Sealed Heating System Commissioning: Method Statement Requirements for Pressurisation and Expansion Vessel Installation

Table of Contents

• Introduction: Why Commissioning Method Statements Matter
• Pre-Commissioning Checks
• Expansion Vessel Pre-Charge Verification
• Fill and Vent Sequence
• System Pressurisation and PRV Testing
• PSSR Records and Handover Documentation
• What Your Method Statement Must Cover
• Frequently Asked Questions

Introduction: Why Commissioning Method Statements Matter

Commissioning a sealed heating system — particularly a commercial system with pressurisation unit and expansion vessel — is a phased operation that carries specific hazards at each stage. A method statement that simply says "commission in accordance with manufacturer's instructions" is inadequate because it provides no information about the hazards and controls for the specific system being commissioned.

Principal contractors and PSSR competent persons require a detailed commissioning method statement that describes the fill, vent, and charge sequence, specifies the PPE and exclusion zone requirements, and confirms the records that will be produced. This guide provides the method statement content for each phase. Full trade RAMS guidance is available at our pressurisation equipment RAMS resource.

Browse all trade RAMS templates on the RAMS AI trade hub — covering 22 specialist construction trades.

Pre-Commissioning Checks

Before any fill water is introduced to the system, the following checks must be completed and recorded:

• All pipework connections confirmed torqued to specification (compression fittings) or pressed (press-fit) — visual inspection of every joint in the system
• System isolation valves in correct position (all open except the pressurisation unit fill valve, which should be closed)
• Automatic air vents open and operational
• Pressure relief valve specification confirmed: set pressure correct, discharge pipe directed safely to drain (not toward persons or electrical equipment)
• Expansion vessel pre-charge pressure verified (see next section)
• Pressurisation unit wired, powered, and controller set to system parameters
• Pressure gauge calibration certificate confirmed current
• Commissioning records template prepared

Expansion Vessel Pre-Charge Verification

The expansion vessel pre-charge pressure is a critical system parameter. An incorrectly pre-charged vessel will not perform as designed — it can result in waterlogging (pre-charge too low) or inadequate expansion acceptance (pre-charge too high), both of which can lead to repeated PRV lifting and potential system damage.

Pre-charge verification procedure:

1. Confirm the specified pre-charge pressure from the system design documentation (typically equal to the system static pressure head at the vessel connection point)
2. Check the vessel using a calibrated nitrogen/air gauge at the Schrader valve — with the vessel disconnected from the system (or the system fully depressurised)
3. If the pre-charge is incorrect:
   • Too low: add nitrogen using a nitrogen cylinder with pressure regulator until the correct pre-charge is achieved. Do not use compressed air from a compressor — nitrogen is preferred for closed systems to prevent corrosion
   • Too high: release nitrogen from the Schrader valve in small increments until the correct pre-charge is achieved
4. Record the measured pre-charge pressure on the commissioning record

Fill and Vent Sequence

1. Connect the temporary filling loop (with double check valve) to the system make-up point
2. Slowly open the fill valve and allow the system to fill by static pressure from the mains — monitor system pressure gauge
3. Open automatic air vents on each section of the system in sequence to release trapped air
4. When each radiator and emitter circuit is full and the automatic air vent is expelling water rather than air, close the vent and move to the next
5. Continue until the system pressure gauge shows the system is filling at the correct rate and air is no longer being purged from the automatic vents
6. Open the pressurisation unit supply valve and allow the pressurisation unit to take over maintaining pressure
7. Close the temporary filling loop and disconnect
8. Check all automatic air vents are in the correct position (open for operation)

System Pressurisation and PRV Testing

Once the system is filled, the pressurisation unit will automatically maintain the set point pressure. Confirm the commissioning sequence:

1. Confirm pressurisation unit set point matches the system design pressure (typically 1.5 bar for a two-storey heating system, higher for multi-storey)
2. Allow the system to reach operating temperature and confirm that the pressurisation unit is maintaining the set point pressure under thermal expansion
3. Observe the pressure relief valve during initial heating up — if the PRV lifts at or near operating temperature, the system pressure, expansion vessel pre-charge, or vessel size may be incorrect and must be investigated
4. PRV test (where required by the written scheme of examination): manually open the PRV test lever briefly to confirm the valve lifts, discharges water to drain, and reseats cleanly when released. Record the result.

During pressurisation and initial heating, maintain the exclusion zone and ensure all personnel are wearing appropriate PPE (face shield, waterproof gloves).

PSSR Records and Handover Documentation

The following records must be produced at commissioning and handed to the principal contractor for retention and handover to the building owner:

• Pre-commissioning inspection record: all pre-commissioning checks, signed and dated
• Expansion vessel pre-charge verification record: measured pre-charge pressure, specified pre-charge pressure, any adjustment made
• System fill and commissioning record: fill date, fill water conditions (inhibitor type and dose added), initial cold system pressure, hot system pressure at operating temperature, pressurisation unit set point
• PRV test record: PRV set pressure, test result (lifts / reseats satisfactorily), date, signed by competent person
• PSSR examination record: to be produced by the PSSR competent person (not the installing contractor) confirming that the system has been examined in accordance with the written scheme

What Your Method Statement Must Cover

• Pre-commissioning checklist: connections, valves, vents, PRV, vessel pre-charge, gauge calibration
• Expansion vessel pre-charge procedure: specified pressure, measurement, nitrogen adjustment
• Fill and vent sequence: filling loop connection, air purging, pressurisation unit handover
• Operating pressure confirmation: set point, thermal expansion monitoring, PRV response
• PRV test procedure: where required by written scheme, test method, record
• PPE during commissioning: face shield, gloves, footwear
• Exclusion zone during pressurisation and initial heating
• PSSR records: list and format, responsibility for each record
• Handover documentation package to principal contractor

Frequently Asked Questions

Can we use mains water for initial system fill without adding inhibitor?

For the initial fill and commissioning test, mains water alone is sometimes used — but inhibitor must be added before the system is left in normal operation. The type and dose of inhibitor must be specified in the system commissioning specification and added at the correct dose rate based on the system volume. Record the inhibitor type, the dose added, and the system volume on the commissioning record. The inhibitor concentration should be confirmed using a test kit before handover.

What happens if the PRV fails to reseat after the test?

If the PRV fails to reseat cleanly after the test (i.e., it continues to drip or discharge water rather than closing fully), it must be replaced before the system is handed over. A PRV that does not reseat correctly will continue to discharge water, draining the system and potentially flooding the plant room. Do not attempt to force the PRV closed — replace it. Document the failure and the replacement on the commissioning record.

Who is responsible for arranging the PSSR written scheme of examination?

The responsibility for arranging the written scheme rests with the user or owner of the pressure system — in most cases, the building owner or the principal contractor acting on their behalf. The installing contractor should raise this requirement with the principal contractor at the pre-commencement stage to ensure that a competent person has been appointed to prepare the written scheme before commissioning takes place. Do not commission a PSSR-applicable system without confirming that the written scheme is in place.