Boiler Expansion Vessel Servicing London

The vessel inside your boiler

Combi and system boilers contain a small expansion vessel behind the case, typically eight to twelve litres, sized for a modest system. It absorbs the expansion of the heating water and keeps the pressure within the range between fill pressure and the three bar safety valve. Because it is hidden, nobody checks it, and by the time the gauge is misbehaving the vessel has usually been flat for a long time. Checking the pre-charge takes a few minutes at the annual service and prevents the whole chain of problems that follows a flat vessel.

• Internal vessel located and access assessed
• Pre-charge tested with the water side drained
• Recharged to the correct cold fill pressure
• Checked as standard at every boiler service

Boiler pressure rising and falling

The pattern is unmistakable. From cold the gauge sits at about one bar; as the heating runs it climbs towards two and a half or three bar; the safety valve lifts and water goes out through the discharge pipe on the outside wall; when everything cools the gauge falls below one bar and the boiler may lock out on low pressure. Topping up through the filling loop restores it for a few days and the cycle repeats. That is an expansion fault, and the boiler itself is usually perfectly healthy.

• Pressure climbing to 3 bar when hot
• Pressure dropping below 1 bar when cold
• Discharge pipe dripping outside
• Low pressure lockouts and repeat top-ups

External expansion vessel installation

On many modern boilers the internal vessel is buried behind the heat exchanger and fan, and stripping the appliance to recharge it costs more in labour than the alternative. In those cases we fit an external expansion vessel on the return pipework — additional capacity, mounted on a bracket with its own service valve so it can be checked in future in minutes. It is also the right answer where the system has grown beyond what the boiler vessel was sized for, which is common after extensions or added underfloor heating.

• External vessel fitted where internal access is poor
• Additional capacity for enlarged systems
• Service valve fitted for future maintenance
• Neatly mounted and supported, not hung on pipework

Central heating expansion vessel replacement

Where the diaphragm has failed the vessel is replaced. On boilers with accessible vessels we fit the manufacturer part; where access is impractical we blank the internal vessel and fit an external unit of the correct capacity. Replacement includes recalculating the size against the actual system volume, setting the pre-charge to match the fill pressure for the height of the property, refilling and venting the system, and checking the safety valve and pressure gauge, which have often been damaged by months of repeated discharge.

• Manufacturer parts where access allows
• External replacement where it does not
• Capacity recalculated for the real system volume
• Safety valve and gauge checked and renewed if passing

Keeping it right — annual checks

A pre-charge check costs almost nothing when it is part of an annual boiler service, and it is the single most effective way to avoid winter pressure faults. We include it as standard, record the reading, and tell you when a vessel is starting to drift so it can be recharged before it causes a discharge. Landlords with several properties and blocks with multiple boilers can have the checks scheduled together so they are documented consistently across the portfolio.