the pipework is existing" is not a valid reason. If you've disturbed any joint, you test.
• Forgetting to check all appliances — when you've worked on shared pipework, every connected appliance needs its gas rate and pressures re-checked.
• Incomplete records — recording the burner pressure but not the standing or working pressure. All three matter.
• Leaving the benchmark unpopulated — most boiler manufacturers now require the benchmark commissioning checklist to be completed for the warranty to be valid. No benchmark, no warranty claim.

Key References

• Gas Safe Register — registration and notification portal.
• BS 6891 — Specification for the installation and maintenance of low pressure gas installation pipework.
• BS 5440-1 & BS 5440-2 — Flues and ventilation for gas appliances.
• IGEM/UP/1B — Tightness testing and direct purging of small gas installations.
• Gas Safety (Installation and Use) Regulations 1998 — the overarching legal framework.

Published 2026-07-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.