Megaflo Cut Out: What It Is and What to Check

What is a Megaflo cut out?

Megaflo is a brand name that many people use for any unvented stainless steel cylinder, so this article is about the type of cylinder rather than one product line. London Cylinder Specialist is not affiliated with, endorsed by or approved by any cylinder manufacturer. Direct versions of these cylinders carry factory-fitted immersion heaters. Each heater has two thermal controls in series: a thermostat that does the everyday switching and a separate thermal cut out that is the last line of defence. A fault in the expansion vessel is a different matter: it shows as a discharge at the tundish, not as a trip.

The cut out is a manual-reset device. Unlike the thermostat, which switches itself on and off as the water cools and heats, the cut out opens its contacts when the heater body reaches a set safety temperature and stays open until someone presses a small reset button. Manufacturers’ instructions for cylinders of this type forbid bypassing the thermal cut out, and that rule applies to everyone.

On indirect cylinders heated by a boiler, there is no immersion cut out in the same sense, but the cylinder thermostat, the motorised valve and, on many models, a thermal cut out on the heat source circuit do the same job. The checks here apply wherever the hot water stops and a reset button is involved.

Why a cut out trips

A cut out trips because the water or the heater reached a temperature the thermostat should have stopped earlier. That is a symptom, and several different faults produce it. Think of the reset as a warning light being cleared, not a repair.

The cut out can also trip for reasons that are not a fault at all. A thermostat that has stuck in the closed position, or a heater left on boost, can take the heater past its limit.

• A failed or stuck cylinder thermostat that keeps the heater on past the set temperature.
• A scaled-up element running hotter at the sheath than the water around it, common in hard water areas.
• A heater that has been left on boost for long periods, with the thermostat set at its top setting.
• An element that has partly lost its water cover because the cylinder was drained and the heater switched back on before it refilled.
• A faulty cut out that trips below its rated temperature.

Checks before you reset anything

Start with what you can see without opening anything. Check the programmer and any boost button, because a cylinder left on boost for hours can legitimately go too hot. Check the thermostat setting on the dial outside the cover, where one is fitted. It should sit around the temperature in your manual, not wound to maximum.

Look for obvious signs of a problem near the cylinder: a hot smell, a discoloured cover or any sign of water around the immersion terminals. If you find any of these, switch off the immersion supply at the isolator and stop. Nothing inside an electrical cover, and nothing inside a sealed safety valve, is a householder job.

Check the room. A recently refilled cylinder, or one drained for plumbing work, should have been full before the heater was switched on. If somebody drained it recently, that is the most likely reason for a trip and worth telling the engineer.

Also listen. Rumbling or a kettle-like noise from the cylinder suggests scale on the element, which points to the heater itself as the fault, and the noisy cylinder guide explains how to tell it from pipework noise. Read unvented cylinder tripped cut out for how this fits the wider unvented fault picture.

The single reset

If the checks above show nothing alarming, you may press the reset button once, but only if it can be reached without removing a cover. Isolate the supply to the heater first at the fused spur or double-pole switch, press the button firmly until it clicks, then restore power. Your manual shows where the button is on your model and whether a householder reset is allowed. If it can only be reached by removing an electrical cover, leave it for an engineer.

Then wait. A trip that resets and holds for a full heat-up cycle was probably a one-off, perhaps a boost left on too long. A cut out that trips again on the same day, or within a few days, is telling you the fault is real. Do not keep resetting it. Each reset tests a safety device that was designed to be used rarely, and repeated trips increase the chance of the water reaching an unsafe temperature.

If it will not reset at all, the button clicks but nothing returns, or the heater stays cold after a successful reset, the cut out or the element has failed. Leave the supply off and describe exactly what you did and saw.

What the engineer checks

With the power isolated and proved dead, the engineer tests continuity through the cut out and the thermostat separately, then checks the element for resistance and for leakage to earth. A scaled or burnt-out element shows as a low or open reading, or as insulation failure. They also check that the thermostat is switching at its dial setting by comparing the water temperature at the draw-off against the pocket reading.

On the water side, they look at the expansion vessel and the valve group, because a cylinder that cannot expand properly can overheat or discharge under pressure. A tripped cut out sometimes comes with a dripping tundish, and that combination points towards a heat control fault rather than a valve fault. See tundish dripping: causes and checks for the discharge side.

Diagnosis sets the answer. A failed thermostat or cut out is a small replacement part. A scaled element is a heater replacement. A cylinder with a failed shell, or one past a sensible repair point, becomes a replacement conversation later. For this fault specifically, unvented cylinder fault repair covers the work, and the hot water cylinder diagnosis page covers the test order.

When to stop and not reset

Do not reset the cut out if the cylinder has been discharging from the tundish, if the pipes near the cylinder are very hot to the touch, or if you hear boiling noises. Do not reset it if the cut out has tripped more than once. Do not attempt to short it out, tape the button down or bridge the terminals, however tempting that seems to restore hot water in the meantime. That removes the final protection on a pressurised vessel.

The same applies to the relief valve and the tundish. Capping a dripping discharge pipe or blocking a relief valve is dangerous and is never a repair.