Megaflo Service and Repair: What Actually Happens

What does a Megaflo service actually involve

A Megaflo service is a set of checks on the sealed vessel and everything bolted to it, carried out with the cylinder left where it stands. The engineer isolates and drains to the point where air re enters the vessel and the internal air gap is restored, then refills and rechecks. Around that sit the checks common to any unvented cylinder: exercising the temperature and pressure relief valve and the expansion relief valve so they seat cleanly, cleaning the strainer in the inlet control group, confirming the stored temperature is where it should be, confirming the thermal cut out has not tripped and resets correctly, and following the discharge pipe from the tundish out to its visible termination.

The air gap step is what makes this brand specific. Most unvented cylinders take expansion into a separate vessel with a rubber diaphragm on the cold feed. A Megaflo takes it into a pocket of air trapped inside the top of the vessel behind an internal baffle. Air dissolves slowly into water, so that pocket shrinks over months whether the household notices or not. Restoring it is a procedure rather than a part, so there is nothing to order in advance, but it also means it only ever happens if somebody attends and knows to do it.

The visit should end with a written record: what was checked, what was found, the stored temperature, the state of the relief valves, and anything flagged for next time. Manufacturer warranty terms on unvented cylinders generally require annual servicing with evidence, and a folder of dated records is that evidence.

• Restore the internal air gap and recheck after refill.
• Exercise and observe the temperature and pressure relief valve.
• Exercise and observe the expansion relief valve on the inlet group.
• Clean the strainer, check the pressure reducing element and the check valve.
• Check the stored temperature and the operation of the thermal cut out.
• Inspect the tundish, the discharge route and its visible termination outside.

Megaflo is a trade mark of its manufacturer. We are an independent plumbing and hot water firm with 20 engineers who work on these cylinders every week. We are not an approved, authorised or official agent of any manufacturer.

How often should a Megaflo be serviced

Once a year. That is the interval every unvented cylinder manufacturer specifies, and on a Megaflo the case for keeping to it is stronger than average because the air gap restoration has no other opportunity to happen. Skip two years on a Megaflo and the expansion capacity is quietly disappearing the whole time.

The other reason for an annual interval has nothing to do with the brand. The relief valves on an unvented cylinder are the protection between a store of very hot water and a sealed steel vessel, and valves that never move can seize or start to weep. An annual exercise keeps them honest.

Book it in a quiet month rather than waiting for a symptom. The same engineer attending the same cylinder in December because the shower has gone cold is a more expensive morning for everybody.

If you have just moved into a London flat or house with an unvented cylinder and nobody handed you service records, assume it has not been done and start the clock now.

What happens on the day, step by step

Expect the engineer to want the cupboard clear and the cylinder reachable before anything starts. The first five minutes are reading: the rating plate for model, capacity, serial number and manufacture date, then the pipework around the top to see what has been done to this installation over the years. Photographing the plate before the visit means the right service parts are already in the van.

Then the checks run in order. Incoming pressure and flow get measured at the property, because an unvented cylinder can only deliver what the main feeds it and a complaint about weak hot water is often a supply question. The relief valves are exercised and watched to see they reseat rather than dribble. The strainer comes out and gets cleaned, which on London water is rarely a formality. The stored temperature and the thermal cut out are checked, and the discharge pipework is followed to its termination.

The air gap restoration happens with the supply isolated and a drain point open to a set level so air re enters the vessel, followed by a refill and a recheck under pressure. The cylinder is then brought back up to temperature while the engineer watches the tundish. A visit that ends without anybody watching a full reheat has skipped the test that matters most.

• Identify the cylinder from the rating plate before touching anything.
• Measure incoming static pressure and flow rate at the property.
• Exercise the safety valves and confirm they reseat.
• Clean the strainer and check the inlet control group.
• Restore the air gap, refill, then watch a full reheat for discharge.

What fails on a Megaflo between services

Four things account for most of what we attend: a failed immersion element, a safety valve that has started to pass, a silted or blocked strainer in the inlet group, and a lost air gap. None of those is a failure of the vessel, and all four are repairs carried out with the cylinder in place. The duplex stainless vessel routinely outlives several sets of the parts attached to it.

Immersion elements are the most frequent single item in London, and the reason is the water rather than the manufacture. Scale forms on the hottest surface in the cylinder, the crust insulates the element, the insulated element runs hotter than it was designed to, and eventually it gives up. On a Megaflo the element is a manufacturer specific part rather than a plain merchant element, so the model has to be identified before one is ordered. That is a serviceability point worth knowing rather than a defect: it means an engineer who turns up without the model number is unlikely to finish in one visit.

On indirect cylinders there is a fifth group worth naming, which is the heating side control gear. A motorised valve that no longer opens, a failed wiring centre connection or a cylinder control that never calls the boiler will all present as no hot water while the cylinder itself is perfectly sound. That is why a decent repair visit starts with finding out whether heat is arriving at the coil at all, before anybody starts pricing components.

• Immersion element, often scale related in London.
• Expansion relief valve or temperature and pressure relief valve passing.
• Strainer silted, restricting flow into the cylinder.
• Lost internal air gap, restored rather than replaced.
• On indirect cylinders, heating side controls failing to bring heat to the coil.

Why is water dripping from the tundish under my Megaflo

Two causes account for nearly all of it, and when the drip happens tells you which. Water running through the tundish during or shortly after a reheat usually means the internal air gap has gone, so the expanding water has nowhere to go and lifts the expansion relief valve instead. Water dripping steadily when nothing is heating usually means a valve is passing and needs changing. Either way the cylinder is behaving correctly by discharging, and the fault is upstream of the valve.

