Air Conditioning Commissioning: Procedure and Documentation

Why Commissioning Matters More Than Installation

A well-installed AC system can still underperform or fail early if commissioning is rushed or skipped. Commissioning is the process of verifying that the system operates correctly, safely, and efficiently before handing it over to the client. It's also a legal requirement under F-Gas Regulation (EU) 517/2014 as retained in UK law — you must document refrigerant charges, leak checks, and system parameters, and only F-Gas certified engineers may handle refrigerants.

This guide walks through commissioning a split or multi-split AC system from the point where pipework and electrical connections are complete, through to issuing your commissioning certificate.

Pre-Commissioning Checks

Before you power anything up, work through these physical checks:

• Pipework integrity: Confirm all pipe joints are brazed (not soldered), insulated, and correctly supported per manufacturer spacing requirements. Check for kinks or traps that will restrict refrigerant flow.
• Pipe lengths and lifts: Verify the total pipe run and any vertical lift is within the manufacturer's limits. Exceeding these without additional refrigerant charge is a common cause of poor performance.
• Electrical supply: Confirm supply voltage, phase, and cable sizing match the unit's data plate. Most residential split systems are single-phase 230V; larger units may require 3-phase. Verify the isolator and MCB/RCBO ratings are correct.
• Drain connections: Check the condensate drain is properly connected, falls away from the unit (minimum 1:50 gradient), and terminates correctly — not over a sink used for food preparation.
• Indoor unit installation: Confirm the unit is level (or very slightly back-tilted to assist drainage), securely mounted, and has adequate clearances front, top, and sides as per the installation manual.
• Communication wiring: For multi-split systems, check inter-unit communication cables are correctly terminated and polarity is observed where required.

Pressure Testing and Evacuation

If you have made any pipe connections on site (rather than using factory-charged units with pre-flared connections), the system must be pressure tested and evacuated before charging.

Pressure Test

Using dry nitrogen (never oxygen or compressed air), pressurise the system to the manufacturer's test pressure — typically around 40 bar for R410A and R32 systems, but always use the figure stated in the installation manual. Hold pressure for a minimum of one hour and confirm there is no drop. Mark the start pressure and end pressure on your commissioning sheet, along with ambient temperature at the start and end of the test (temperature changes of more than a few degrees can cause minor pressure variation).

Evacuation

Connect a two-stage vacuum pump and pull the system down to below 500 microns (ideally below 200 microns). Use an electronic micron gauge — analogue compound gauges are not accurate enough for this. Once target vacuum is achieved, isolate the pump and monitor for 15–30 minutes. A rising vacuum indicates moisture or a leak; a stable reading confirms the system is ready to charge.

Refrigerant Charging

Many split systems are factory-charged for a set pipe length — typically 5–7.5 metres. If your pipe run exceeds this, you'll need to add refrigerant per the manufacturer's specified top-up rate, usually expressed in grams per additional metre of liquid line.

To calculate additional charge:

1. Measure total liquid line length in metres
2. Subtract the factory-charged pipe length from the specification sheet
3. Multiply the difference by the manufacturer's top-up rate (e.g. 20 g/m for a 2.5 kW R32 unit)
4. Add refrigerant by weight using calibrated digital scales — never by pressure alone

Record the following on your commissioning sheet: refrigerant type, quantity added (grams), total system charge, and the serial number of the refrigerant cylinder used. This is a UK F-Gas record-keeping requirement for systems with a charge of 5 tonnes CO2 equivalent or more — but best practice is to record it for all systems regardless of size.

Electrical Tests

Carry out the following before first start-up, and record results:

• Insulation resistance: Test between live conductors and earth at 500V DC. Record in MΩ. Manufacturer minimums vary — typically 1 MΩ minimum, though modern inverter units can have lower tolerance to insulation testing due to capacitors; check the manual and disconnect inverter terminals if required.
• Earth continuity: Confirm earth conductor continuity from unit chassis back to the distribution board.
• Supply voltage: With the unit powered but not running, confirm voltage is within the unit's tolerance band (typically 230V ±10% for single phase).
• RCD operation: Test RCD trip at correct rating.

These tests align with BS 7671:2018+A2:2022 (18th Edition IET Wiring Regulations). You do not need to be a Part P-registered electrician to connect a dedicated AC circuit in most commercial contexts, but domestic work in England and Wales may require notification or self-certification under Building Regulations Part P.

First Start-Up and Performance Verification

Power the system on and allow it to run in cooling mode for at least 20 minutes before taking readings. Record the following:

Airflow and Temperature

• Supply air temperature: Measure at the indoor unit supply grille. Typical cooling supply air should be 10–14°C below return air temperature.
• Return air temperature: Measure at the return grille, away from the supply stream.
• Temperature difference (ΔT): Should be within the manufacturer's specified range (usually 8–12°C in cooling). A ΔT significantly below this suggests undercharge, airflow restriction, or a refrigerant circuit issue.

Refrigerant Circuit

• Suction pressure and saturation temperature: Record from your manifold gauge set. Compare suction saturation temperature against return air temperature — superheat should typically be 5–8°C for fixed-orifice systems (inverter systems vary).
• Discharge pressure: Record and confirm within normal operating range for the ambient temperature.
• Subcooling at liquid line: For TXV systems, subcooling should typically be 3–5°C. Excessive subcooling with low suction pressure suggests overcharge.

Current Draw

Measure running current with a clamp meter and compare to the data plate FLA (Full Load Amps). High current at low ambient suggests a fault; low current may indicate undercharge or an unloaded compressor.

Leak Check After Commissioning

Under UK F-Gas Regulations, a leak check is required on systems containing 5 tonnes CO2 equivalent or more before the system is handed over. For smaller systems it is still best practice and should be documented. Use an electronic refrigerant leak detector calibrated for the refrigerant type in use — R32 and R410A detectors are not interchangeable. Check all flare connections, valve cores, and any brazed joints.

What Goes on the Commissioning Certificate

Your commissioning certificate should capture everything needed to verify the system was installed and started correctly. As a minimum, include:

• Site address and system location
• Manufacturer, model numbers, and serial numbers (indoor and outdoor units)
• Refrigerant type and total system charge (grams)
• Quantity of refrigerant added on site and cylinder serial number
• Pipe run length (liquid and suction lines)
• Vacuum readings (microns) and duration held
• Pressure test result (start and end pressure, duration, temperature)
• Insulation resistance and earth continuity test results
• Supply voltage reading
• Supply and return air temperatures and ΔT in cooling mode
• Suction and discharge pressures, superheat and subcooling
• Running current vs. rated FLA
• Leak check result and method
• Engineer name, F-Gas registration number, company name
• Date of commissioning and signature

For multi-split systems, record individual indoor unit airflow readings and temperatures separately. If the system contains 5 tonnes CO2e or more, you must also log the commissioning in the equipment logbook and retain F-Gas records for at least five years.

Handing Over to the Client

Leave the client with the manufacturer's user manual, a copy of the commissioning certificate, details of the F-Gas maintenance schedule (systems with ≥5 tCO2e require annual leak checks), filter cleaning intervals, and your contact details. Walk them through basic operation — particularly the thermostat setpoints and how to recognise fault codes. A ten-minute handover prevents a lot of unnecessary callbacks.

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.