Megaflo No Hot Water: What to Check, and in What Order

Why has my Megaflo stopped making hot water?

Because something in the chain that heats the stored water has stopped working, and in most cases that something is not the cylinder. A Megaflo stores water under mains pressure and keeps it hot. The heat comes from elsewhere, normally a boiler pumping hot water through a sealed coil inside the cylinder, with an electric immersion heater as a second route. If the taps run cold, either nothing is asking for heat, nothing is supplying it, or the heat cannot get into the water.

That is why guessing is expensive. A cylinder that is cold because a programmer lost its time after a power cut looks exactly the same at the tap as a cylinder that is cold because a motorised valve has seized. Same symptom, completely different job. Working through the chain in order separates them quickly.

It also helps to be precise about what you are seeing. No hot water at all, hot water that runs out much faster than it used to, and hot water that only ever gets lukewarm are three different faults with three different sets of causes. Deciding which of the three you actually have, before you start, saves the most time of anything on this page.

Megaflo is the registered trade mark of its manufacturer. We are an independent London plumbing and hot water firm with 20 engineers, and we repair and replace these cylinders. We are not an approved, authorised or official installer or service agent for any manufacturer.

First, check whether it is the hot water or the whole supply

Run a cold tap before you touch anything else. If the cold is also weak or absent, the problem is the incoming water supply and not the cylinder at all. A Megaflo is fed straight from the cold main, with no storage tank in the loft holding a reserve, so anything that stops the mains stops the hot water within minutes of the last draw off.

The usual culprits are a stopcock turned down or off by somebody working elsewhere in the property, and works on the street outside. In a converted flat the stopcock serving you may sit in a communal cupboard rather than in your own kitchen, which is worth knowing before the day you need it.

Check more than one outlet too. If the hot runs fine at the bath but not at the kitchen mixer, you do not have a cylinder fault, you have an outlet fault: a blocked cartridge, a partly closed service valve, or a scaled aerator. On a mains pressure system every hot tap in the house is fed by the same cylinder, so a problem at one tap and nowhere else points away from the cupboard.

Check the programmer, the timer and the hot water schedule

This is the single most common finding, and it costs nothing to rule out. Go to the programmer or the app controlling the system and confirm the hot water channel is actually calling. Hot water and heating are separate channels on almost every system boiler installation, so heating that works tells you nothing about whether hot water has been asked for.

Time and date are the things to look at hardest. A programmer that lost power, even briefly, can come back on the wrong day or stuck at midnight, in which case every schedule in it is now firing at the wrong hours. Clocks that do not adjust themselves in spring and autumn drift by an hour and quietly move the morning heat to after everybody has left. A schedule someone changed months ago behaves the same way.

Put the hot water to constant or press the boost, then wait properly. A cylinder that has been cold for a day is starting from nothing, and a full reheat from cold takes a good while rather than minutes. Judging the result after five minutes at the tap is how people convince themselves a working system is broken.

• Confirm the hot water channel is on, separately from the heating channel.
• Check the time and the day on the programmer, not just the schedule.
• Look for a holiday or away mode that somebody left running.
• Put it to constant or boost and give the cylinder an hour before judging.
• In a flat with a landlord fitted controller, check whether the schedule is yours to change at all.

Is the boiler actually firing when the hot water calls?

Stand next to the boiler, put the hot water to constant, and listen. A boiler that fires and settles into a run is supplying heat, which moves the fault further down the chain. A boiler that stays silent while the programmer says it is calling has either not received the demand or cannot respond to it, and that is a boiler side investigation rather than a cylinder one.

The in between case is the informative one. If the boiler runs happily for the radiators but does nothing when only hot water is called, the demand is getting lost somewhere between the controls and the boiler. On a system with a cylinder that usually means the motorised valve that sends heat to the cylinder has stuck, its microswitch has failed, or a wiring centre connection has gone. Those are all repairable parts, and they are the classic cause of heating that works while hot water does not.

If the boiler is showing a fault or has locked itself out, that is a boiler matter and belongs with whoever looks at your boiler. Note what the display says and leave it there. Repeatedly resetting an appliance that keeps locking out does not fix the reason it locked out, and it removes the evidence an engineer would have used.

What the immersion heater tells you

Switch the immersion on for an hour and see what happens, because the result splits the problem in half. If the water gets hot, the cylinder is sound and holding heat, and the fault lies on the boiler side of the system. If the immersion gives you nothing either, the fault is more likely to sit with the cylinder, its electrics, or the stored water itself.

An immersion heater is an electric element screwed into the cylinder, with a thermostat of its own that cuts the supply once the water reaches temperature and cuts it permanently if something has gone wrong. Elements fail, usually after years of being the hottest surface in a hard water system, and thermostats trip. Neither is dramatic and both are ordinary replacement parts on most cylinders.

Two practical notes. The immersion is a useful stopgap while you wait for an engineer, so a household with a dead boiler can still get baths, but it is a slower and more expensive way to heat a full cylinder than the boiler is. And on some cylinder models the immersion is a manufacturer specific part rather than the plain screw in element a merchant keeps on the shelf, which is a serviceability point worth knowing: it can mean a wait for the right component instead of a same visit fix.

Switching an immersion on at its own isolator is a normal household action. Anything beyond that, including removing the cover, testing the element or replacing a thermostat, is electrical work on a fixed appliance and is not a homeowner job.

