What is a Megaflo, and is it worth it over a combi?
A Megaflo is an unvented hot water cylinder made by Heatrae Sadia. It stores hot water fed directly from the cold mains, so every tap and shower gets hot water at mains pressure, with no cold water tank in the loft. It is worth it over a combi when a home has two or more bathrooms, or showers are often used at the same time, and the mains supply has enough flow. For a flat or small house with one bathroom, a combi is usually simpler and takes less space. Megaflo is a brand name: other manufacturers make unvented cylinders that do the same job, and the choice between them matters less than getting the size and installation right.
Loyal Plumbers · Updated
Plan a cylinder installationHow a Megaflo works
The cylinder is a sealed, insulated stainless steel vessel connected to the incoming cold main through a pressure-reducing valve. Water inside is heated either by a coil carrying hot water from a gas boiler (an indirect cylinder) or by electric immersion elements (a direct cylinder). When you open a hot tap, mains pressure pushes the stored hot water out and cold water refills the cylinder from the bottom.
Because water expands as it heats, an unvented cylinder needs somewhere for that extra volume to go. Depending on the model, that is an internal air gap at the top of the cylinder or an external expansion vessel on the wall beside it. Our guide to expansion vessels and internal air gaps explains the difference and what goes wrong with each.
Heatrae Sadia sells versions for boiler systems, for direct electric heating and for solar or heat pump input, in a range of sizes.
The safety devices that come with it
Storing hot water under pressure is safe when the controls work, and the cylinder carries several layers of protection. These are why an unvented cylinder must be fitted by an installer with the G3 unvented hot water qualification and serviced regularly.
- Thermostat: controls the stored water temperature
- High-limit cut-out: shuts off the heat source if the thermostat fails
- Temperature and pressure relief valve: releases water if the cylinder gets too hot or pressure rises too far
- Expansion relief valve: protects the cylinder from excess pressure in the cold supply
- Tundish and discharge pipe: shows when a valve is releasing water and takes it safely outside
Hot water or steam coming out of the tundish or discharge pipe means a safety device is operating. Turn off the boiler and immersion heater and get an engineer out. Do not block the pipe or cap the tundish.
Megaflo or combi: the real differences
Both give you mains-pressure hot water with no loft tanks. The difference is that a combi heats water as it flows, while a Megaflo heats it in advance and stores it.
| Question | Megaflo with a system boiler | Combi boiler |
|---|---|---|
| Two showers at once | Good, until the stored water is used | Often weak, limited by boiler output and mains flow |
| Space | Boiler plus a cylinder cupboard | Boiler only |
| Hot water if the boiler fails | Immersion heater as backup | None until the boiler is repaired |
| Running out | Possible after heavy use; needs time to reheat | Never runs out, but flow is limited |
| Mains supply | Needs adequate flow and pressure | Also needs adequate flow and pressure |
| Future heat pump or solar | Cylinder can be chosen to suit | Would need a cylinder adding |
| Annual servicing | Boiler and cylinder | Boiler only |
When a Megaflo is worth it
A Megaflo earns its keep in family houses where hot water is used in more than one place at a time: two bathrooms in the morning, a bath filling while the kitchen tap runs, or a large rainfall shower that a combi cannot keep up with. It is also a common upgrade in older London houses with a vented cylinder and loft tanks, where upstairs showers are weak because pressure depends on the height of the tank.
It is less likely to be worth it in a one-bathroom flat with good mains pressure, where a combi does the job without giving up a cupboard, or in a property where the incoming mains is poor. An unvented cylinder cannot deliver more water than the mains supplies, so a strong-looking pressure reading with a weak flow still disappoints.
- Two or more bathrooms, or showers used at the same time: usually worth it
- Replacing a vented cylinder with weak upstairs pressure: often worth it, if the mains is good
- Planning a heat pump or solar thermal later: worth choosing a compatible cylinder now
- One bathroom, small household, little storage space: a combi is usually the better fit
- Weak mains flow at the stopcock: fix or measure the supply first
What the installer needs to check first
The survey is where a Megaflo installation succeeds or fails. The installer measures the flow rate and pressure at the incoming main, because the manufacturer sets minimum supply requirements and the cylinder's performance depends on them. They also size the cylinder to the household's peak demand, not the number of bedrooms.
Next comes the route for the discharge pipe from the tundish to a safe outside point, which in flats and terraces can be the hardest part. Then space: the cylinder, its valves, expansion arrangement and access for servicing all need room. The installation is notified to building control, usually through a competent person scheme, and you should receive the commissioning paperwork and a completed Benchmark checklist.
Our guides to choosing a hot water cylinder size and to planning the discharge route go into each of these in more detail.
Upkeep and lifespan
Heatrae Sadia recommends an annual service for unvented cylinders, and the manufacturer's guarantee is conditional on the installation and servicing requirements being met, so keep the service records. A service checks the relief valves, the expansion arrangement, the pressure-reducing valve and the thermostats and cut-outs, and makes sure the discharge route is clear.
In hard water areas like most of London, scale on the coil and immersion element is the usual cause of slow reheating and early element failure. A water softener or scale protection on the supply helps. With servicing, the stainless steel vessel itself is built to last many years; the valves and controls around it are the parts that wear and get replaced.
Get a Megaflo fitted, serviced or repaired
Loyal Plumbers installs, replaces and services Megaflo and other unvented cylinders across London and everywhere inside the M25. We check your mains supply, household demand, space and discharge route before recommending a size, and gas work on the boiler side is carried out by Gas Safe registered engineers.
Call us or send an enquiry with your current hot water set-up, the number of bathrooms and where the cylinder would go. If your existing unvented cylinder is dripping from the tundish or has lost hot water, tell us that too so we can prioritise the visit.
Common questions
- Is a Megaflo the same as an unvented cylinder?
- A Megaflo is one brand of unvented cylinder, made by Heatrae Sadia. People often use the name for any unvented cylinder, in the same way as Hoover for a vacuum cleaner.
- Does a Megaflo need a boiler?
- Not always. Indirect models are heated by a boiler or heat pump through a coil. Direct models are heated only by electric immersion elements, which can suit flats without gas.
- Can a Megaflo run out of hot water?
- Yes. It holds a fixed amount of stored hot water. Once that is used it has to reheat, which is why sizing the cylinder to your busiest period matters.
- Does a Megaflo need servicing every year?
- The manufacturer recommends an annual service, and keeping to it is part of the guarantee conditions. The service checks the safety valves, expansion arrangement and controls.
- Can any plumber fit a Megaflo?
- No. Unvented hot water systems must be installed by someone holding the G3 unvented qualification, and the installation must be notified to building control.
Sources and further reading
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