(For Gas Safe Engineers Only)

This guide explains how to recharge an expansion vessel safely and correctly and is intended strictly for Gas Safe-registered engineers. Always follow manufacturer instructions and unsafe situation procedures where applicable.

An expansion vessel must only be recharged if no water is present at the Schrader valve. If water is discharged, the internal diaphragm has failed, and the vessel must be replaced.

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Why Recharging an Expansion Vessel Matters

Recharging a boiler expansion vessel is a critical part of servicing and fault diagnosis. A vessel with insufficient charge cannot absorb system expansion, leading to:

• Rapid pressure rises
• Pressure relief valve discharge
• Repeated boiler lockouts
• Customer complaints of pressure loss

Correct vessel pressure stabilises system operation and prevents unnecessary component failure.

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Pre-Checks Before Recharging

Before attempting to recharge:

• The boiler must be switched off
• System pressure must be fully released
• Schrader valve must be tested for water

If water is present, do not proceed — the vessel has failed.

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Step-by-Step Guide to Recharging an Expansion Vessel

1. Isolate and Drain the System

• Turn the boiler off electrically
• Isolate flow and return where possible
• Open the drain valve
• Allow pressure to fully drop to zero

Confirm the boiler pressure gauge reads 0.0 bar.

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2. Locate the Schrader Valve

Locate the Schrader valve on the expansion vessel.
Attach a calibrated pump with an accurate pressure gauge.

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3. Recharge to the Correct Pressure

Recharge the vessel to just under 1 bar, unless the manufacturer specifies otherwise.

Typical cold charge range:

• 0.8 – 1.0 bar

Do not exceed manufacturer limits.

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4. Confirm the System Is Fully Drained

Ensure:

• No water is exiting the drain point
• Boiler pressure remains at zero

Failure to fully drain will result in a false vessel charge.

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5. Leak Test the Schrader Valve

After pumping:

• Remove the pump
• Apply leak detection spray
• Check for bubbling

If leaking:

• Replace the valve core
• Retest

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6. Refill and Repressurise the System

• Close the drain valve
• Open isolation valves
• Refill the system
• Set the cold system pressure correctly

Observe pressure stability during heat-up.

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Bonus Check – Expansion Hose Blockages

Expansion vessel connecting hoses frequently block with sludge or magnetite.

While the system is drained:

• Inspect the expansion pipework
• Clear any restrictions
• Confirm free flow back to the vessel

Blocked hoses will cause pressure faults even with a correctly charged vessel.

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Common Signs of Expansion Vessel Failure

• Pressure rises rapidly on heat-up
• PRV discharges repeatedly
• Pressure drops after cooling
• Schrader valve releases water

In these cases, replacement is required.

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Summary – Expansion Vessel Recharging

• The vessel must be empty before charging
• Water at the Schrader valve = failed diaphragm
• Charge to just under 1 bar (unless stated otherwise)
• Always leak-test after charging
• Check expansion hoses for blockages

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Support for Gas Safe Engineers in Surrey & London

Certificates4UK Ltd supports engineers and property professionals with:

• Boiler diagnostics
• Expansion vessel testing and replacement
• Boiler servicing
• Gas Safety Certificate CP12 inspections

Operating across Molesey, Surrey & London.

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