Megaflo No Hot Water: Checks in the Right Order

What type of Megaflo-style cylinder do I have?

Megaflo is a brand name that people use for any unvented cylinder, and it is used here only to describe the type. London Cylinder Specialist is not affiliated with, endorsed by or approved by any cylinder manufacturer. An unvented cylinder is fed straight from the mains and sits under mains pressure. It has no loft tank and no vent pipe, but has a safety group of valves, an expansion vessel or internal air space, and a discharge route through a tundish. Look for a label on the cylinder or a boxed set of valves on the cold inlet, and an open funnel-shaped tundish below the discharge pipe.

Then find out how it is heated, because the checks differ. A direct unit has only an electric immersion element or two. An indirect unit has a coil connected to the boiler, and may also have an immersion element as a back-up. Look for two pipes from the heating system into the cylinder, and a motorised valve beside it, as well as a wired immersion cover.

Part G of the Building Regulations covers unvented hot water storage, which is why this type of cylinder calls for an installer with the right skills and why this page gives you a short list of things to look at and a clear list of things to leave. Unvented cylinder regulations: Part G explains the background.

What should I check first when there is no hot water?

Start where the fault is most often a switch, a timer or a fuse, not a failed part. Go through these in order.

• Is the immersion switched on at the isolator, usually a double-pole switch on the wall or in the airing cupboard, and is its neon on?
• Is the programmer calling for hot water, and are its times what you expect after a power cut or clock change?
• Has the consumer unit tripped on the circuit for the cylinder, or has the fuse in a fused spur blown?
• Is the cylinder thermostat dial, if it has an outside control, set to a sensible temperature and not at its lowest setting?
• On an indirect unit, is the heating working, and does the motorised valve beside the cylinder move when hot water is called for?

Why does the cut-out trip, and can I just reset it?

An unvented cylinder carries a thermal cut-out as a safety device, and a tripped cut-out is a recognised reason for no heat and no hot water. It is an overheat protection. Its job is to stop the heat source if the stored water gets hotter than it should, and it stays off until it is reset by hand. Where the reset button sits depends on the model, and the manual for your cylinder is the place to find it.

The rule is the same as on the reset page. Press it once, only if it is reachable without removing any cover, with the circuit isolated at the consumer unit first and the water allowed to cool. If reaching it means taking off a terminal cover, that is an electrical enclosure and the reset is a job for an engineer. If hot water returns and stays, a one-off trip is plausible. If it trips again, it is telling you something: a failed thermostat, a scaled element or a boiler-side control that is not stopping. Repeated resetting hides a fault that can turn dangerous, so it is the one thing you should not do. How to reset a hot water cylinder covers the one safe press and when to stop, and unvented cylinder tripped cut-out goes through the causes.

Is it the element, the thermostat or the boiler side?

If power reaches the cylinder and the cut-out is not tripped, the heat source is next. On an electric unit the element and its thermostat are the parts that fail. An element can burn out, or the thermostat can open and not close again. A cylinder with two elements may keep a little hot water at the top when the lower one has failed, so the symptom is hot water that runs out fast rather than none. Immersion heater not working: what to test sets out the test order, with the safe checks separate from the engineer ones.

On an indirect unit, the heat source is the boiler. The two-port valve, the pump and the cylinder thermostat decide whether heat reaches the coil. A cold flow pipe at the cylinder means the heat is not arriving, and the fault is in the valve or controls, which two-port valve and no hot water explains. A hot flow pipe with a cold cylinder points to the coil. See cylinder heating coil fault.

There is one more heat source to rule out: a cylinder with both an immersion and a coil can have one working and the other not. If the cylinder warms a little only on the days the heating is on, the immersion is likely off or failed.

What if the cylinder is hot but nothing comes out of the taps?

Unvented cylinders depend on the mains. If the mains inlet to the cylinder is restricted, the hot taps run weakly or not at all even though the cylinder is full of hot water. The cause can be a closed stopcock or isolation valve, a blocked strainer in the inlet group, or a pressure reducing valve that has failed. Cold taps that are also weak, or that stop, point at the mains rather than the cylinder.

You can check that the stopcock and the isolation valve on the cylinder inlet are fully open. Beyond that, the inlet group, the pressure reducing valve and the expansion vessel are safety parts that need an engineer. Do not cap, plug or bypass any of the valves or the tundish discharge to get flow back. Low pressure from taps on an unvented cylinder deals with that pattern in detail.

A cylinder discharging steam or very hot water is a different situation. Turn off the heat source at the isolator, keep clear and call an engineer, and see TPRV dripping on an unvented cylinder for why the valve discharges.

What does the engineer check, and is it repair or replace?

The engineer follows the same order: power and controls, cut-out, element or coil with its thermostat, then the inlet group and valves. Each step rules out a part before the next is touched, so the repair matches the fault.

The usual outcomes are a replaced element, thermostat or cut-out, a replaced motorised valve, or a cleaned or replaced inlet group component. A new cylinder is worth discussing when the shell is leaking, when the coil has failed, or when repeated failures show the cylinder is at the end of its life. unvented cylinder fault repair covers the work, and cylinder repair or replace sets out how the decision is made.