Expansion Vessel Recharge London

How an expansion vessel recharge is done properly

The vessel has air or nitrogen on one side of a rubber diaphragm and system water on the other. To measure the pre-charge you must remove the water pressure first, otherwise the gauge simply reads the system. We isolate the vessel where a service valve exists, drain the water side down, then read the pre-charge at the Schrader valve. If it has dropped we pump it back to the target figure — normally matching the cold fill pressure — recheck after it settles, reconnect, refill the system and bleed air from radiators and high points. Finally we run the system up to temperature and watch the gauge.

• Vessel isolated and water side drained
• Pre-charge read at the Schrader valve
• Recharged to the correct pre-charge pressure
• System refilled, vented and re-pressurised
• Behaviour proved hot and cold before we leave

Pre-charge pressure and pressure adjustment

Pre-charge is not a universal number. On a sealed domestic heating system it should equal the cold fill pressure, usually around 1 to 1.5 bar depending on the height of the building above the boiler — a tall Victorian conversion with radiators three floors up needs a higher figure than a ground-floor flat. On potable and unvented systems it is set relative to the incoming main and the pressure reducing valve setting. Too low and the vessel fills with water; too high and the system cannot pressurise at all. We calculate and set the figure that suits your property.

• Pre-charge set against cold fill pressure
• Building height and static head allowed for
• Potable vessels set against the pressure reducing valve
• Pressure adjustment after boiler or system changes

When a recharge fixes it and when it does not

Vessels lose a small amount of pre-charge naturally over years, and those are the ones a recharge fixes permanently. Where pressure has gone in weeks, the diaphragm has split and the air is escaping into the system water — you can often confirm it by pressing the Schrader valve and getting water instead of air. A leaking valve core is a cheap fix; a failed diaphragm on a sealed vessel is not repairable and the vessel must be replaced. We tell you which you have on the first visit instead of booking a series of recharges.

• Gradual pressure loss — recharge holds
• Water at the air valve — diaphragm failed
• Leaking Schrader valve core replaced on site
• Replacement quoted where recharging is pointless

Recharging boiler, potable and unvented vessels

Combi and system boilers have an internal vessel behind the case that can usually be recharged in place, though on some models access is poor enough that fitting an external vessel is the better long-term answer. Unvented cylinders such as Megaflo, Santon and OSO use either a separate potable expansion vessel or an internal air bubble that needs re-establishing. Both are routine work for us. Potable vessels must be WRAS-approved with a diaphragm rated for drinking water, so we recharge or replace like for like and never fit a heating vessel to a potable system.

• Boiler internal expansion vessel recharge
• Megaflo and unvented cylinder expansion vessel recharge
• Air bubble re-established on bubble-top cylinders
• Potable vessels kept WRAS compliant

What it costs and what you get

A recharge is charged at our standard visit rate and usually completed within the first chargeable period, including the pressure test and the checks on the safety valve, filling loop and gauge that should accompany it. Where the vessel turns out to be waterlogged we will give you the replacement price on the spot so you can decide there and then rather than waiting for a quote. Commercial and plant room vessels are quoted after a short survey because access, isolation and vessel size vary far more than on domestic work.