How the gas cushion takes up expansion, what the pre-charge should match, how to check it, and what weeping on every heat-up means.
On a sealed unvented system there is no open vent to take the extra volume when water heats, and the check valve stops it going back to the main. Without somewhere to go, the pressure would rise sharply on every heat-up until the relief valves discharged. The expansion vessel, or an internal air gap built into some cylinders, is the normal provision; the relief valves are only back-ups.
How the vessel works
A flexible diaphragm separates the water side, connected to the system, from a gas side pre-charged with air or an inert gas. As the water heats, it pushes on the diaphragm and compresses the cushion, taking up the extra volume and limiting the pressure rise. When pressure falls, the compressed gas pushes accepted water back into the system. The cylinder still needs adequate supply pressure and flow. Water hammer is a separate design check: a suitable gas-cushioned device in the right place can damp a surge, but a vessel sized for thermal expansion may not stop shock from a distant tap.
Why the pre-charge matters
Set the pre-charge to the manufacturer's specified value. Where that value corresponds to the regulated cold inlet pressure, the vessel starts accepting water as soon as the pressure rises above that setting on heat-up.
- Too high: water is kept out until the pressure climbs further, so it enters late.
- Too low or flat: less usable cushion is left, so the pressure spikes on heat-up and can lift the relief valve.
Check the pre-charge with the water system isolated and fully depressurised: water pressure on the diaphragm changes the gas-side reading. A serviceable vessel with a reduced charge can be recharged; a failed diaphragm or damaged vessel needs repair or replacement.
Reading the symptom
- Weeps on every heat-up: lost charge or a failed vessel is the classic cause. A failed pressure-reducing valve letting the inlet pressure climb can do the same, so check both.
- Weeps even when cold: check whether the reducer is allowing excessive downstream pressure, whether the relief valve is correctly rated, and whether its seat is damaged or dirty. When it weeps tells you where to look, not what the cause is.
A cylinder with an internal air volume
The Megaflo Eco has an internal air volume instead of a separate expansion vessel. Its fault chart distinguishes the valve and discharge pattern:
- Expansion relief, intermittent: the air volume has reduced.
- Expansion relief, continuous: check the 3 bar pressure-reducing valve and the relief valve seat.
- T&P relief, intermittent: either the air volume has reduced or the 8 bar expansion relief valve is faulty. Recharging the air may not fix it.
For the prescribed recharge, stop the heat source, isolate the incoming water and open the lowest hot tap. Hold the T&P valve open until the tap stops running and the gurgling ceases, then release the valve. Air enters as water leaves; restoring the supply refills the cylinder and compresses the renewed air space. Close the hot tap when water flows again. If it keeps discharging, a competent person needs to investigate further. Turn the heating back on only when the cylinder is full and the required checks are done.