Summary

Boiler commissioning is not simply "lighting it up and making sure it heats the house." It is a formal, documented process that verifies the installation is safe, correctly sized, and performing as designed. The Benchmark Commissioning Checklist is the industry-standard document used across all major UK boiler manufacturers and is a condition of warranty for most brands.

Failing to complete the Benchmark checklist properly — or not completing it at all — is not just a compliance issue. If the system later develops problems, an incomplete Benchmark can invalidate the manufacturer's warranty. It also means you have no documentary evidence that the installation was commissioned correctly, leaving you exposed if a complaint or claim arises.

The HHIC (Heating and Hotwater Industry Council) operates the Benchmark Scheme. Boiler manufacturers submit their commissioning documentation to the scheme, which produces standardised checklists. Gas Safe registered engineers must hold appropriate categories to commission boilers.

Key Facts

  • Benchmark Commissioning Checklist — industry standard; issued with every new boiler; must be completed at commissioning; separate from installer certificate but equally important
  • Gas Safe registration — commissioning engineer must hold the appropriate ACS (Accredited Certification Scheme) categories: CENWAT (central heating and hot water), CCN1 (natural gas core), and applicable appliance categories
  • Gas rate check — verify boiler's actual gas consumption matches rated input; measured at meter with a suitable test meter or by observing meter dials over a timed period
  • Flue gas analysis — measure CO, CO2, O2, and efficiency at full output; CO/CO2 ratio should be <0.004 for a correctly burning gas boiler; efficiency reading confirms good combustion
  • System pressure — fill sealed system to 1.0–1.5 bar cold; check all air purged; verify expansion vessel pre-charge
  • Inhibitor — add BS 7593-compliant corrosion inhibitor (e.g., Fernox F1, Sentinel X100); record type and concentration tested; minimum 300ppm
  • Water treatment — BS 7593:2019 requires: clean system (or flush if replacing boiler), add inhibitor, fit filter (magnetic and/or scale reducer in hard water areas)
  • Flow rate — verify boiler achieves design flow rate through all zones; check pump operation
  • Controls commissioning — set programmer schedule, room thermostat, TRVs, and any weather or load compensation controls; verify boiler interlock is operating
  • Boiler Plus compliance — confirm the installation meets Part L controls requirements (room thermostat, TRVs, boiler interlock, time control, plus one additional measure)
  • Handover — demonstrate controls to customer; explain how to repressurise the system; leave Benchmark checklist, manufacturer's manual, and any relevant certificates
  • Noise test — listen for unusual sounds: whirring (pump/fan), gurgling (air in system), kettling (limescale on heat exchanger in hard water areas)
  • DHW performance — for combi boilers: verify hot water flow temperature at tap (target 50–55°C within 30 seconds at 10 litres/minute); check temperature stabilises

Quick Reference Table

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Commissioning Task Pass Criterion Test Method
Gas rate verification ±5% of rated input Time 0.02m³ at meter; compare to rating plate
Flue gas analysis (CO/CO2 ratio) <0.004 Flue gas analyser
Combustion efficiency >85% (target 90%+) Flue gas analyser
CO in flue gases <200ppm (usually <50ppm for healthy boiler) Flue gas analyser
System pressure (cold) 1.0–1.5 bar Pressure gauge on boiler
System pressure (hot) ≤2.5 bar Pressure gauge on boiler
Inhibitor concentration ≥300ppm (check with test strip) Inhibitor test strip
Hot water temperature (combi) 50–55°C at outlet within 30 seconds Calibrated thermometer
All radiators heating Equal temperature differential TRV + infrared thermometer

Detailed Guidance

Pre-Commissioning Checks

Before lighting the boiler:

  1. Gas supply check:

    • Emergency control valve (ECV) open
    • Carry out a tightness test on all new pipework installed (IGE UP/1B)
    • Purge air from new pipework at the boiler gas cock
    • Check gas pressure at the boiler inlet: should be 17–21 mbar for natural gas (NG), 20 mbar nominal

  2. Hydraulic check:

    • All pipework connections secure and leak-free (check visually before pressurising)
    • Drain cocks closed
    • Filling loop connected and both valves closed after filling
    • System filled and pressure at 1.0–1.5 bar
    • All radiator air valves closed
    • Circulate water and bleed all radiators; refill to pressure if it drops

  3. Electrical check:

    • Boiler correctly wired to fused spur
    • Zone valves wired correctly
    • Controls connected and set to operating mode

  4. Flue check:

    • Flue installed per manufacturer instructions
    • All joints made and supported per manufacturer guidance
    • Terminal location complies with Approved Document J clearances
    • Any room-sealed flue (balanced flue): room-sealed integrity verified

Gas Rate Verification

Gas rate check confirms the boiler is drawing gas at approximately the rated input.

