Potable & Hot Water Expansion Vessel Servicing

Potable vessels on the cold feed to an unvented cylinder must be WRAS approved with a butyl diaphragm suitable for drinking water. Servicing means isolating the cold feed, draining the vessel, checking the pre-charge against the pressure reducing valve setting and recharging to match, then re-testing the expansion relief and temperature relief valves as part of the same visit.

• WRAS-approved potable vessels only
• Pre-charge matched to the PRV setting
• Expansion and temperature relief valves tested
• Benchmark-style service record left on site

A hot water expansion vessel that has lost its charge shows itself as water dripping from the tundish or the outside discharge pipe whenever the cylinder reheats, and sometimes as banging pipework. We confirm whether the discharge is coming from the expansion relief valve (usually a vessel fault) or the temperature relief valve (a control fault) before recommending any work.

• Tundish and discharge pipe faults diagnosed
• Expansion vs temperature relief discharge identified
• Pressure reducing valve checked and reset
• Cylinder controls and thermostats verified

Unvented cylinder expansion vessel recharge is the most common fix for a dripping tundish. The vessel is isolated from the cold main, drained through the service valve and recharged, typically to around 3.0-3.5 bar to match the pressure reducing valve. Where the internal bubble-top air gap on an integral unit has collapsed, we re-establish it as part of the same visit.

• Recharge to match the PRV set pressure
• Integral air gap re-established on bubble-top cylinders
• Cold main pressure and flow recorded
• Tundish re-tested through a full reheat

Megaflo cylinders use an internal air bubble on some models and an external potable vessel on others. We recharge and replace both. Megaflo expansion vessel recharge takes under an hour on most installations; where the external vessel has waterlogged we fit a matched WRAS replacement and re-commission the cylinder.

• Megaflo, Santon, Telford, Gledhill, OSO and Heatrae Sadia
• Internal air gap re-charge and external vessel swap
• Matched WRAS replacement vessels
• Cylinder re-commissioned and logged

When a potable vessel will not hold its charge, replacement is the only safe option — a split diaphragm on a drinking water vessel is also a hygiene concern. We fit a correctly sized WRAS-approved vessel with a service valve so future recharges take minutes, and pressure test the cold feed before handover.

• Correct sizing from cylinder capacity
• Service valve fitted for easy future recharge
• Cold feed pressure tested after fitting
• Old vessel removed and disposed of