Megaflo is a registered trade mark of Heatrae Sadia, a trading name of Baxi Heating UK Limited. We are an independent London leak firm with no connection to that company — we are not approved, appointed or endorsed by them, and we are not their service agent. What follows is a description of how these cylinders behave when they lose water, written from the repair end.
The word is used loosely, too. Plenty of people say "my Megaflo is leaking" about an unvented cylinder made by somebody else entirely, because the brand has become the generic term for the tall white cylinder in the airing cupboard. Most of what follows applies to any unvented cylinder; the air gap section is the part genuinely specific to this range.
Three places a Megaflo shows water, and they mean different things
An unvented cylinder is a sealed steel vessel sitting at mains pressure, with a set of safety devices whose entire purpose is to let water out in a controlled way. So before deciding anything has failed, work out which of these you are looking at.
- Water from the discharge pipe, via the tundish. The tundish is the small open funnel in the pipework above the cylinder, usually with a visible air break. Water running through it means a relief valve has lifted. The cylinder is intact; something upstream of it is wrong.
- Water appearing at the base of the cylinder, with the top dry. The vessel is wrapped in sprayed foam insulation, so you cannot see the steel. Anything weeping higher up runs down inside that foam and emerges at the floor. A wet base tells you almost nothing about where the leak is.
- Water on the pipework around it. Compression joints on the inlet group, the hot outlet, a blending valve, the secondary return, the boiler coil connections on an indirect model. These are ordinary plumbing leaks that happen to be next to a cylinder.
The discharge pipe: read which valve lifted
There are normally two devices that can dump water, and they fail for different reasons. On an unvented cylinder the inlet control group carries a pressure reducing valve and an expansion relief valve; a combined temperature and pressure relief valve is screwed into the cylinder body itself. Both discharge through the same tundish, which is exactly why the tundish is installed somewhere you can see it.
| What you observe | What it usually points to |
|---|
| Trickle from the tundish every time the water heats, stopping when it cools | Expansion has nowhere to go. On an air-gap model, the air gap has been absorbed into the water. On a vessel model, the expansion vessel has lost its charge. |
| Constant dribble with the cylinder cold and nothing running | Debris on a valve seat, or a pressure reducing valve passing and letting mains pressure creep up behind it. |
| Hot or steaming discharge, cylinder very hot to the touch | A control failure. The thermostat and the thermal cut-out have both let the cylinder overheat and the relief valve is doing the only thing left. Switch off the heat source and get someone out. |
| Discharge that started right after a new immersion heater or a valve change | Something was disturbed. Usually a seat, a seal or an isolating valve left in the wrong position. |
One rule cuts through all of it: never plug, cap or wind down a discharging relief valve to stop the noise, and do not let anyone "just swap the valve" as the fix. The valve is a symptom. Replacing it because it is wet is like replacing a smoke alarm because it keeps going off.
The air gap, which is the part people get wrong
Water expands when it is heated, and in a sealed vessel that expansion has to go somewhere. Most unvented cylinders solve this with an external expansion vessel: a metal can plumbed in beside the cylinder, with a rubber diaphragm and a Schrader valve on the end, charged with air like a tyre. When it fails you take the pressure off the system, put a gauge and a pump on the Schrader valve, and re-charge it — or replace it, because a perished diaphragm cannot be pumped back up.
A large part of the Megaflo range does it differently. Instead of a separate vessel there is a pocket of air held inside the top of the cylinder — the air gap. There is no can, no diaphragm and no Schrader valve, so if you go looking for an expansion vessel to pump up you will not find one. Over months and years, air dissolves into the water and that pocket shrinks. When it has gone, the first hot cycle of the day has nowhere to expand into, pressure rises, the expansion relief valve lifts, and you get the classic trickle out of the tundish whenever the water heats.
Note that the range is not uniform on this — different Megaflo models have been built over the years, and some variants are supplied with an external vessel instead. Look at the cylinder before assuming. If there is a can strapped to the pipework beside it, you are on a vessel, not an air gap.
How the air gap is re-charged
Heatrae Sadia publish the procedure, and it is deliberately simple. In outline: the mains cold supply to the cylinder is turned off, the lowest hot tap in the property is opened, and the temperature and pressure relief valve is held open until the gurgling stops and water stops coming out of both the tap and the valve. The valve is then closed, the cold supply restored, and the tap closed once water flows again. That drops the water level, lets air back into the top of the vessel, and re-forms the gap.
Two cautions matter more than the steps. Holding open a relief valve on a hot cylinder sends scalding water down the discharge pipe, so turn the heat off and let it cool first. And if the air gap collapses again within weeks rather than years, re-charging it is not the answer — something else is putting pressure into the vessel, usually a pressure reducing valve that has stopped reducing.
The immersion bosses, and why the leak hides
Immersion heaters screw into bosses in the cylinder wall and seal on a gasket. Gaskets perish. Elements get changed, sometimes by someone who overtightened or cross-threaded the replacement, and the seal never recovers. Heavy scale — a real issue across much of London's water supply — makes element changes more frequent, which means more chances for a boss to be disturbed.
What makes this awkward is the foam. A boss weeping a few millilitres an hour produces nothing you can see at the boss, because the water is absorbed into the insulation and tracks down inside it. It shows up as a damp floor, a stained ceiling in the room below, or a musty airing cupboard weeks later. If the cylinder is dry at the top and wet at the bottom, the immersion bosses are one of the first things to check, and they are checked by exposing them, not by looking.
When the cylinder itself has gone
This is the uncommon case, and it is the one that costs money. The vessel is stainless steel and it is welded. If it has holed or cracked, there is no repair: you cannot braze, solder or patch a pressurised stainless vessel and expect it to hold, and nobody reputable will try. The cylinder is replaced.
The signs are a persistent wet base with the bosses, valves and every joint confirmed dry, and often a flow rate or recovery that has quietly got worse. One manufacturer note worth knowing if you have a softener: Heatrae Sadia's own guidance sets a chloride limit of 250 mg per litre on softened water going into a Megaflo. Stainless does not like chlorides, and a softener set up without regard to that is not doing the cylinder any favours.
What we do and what we do not
We are leak specialists. If water is appearing near a cylinder and nobody can say why, that is our work: isolate, pressure-test, and trace it — including the buried hot and cold pipework and the heating circuit feeding an indirect coil, which is where a surprising number of "cylinder leaks" turn out to live. We work at £150 per hour agreed before we attend, with no call-out fee, and if we do not find the leak you do not pay for the time we spent looking. Where the leak has damaged a floor or a ceiling, our own handymen and builders put the fabric back rather than leaving you with a hole.
What we do not do is present ourselves as a manufacturer's agent. Work on unvented hot water storage is controlled under Building Regulations and needs an engineer holding the unvented qualification — G3 in England and Wales, P3 in Scotland, P5 in Northern Ireland. If your cylinder is in warranty, the sensible first call is the manufacturer's own support line, not us. If it is out of warranty, or the problem is a leak rather than the appliance, that is what we are for.
If the water has already reached a neighbour, a ceiling or a floor, a detection visit includes a written report prepared to support a trace-and-access claim. We can only document what we find; what your policy covers is the insurer's decision, and anyone promising you a payout is guessing.