Lukewarm is a different fault from stone cold

Water that gets warm but never hot means heat is arriving and something is limiting it. Water that stays stone cold means no heat is arriving at all. Treating those two as the same symptom sends people down the wrong path more often than any other mistake on this subject.

The limiting causes are worth going through in order. A heating period too short for the cylinder size means the reheat is stopped halfway every day. A boiler that keeps cycling on and off rather than running through never gets the coil properly hot. A thermostatic blending valve on the cylinder outlet, fitted on a lot of newer installations, can drift and send tempered water to the taps while the cylinder behind it is perfectly hot. And a coil furred with scale passes heat slowly, so the water climbs but never quite arrives.

There is also the honest, dull explanation: demand outgrew the cylinder. A Megaflo stores a fixed volume, and a household that has gained a person, a second bathroom or a much bigger shower head can empty it faster than it reheats. That shows as hot water that fades halfway through the second shower, which is a sizing conversation rather than a fault.

When the fault is the coil inside the cylinder

The coil is the last thing to suspect and the most serious thing to find. It is a sealed tube carrying hot water from the boiler through the middle of the stored water, transferring heat without the two ever mixing. When a coil stops working properly there are two versions: it is fouled, so heat transfer has dropped away, or it has failed through, so the two bodies of water are now connected.

A fouled coil behaves predictably. The boiler fires, the flow pipe into the cylinder gets hot, the return pipe comes back nearly as hot because the heat is not being given up, and the stored water takes far longer to get there than it used to. An engineer feels that at the pipework within a couple of minutes of the boiler running, which is why we ask for the boiler to be left off before a visit rather than run all morning.

A coil that has failed through announces itself differently, usually through the heating side behaving strangely at the same time as the hot water: pressure on the sealed heating system moving on its own, discoloured water at the hot taps, or both together. That combination is worth reporting exactly as you saw it, because it changes the job from a part to a decision about the cylinder. A coil is not a component that gets renewed inside an existing vessel.

Hard water, London homes, and the slow failure nobody notices

Almost the whole London supply area is hard, and scale forms on the hottest surfaces in the system, which means the immersion element and the boiler coil. That is the background reason a great many of these calls happen at all. Scale does not break anything on a particular Tuesday. It adds a few minutes to the reheat every year until one winter morning the family runs out and everyone decides the cylinder has failed overnight.

Where the cylinder lives shapes what happens next. In a Victorian terrace or a conversion flat it is usually in a first floor cupboard off a landing, often behind a doorway narrower than the cylinder, sitting directly above a ceiling. That matters for access on the day, and it is why a slow drip at the base of a cylinder up there deserves more urgency than the same drip in a garage would.

Purpose built blocks are the place to be careful about assumptions. A tall white cylinder in a hall cupboard may be an unvented cylinder, or it may be a thermal store or part of a communal heat arrangement, which looks broadly similar and diagnoses completely differently. New builds usually have the cylinder in a plant cupboard beside an underfloor heating manifold with pipework buried in liquid screed, where nobody should be lifting a floor on a hunch. Reading the label on the front of the unit before anything else is the cheapest step in this whole process.

What you can safely check yourself, and where it stops

You can check anything you can see and switch. You should not open, adjust or test anything that holds pressure or carries a fixed electrical supply. That line is not about qualifications for their own sake: a Megaflo holds a large volume of water above the temperature at which it will scald, at the pressure of the street main, usually on the floor above a ceiling.

Safe to do yourself, in this order:

• Run a cold tap to confirm the mains supply is present.
• Check the hot at more than one outlet, so one bad tap is not mistaken for a cylinder fault.
• Check the programmer time, date and hot water schedule, then set it to constant.
• Listen at the boiler for whether it fires on a hot water demand.
• Switch the immersion heater on at its isolator and give it an hour.
• Photograph the label on the front of the cylinder, including the model name and litre capacity.
• Look at the tundish, the small open funnel in the pipework above the cylinder, and note whether anything is running through it.

Stop and call someone qualified if you see water running from the tundish or from a pipe outside, water anywhere around the base of the cylinder, scorching or a burning smell at the immersion, or if the boiler keeps locking out. If the cylinder is leaking from the body of the vessel rather than from a fitting, turn the water off at the stopcock first.

How an engineer works through it on site

The same order, with instruments. We confirm there is a genuine demand at the controls, then test whether that demand is reaching the boiler, usually at the wiring centre and the motorised valve, because that is where a hot water only fault most often hides. Then we look at what the boiler is doing when it runs, and what the flow and return pipes at the cylinder tell us about whether the coil is accepting heat.

After that the electrical side of the cylinder gets tested properly: the immersion element, its thermostat and the supply feeding them. Only when the controls, the boiler side and the electrics are all cleared does attention move to the vessel itself. Diagnosing in that order matters, because the cheapest and commonest causes sit at the start, and a component swapped on a guess at the end does not fix a fault that began at the beginning.

Most outcomes are repairs. Valves, thermostats, immersion heaters and controls are all serviceable parts, and a cylinder that is sound in itself is normally worth putting right rather than replacing. The conversation changes when the vessel is leaking from the body or the coil has failed through, because neither is renewable inside an existing cylinder. What a replacement costs depends on the capacity the household needs, what the incoming main can feed, how much pipework has to change, and how the old unit gets out of a first floor cupboard, which is why our rate lives on the pricing page rather than in an article.