Procedure (with mechanical meter):

  1. Ensure only the boiler is running — turn off all other gas appliances
  2. Put boiler into maximum output (heating and/or hot water)
  3. Observe the 0.01m³ test dial on the meter (fastest moving dial)
  4. Time how long it takes to rotate (e.g., 0.01m³ in X seconds)
  5. Calculate: gas consumption rate = (0.01m³ ÷ time in seconds) × 3600 = m³/hr
  6. Convert to kW: m³/hr × CV (m³) (typically 10.35 kWh/m³ for natural gas)
  7. Compare to boiler rated input on the data plate; should be within ±5%

Example: 0.01m³ in 24 seconds → (0.01/24) × 3600 = 1.5 m³/hr → 1.5 × 10.35 = 15.5 kW. If boiler is rated at 18kW, this indicates the boiler may be de-rated or gas pressure issue — investigate.

Smart meters: Many SMETS2 meters do not have a test dial. Use the meter's display to read gas consumption in m³ at the start and end of a timed period. This is less precise but acceptable.

Flue Gas Analysis

A calibrated flue gas analyser is required. Common analysers: Kane 455 Pro, Testo 320, Wohler A450.

Procedure:

  1. Insert sampling probe into the flue analysis point (manufacturer provides access)
  2. Allow boiler to run at full output for minimum 5 minutes to stabilise
  3. Record: CO (ppm), CO2 (%), O2 (%), efficiency (%), flue gas temperature
  4. Calculate CO/CO2 ratio: should be <0.004 for a healthy boiler

Interpreting results:

CO (ppm) CO2 (%) Ratio Assessment
<10 9–10.5 <0.004 Excellent combustion
10–50 9–10.5 <0.004 Good
50–200 9–10.5 <0.004 Acceptable; monitor
Any Any >0.004 Investigate; potential safety issue
>200 — — IMMEDIATELY DANGEROUS — shut down

If CO is high or CO/CO2 ratio exceeds 0.004, the boiler must NOT be left in service. Common causes:

  • Heat exchanger contamination or leakage (flue gas recycling)
  • Incorrect gas/air ratio (burner or gas valve fault)
  • Restricted combustion air supply

System Water Treatment (BS 7593)

BS 7593:2019 Treatment of water in domestic hot water central heating systems:

When replacing a boiler on an existing system:

  1. Chemical flush the existing system before installing new boiler (or demonstrate clean system)
  2. Add inhibitor: Fernox F1, Sentinel X100, or equivalent at manufacturer's specified dose rate
  3. Test inhibitor concentration with the supplied test strips — must show ≥300ppm
  4. Fit a magnetic filter (e.g., Magnaclean, Fernox TF1, Sentinel TX22) on the return pipe before the boiler
  5. In hard water areas (hardness >200 ppm): fit a scale reducer or add a scale inhibitor to the system water

Record on the Benchmark checklist:

  • Inhibitor product and batch number
  • Concentration tested and result
  • Filter type and location
  • Any flush products used

Failure to comply with BS 7593 is a common warranty invalidation trigger — boiler manufacturers check this when warranty claims are made.

Benchmark Completion and Handover

The Benchmark checklist booklet comes inside the boiler packaging. Key sections:

  • Section 1: Installer details, commissioning date, Gas Safe number
  • Section 2: Boiler details, serial number, location
  • Section 3: Commissioning checks with tick boxes and values (gas pressure, gas rate, flue analysis, system pressure, water treatment)
  • Section 4: Controls settings and Boiler Plus compliance
  • Customer handover section: Explanation of operation, repressurise instructions, service schedule

After completing:

  • Sign and date all sections
  • Leave the Benchmark booklet with the customer — it stays with the property
  • Photograph the completed checklist (keeps a copy for your records)

Handover demonstration must include:

  1. How to operate the controls (programmer, thermostat, TRV adjustment)
  2. How to read and repressurise the system (to 1–1.5 bar if it drops below 1 bar)
  3. What the pressure gauge is and where it is
  4. How to reset the boiler if it shows a fault code
  5. Annual service reminder — advise booking 12 months from commissioning

Frequently Asked Questions

Is Benchmark mandatory?

The Benchmark scheme is not a legal requirement under Statute, but:

  • Most boiler manufacturers require a completed Benchmark checklist as a condition of warranty
  • HHIC and Gas Safe recommend it as a professional standard
  • Many developers and social housing providers specify it in their contracts

In practice, completing Benchmark is the professional standard for all new boiler installations and is expected by all reputable customers.

Can I commission a boiler that someone else installed?

Yes, if you are Gas Safe registered with appropriate categories. You take responsibility for your commissioning checks, not the installation. If you find the installation is unsafe or non-compliant, you must not commission it and must advise the customer.

How often should the flue gas analyser be calibrated?

Per the analyser manufacturer's guidance — typically annually. Use an out-of-calibration analyser and your flue gas analysis results have no evidential value. Keep calibration certificates with the analyser.

Regulations & Standards