Expansion Vessel Recharge & Repressurisation
Expansion vessel recharging service for heating, boiler and potable systems across London. We drain the water side, reset the air-side pre-charge pressure to the manufacturer figure and repressurise the system so it holds steady instead of climbing and dropping through every heating cycle.
What this service covers
Expansion vessel recharge
Recharging restores the cushion of air that absorbs expansion as the system heats. The water side is isolated and drained, the pre-charge is measured at the Schrader valve and air or nitrogen is added until the vessel reaches its design figure. The system is then refilled, vented and run up to temperature so we can confirm the pressure gauge behaves correctly across a full cycle.
- Water side drained before charging
- Air or nitrogen charge to design pressure
- System refilled, vented and run up to temperature
- Full heating cycle observed before handover
Pre-charge pressure & pressure adjustment
Expansion vessel pre charge pressure has to match the static head of the system, not a default number. Set too low and the vessel waterlogs quickly; set too high and the vessel cannot accept expansion, so the pressure relief valve discharges. We calculate the correct figure from the system height and fill pressure, then make the pressure adjustment and label the vessel with the set value and date.
- Pre-charge calculated from static head and fill pressure
- Adjustment up or down to the correct figure
- Vessel labelled with set pressure and service date
- Filling loop and gauge checked at the same visit
Expansion vessel repressurisation
If your boiler pressure keeps dropping to zero or climbing to 3 bar, repressurisation on its own will not hold. We look at the vessel first, then the filling loop, the pressure relief valve and any system leaks, so the repair addresses the cause rather than topping the system up every few weeks.
- Root cause found before topping up
- Pressure relief valve seat checked for passing
- Filling loop isolation confirmed
- Advice on safe home repressurisation
Heating & boiler expansion vessel recharge
Heating expansion vessel recharge covers the internal vessel inside a combi or system boiler as well as external vessels on the return pipework. Internal boiler vessels are often too small for the system they serve, so where recharging cannot keep up we quote an external expansion vessel installation instead of repeated call-outs.
- Internal boiler vessel recharge where accessible
- External vessel recharge on heating returns
- Undersized vessel identified and quoted
- Works on Worcester, Vaillant, Ideal, Baxi and Glow-worm systems
Related expansion vessel services
Pressure, heating and control faults often overlap — these pages cover the rest of the system.
Expansion Vessel Servicing, Inspection & Testing
Annual expansion vessel servicing for heating systems, boilers, unvented cylinders and plant rooms across London and inside the M2…
Expansion Vessel Replacement & Repair
Replacement and repair of failed, waterlogged and leaking expansion vessels on heating systems, boilers and unvented cylinders acr…
Potable & Hot Water Expansion Vessel Servicing
Servicing, recharge and replacement of potable (WRAS-approved) expansion vessels on unvented hot water cylinders across London — i…
Commercial & Plant Room Expansion Vessel Servicing
Servicing, pressure testing and replacement of commercial heating expansion vessels in plant rooms, boiler houses and rooftop plan…
Also see our underfloor heating repair London and heating pump repair pages.
Pressure dropping or relief valve discharging?
Tell us what the pressure gauge is doing and we will send an engineer with the right vessel and charging kit.
Call 07427 826446Frequently Asked Questions
Engineer labour from £120 + VAT per hour, with a 1-hour minimum. Parts and materials are quoted separately before work begins.
What's included in Expansion Vessel Recharge
- Review of the equipment and the outcome needed
- Inspection of condition and operation
- Recommendation of the right service scope
- Written quote before chargeable work
- Record of findings
How Expansion Vessel Recharge works
- 1
Contact us
Describe the equipment and what you need it to do.
- 2
Visit
An engineer inspects the equipment and its operation.
- 3
Recommendation
The right scope is recommended: servicing, repair or installation.
- 4
Written quote
Any chargeable work is quoted before it starts.
- 5
Work and record
Approved work is carried out and recorded.
Expansion Vessel Recharge prices
| Item | Price |
|---|---|
| Engineer labour | £120 + VAT per hour |
| Minimum charge | One hour |
| Parts and materials | Written quote before work starts |
See all rates on our prices page or read why customers choose us.
Expansion Vessel Recharge: scope, checks and next steps
An expansion vessel accommodates changes in water volume. Rising pressure while heating and discharge from a relief valve can have several causes; they do not prove the vessel needs replacement.
Separate the possible causes
Pre-charge measurement requires the water-side pressure to be relieved correctly. Compare the specified charge, vessel condition, valve leakage and available expansion capacity; measuring against a pressurised water side can mislead.
Repair or replacement?
A vessel that holds its charge may need adjustment rather than replacement. A failed diaphragm, corroded shell or inadequate capacity requires a different scope. Do not adjust a safety valve to mask pressure rise.
Choose the right service scope
Inspection, routine maintenance, fault diagnosis and installation answer different questions. Describe the equipment and the outcome needed so the visit can be scoped appropriately rather than treating every request as a replacement job.
Details to include with your job request
Provide the vessel label, system type, cold and hot pressure readings and where any discharge appears. Do not isolate safety devices or attempt a pressure test yourself.