Potable Expansion Vessel Servicing London

Potable vessels and why they differ

A potable expansion vessel carries drinking water, so it must be WRAS approved with a butyl diaphragm suitable for potable use and a construction that will not taint the supply. A heating vessel fitted to a potable system is a contamination risk and a compliance failure, and we see it regularly on unvented installations done cheaply. Potable vessels also sit at mains pressure controlled by a pressure reducing valve, so the pre-charge is set against the PRV setting rather than a heating fill pressure.

• WRAS approved potable vessels only
• Pre-charge set against the pressure reducing valve
• Correct diaphragm material for drinking water
• Incorrect heating vessels identified and replaced

Unvented cylinder expansion vessel recharge

Servicing an unvented cylinder properly means isolating the cold supply, draining down enough to remove pressure from the vessel, testing the pre-charge, recharging it to the correct figure and refilling. On bubble-top cylinders with no separate vessel, the internal air gap is re-established instead by draining the cylinder and letting it refill, which restores the expansion space. We also test the expansion relief valve, the temperature and pressure relief valve, the thermal cut-out and the tundish discharge arrangement while we are there.

• Unvented cylinder expansion vessel recharge
• Internal air bubble re-established where fitted
• Expansion and T&P relief valves tested
• Tundish and discharge pipework checked
• G3 service record issued

Megaflo and branded cylinder vessels

Megaflo, Santon, OSO, Gledhill and Telford cylinders each handle expansion slightly differently — some with an external potable vessel, some with an internal air bubble, some with an integral vessel inside the case. Megaflo expansion vessel recharge is one of our most common calls, usually prompted by water running from the tundish. We know which models can be recharged, which need an external vessel retrofitting, and which manufacturer parts to fit so the cylinder warranty is not compromised.

• Megaflo expansion vessel recharge and replacement
• Santon, OSO, Gledhill and Telford cylinders supported
• External vessels retrofitted where appropriate
• Manufacturer parts used to protect warranties

Water discharging from the tundish

Water dripping or running from the tundish whenever the cylinder heats is the signature of a failed potable expansion vessel. With no expansion space, the pressure rises past the expansion relief valve setting and the excess is dumped. People often have the relief valve replaced first, which stops the drip briefly before it starts again, because the valve was doing its job. We test the vessel before condemning valves, then recharge or replace it and confirm the discharge stops through a full reheat cycle.

• Tundish discharge diagnosed at source
• Expansion relief valve tested rather than assumed faulty
• Result proved across a full reheat
• Discharge pipework corrected where non-compliant

Replacement and hot water expansion vessel servicing

Where the potable vessel has failed we replace it with a correctly sized WRAS unit, fitted with an isolating and draining arrangement so future servicing does not mean draining the cylinder. Sizing is based on the stored volume — a 300 litre cylinder needs considerably more expansion capacity than a 150 litre unit, and undersized vessels are a frequent cause of repeat failures. Annual hot water expansion vessel servicing alongside the G3 cylinder service is the cheapest way to keep the system compliant and quiet.