What matters far more than the diagnosis is what you do in the meantime. Never cap, plug or tighten a discharging valve to stop the noise or the drip. That pipe is the designed escape route for a sealed vessel and the drip is the cylinder telling you something. If discharge is heavy rather than a drip, or if the water coming out is scalding, isolate the heat source and get someone out.

On a cylinder that has not been serviced in years the drip could be either, so the engineer restores the air gap, watches a full reheat and sees whether it stops. Work through it in that order rather than replacing valves speculatively.

A discharge pipe that has been capped, blocked or routed somewhere it cannot be seen is a serious finding. It needs putting right, not living with.

What does a Megaflo repair visit look like

A repair visit starts with diagnosis rather than parts. The engineer establishes what the household is actually experiencing, which is usually one of four things: no hot water at all, hot water that runs out too quickly, weak pressure or flow at the outlets, or water appearing somewhere it should not. Each points at a different half of the system.

No hot water splits immediately into an electrical question and a heating side question. On a direct cylinder the immersion is the entire heat source, so the element, its thermostat and its thermal cut out are the whole story. On an indirect cylinder the first job is finding out whether the boiler is delivering heat to the coil at all, because a perfectly healthy cylinder fed by a motorised valve that never opens looks exactly like a dead cylinder from the bathroom. Hot water that runs out too quickly is usually demand against capacity or a control setting, not a broken part. Weak flow at the outlets sends the engineer to the incoming main, the pressure reducing element and the strainer before anything else.

Once the fault is identified, most Megaflo repairs are component changes done in place. The awkward variable is not the part, it is the cupboard. An immersion element needs real length to withdraw, and in a Victorian conversion where the cylinder is boxed in tight against a wall, a twenty minute job becomes an hour of careful dismantling first. Say on the phone how the cylinder is boxed in.

• No hot water: element and its controls, or heat not reaching the coil.
• Runs out too fast: capacity against demand, or a control setting.
• Weak flow: incoming main, pressure reducing element, strainer.
• Water where it should not be: joint, valve, tundish or vessel, confirmed not assumed.

What hard London water does to a Megaflo between services

Almost the whole London supply area is hard, and hard water is why servicing matters more here than the manufacturer literature implies. Scale comes out of solution on the hottest surfaces, which means the immersion element and the outside of the boiler coil. A scaled coil transfers less heat, so the household notices slow recovery long before anything actually fails. A scaled element runs hot under its own crust and burns out. The stainless vessel is unaffected, which is exactly why a long lived Megaflo can be sound while its third element sits in the back of a van.

Scale also explains findings people misread. Rumbling or ticking during a reheat is usually deposit on a heating surface rather than anything structural, and a strainer that needs real cleaning at every service is the water doing what London water does. Both are maintenance findings.

Three habits reduce the damage, and all three get decided at a service rather than at a replacement. Specify an Incoloy or titanium sheathed element rather than the cheapest copper one, because the better sheath tolerates scaling far longer. Keep the stored temperature at what the household needs rather than at maximum, because every extra degree speeds deposition. And keep the annual interval, because that is when the strainer gets cleaned and the state of the element gets noticed.

Can you service a Megaflo yourself

No. Unvented cylinder work is restricted to engineers holding the relevant qualification for it, and the reason is straightforward physics rather than bureaucracy. A sealed vessel holding a large volume of water above 60 degrees at mains pressure carries layered safety devices for a reason, and the parts of a service that matter most are precisely the parts that involve those devices. Exercising a temperature and pressure relief valve and draining a vessel to restore an air gap are not tasks to learn from a video.

There are two things a householder should do, and both are observation. Look at the tundish now and then, ideally after a reheat, and note whether anything is running through it. Know where the discharge pipe terminates outside, so you can see it if it ever discharges heavily.

The third useful habit is paperwork. Keep the service records together, photograph the rating plate once so the model and serial are always to hand, and note the date of anything replaced. When something does fail, that folder turns a diagnostic visit into a short conversation.

If a cylinder is discharging heavily, or the water from the tundish is scalding, turn off the heat source to the cylinder and call someone. Do not attempt to stop the discharge.

What drives the cost of a Megaflo service or repair

A service on a working cylinder is the predictable end of it: one engineer, one visit, standard consumables, and the variables are access and whether anything is found that needs putting right. A repair costs what it costs because of three things, and none of them is the brand on the front. The first is the part, and on a Megaflo the element and some inlet group components are manufacturer specific rather than generic, which affects availability more than anything else. The second is access, which in a boxed in first floor cupboard can easily be the larger half of the labour. The third is whether one visit is enough, which depends almost entirely on whether the model was identified before the engineer set off.

On an older pre Eco cylinder there is a fourth variable, which is parts support. Components for a superseded range are not always on a shelf, and a straightforward repair turns into a search. That is a serviceability judgement rather than a criticism of the product, but it is where a sensible engineer starts asking whether repeated repairs still make sense.

Our own rates are published on our pricing page rather than quoted in an article, and any written quote should itemise labour, parts and making good separately so you can see what is driving it. The cost genuinely worth avoiding is a cylinder replaced because nobody diagnosed it. In most cases an unvented cylinder repair puts the system right without anybody touching the vessel, and the annual unvented cylinder service is what keeps the repair list short. Where the steel itself has genuinely gone, a hot water cylinder replacement is the correct answer, but that conclusion should follow a diagnosis, not precede one.

• Which part failed, and whether it is manufacturer specific.
• Access to the cylinder and room to withdraw an element.
• Whether the model was identified before the visit.
• On older ranges, whether parts are still readily available.