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 826446