Expansion Vessel Servicing London

What expansion vessel servicing includes

A proper expansion vessel service is a sequence of measurements, not a visual glance. We isolate the vessel and drain the water side so the reading is genuine, test the pre-charge at the Schrader valve with a calibrated gauge, compare it against the cold fill pressure of the system rather than a generic figure, recharge with nitrogen or dry air to the correct pre-charge, and then bring the system back to pressure and watch how it behaves through a full heating cycle. The pressure gauge should rise modestly as the system heats and settle back on cooling; a gauge that climbs to the safety valve lift pressure tells you the vessel is not doing its job.

• Vessel isolated and water side drained before testing
• Pre-charge measured with a calibrated gauge
• Pre-charge set against actual cold fill pressure
• System observed hot and cold to prove the vessel works
• Safety valve, filling loop and gauge checked at the same visit

Expansion vessel inspection, testing and pressure checks

Inspection covers the things that shorten a vessel life long before the diaphragm fails: corroded shells, vessels hanging unsupported from pipework, seized or missing isolation valves, perished flexible connections and Schrader valves that leak as soon as they are touched. Testing is straightforward — a vessel with sound internals will hold its pre-charge and feel light when tapped at the base; a waterlogged vessel is heavy, dull-sounding and will push water back out of the air valve. We record every reading so the next service has a baseline to compare against rather than starting from scratch.

• Expansion vessel inspection and condition report
• Expansion vessel pressure check and pressure test
• Waterlogging test at the Schrader valve
• Support brackets, isolation and flexi connections checked
• Readings recorded for the maintenance log

Expansion vessel maintenance and annual service intervals

Domestic heating vessels should be checked annually, ideally at the boiler service, because a vessel that has lost pre-charge will quietly cook the safety valve and the pressure relief discharge for months before anyone notices. Unvented hot water vessels are part of the annual G3 service and should never be skipped. Commercial plant rooms with large or multiple vessels are best placed on a written maintenance schedule with recorded readings, because a single failed vessel in a bank will overload the rest and shorten their lives too. We can quote an annual maintenance visit or a full maintenance contract with priority attendance.

• Annual expansion vessel maintenance for homes and landlords
• Vessel checks built into boiler and cylinder servicing
• Expansion vessel maintenance contracts for commercial sites
• Written records supplied after every visit

Signs your expansion vessel needs servicing

The classic symptom is a boiler pressure gauge that will not stay put — it climbs towards three bar when the heating runs, then drops below one bar once everything cools, and topping up through the filling loop only repeats the cycle. Water dripping from an outside discharge pipe, a safety valve that has started weeping, or a system that needs refilling every week or two all point the same way. On unvented cylinders the tell-tale is water discharging from the tundish whenever hot water is drawn. None of these are the boiler failing; they are the expansion arrangement failing.

• Pressure rising towards 3 bar when hot
• Pressure dropping below 1 bar when cold
• Discharge pipe dripping outside the property
• Frequent refilling through the filling loop
• Water from the tundish on unvented cylinders

Vessel sizing and system volume

A vessel that is too small will fail again no matter how carefully it is recharged, and small undersized vessels are extremely common where boilers have been swapped without recalculating system volume. We work out the approximate system volume from the boiler output, radiator count and pipe runs, check the acceptance factor against the fill and safety valve pressures, and either confirm the fitted vessel is adequate or recommend an additional external vessel. Adding an external vessel alongside the boiler internal one is usually cheaper and more reliable than stripping the appliance.

• System volume estimated and vessel sizing checked
• External expansion vessel installation where capacity is short
• Correct acceptance factor for fill and relief pressures
• Potable vessels sized to cylinder capacity

Service, repair or replace

Most vessels that hold a recharge for a full heating season are worth servicing. A vessel that loses pre-charge within days has a punctured diaphragm or a leaking air valve, and the honest answer is replacement — repeated recharging is a way of spending money slowly. We carry common domestic vessels and Schrader valve cores on the van and can usually recharge or replace on the first visit, with larger commercial and plant room vessels quoted after survey. Where replacement is right we fit the correct capacity with serviceable isolation so future work does not need the system drained.