If you've ever had a powerful shower with consistently strong pressure, regardless of how many taps are running elsewhere in the house, you've likely experienced the benefit of an unvented cylinder. In London's finest homes, the Megaflo is often the unsung hero of the plumbing system.
What is a Megaflo?
Megaflo is a brand name — specifically, a range of unvented hot water cylinders manufactured by Heatrae Sadia, now owned by Baxi. Like "Hoover" for vacuum cleaners, "Megaflo" has become a generic term in the plumbing industry for any unvented cylinder. Other manufacturers produce equivalent systems: the Gledhill Pulsacoil, the Joule Cyclone, and various own-brand cylinders from Vaillant and Ariston.
An unvented cylinder differs from a traditional vented (gravity-fed) hot water cylinder in one fundamental way: it connects directly to the mains water supply. Hot water is stored under mains pressure, meaning every outlet in the property receives full mains-pressure hot water — powerful showers, fast-filling baths, and consistent performance even when multiple taps run simultaneously.
How Does an Unvented Cylinder Work?
Mains-pressure cold water enters the cylinder and is heated either by an immersion heater, a coil connected to the boiler, or both. The water expands as it heats; an internal or external expansion vessel accommodates this expansion. A series of pressure and temperature safety devices — including a temperature relief valve and a pressure relief valve — protect the system from over-pressurisation.
The result is hot water stored at mains pressure, available on demand across the entire property at consistent, powerful flow rates. For London period townhouses with multiple bathrooms on different floors, this is transformative compared to a conventional gravity-fed cylinder.
Megaflo vs Traditional Gravity-Fed Cylinder
A traditional vented cylinder relies on a cold water tank in the loft to supply it via gravity. This produces lower pressure at high-floor outlets — a problem that worsens in taller properties. Showers in loft conversions and upper-floor bathrooms are particularly affected. The cold water tank also occupies significant loft space and presents a Legionella risk if not properly maintained.
An unvented cylinder eliminates the loft tank entirely, freeing up space, removing the Legionella risk associated with stored cold water, and delivering consistent pressure throughout the property. For the homes RARE typically serves — large Victorian and Georgian townhouses in Mayfair, Belgravia, Hampstead and South Kensington — an unvented system is almost always the correct choice.
Megaflo Servicing: What's Required
All unvented cylinders must be serviced annually by a qualified engineer holding the relevant G3 unvented cylinder qualification. This "CP8 service" checks:
- The temperature and pressure relief valve (TPR valve) — the primary safety device
- The expansion vessel charge pressure
- The pressure reducing valve (PRV) setting
- The check valve and line strainer
- The immersion heater element condition (where fitted)
- The cylinder thermostat calibration
Neglecting annual servicing is a safety risk — an incorrectly functioning TPR valve on an unvented cylinder can, in extreme cases, cause catastrophic failure. It is also likely to invalidate household insurance policies, which typically require evidence that installed plant has been properly maintained.
RARE's engineers hold full G3 qualification and carry out CP8 unvented cylinder services across London. Learn more about our Megaflo and unvented cylinder service.
Common Megaflo Problems
The most common faults we encounter on unvented cylinders include:
- Expansion vessel failure: The internal diaphragm loses its charge, causing the PRV to discharge repeatedly. This is the single most common Megaflo fault and is a straightforward repair.
- TPR valve weeping or discharging: Usually caused by over-pressure from a failed expansion vessel, or a PRV set too high.
- Immersion heater element failure: Element burnout due to limescale accumulation — particularly common in London's hard water areas.
- Slow hot water recovery: Usually indicates either a failing immersion element or incorrect boiler thermostat settings.
Megaflo service or repair needed?
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Frequently Asked Questions
What is a Megaflo?
Megaflo is a brand name for a range of unvented hot water cylinders manufactured by Heatrae Sadia. More broadly, "Megaflo" is used as a generic term for any unvented hot water cylinder. These systems store hot water under mains pressure, delivering powerful hot water flow to multiple outlets simultaneously.
Does a Megaflo need annual servicing?
Yes. All unvented cylinders require annual inspection (a "CP8" service) by a qualified engineer to check the pressure relief valve, expansion vessel, and temperature/pressure safety controls. Neglecting this is both a safety risk and may invalidate your insurance.
How long does a Megaflo last?
Well-maintained Megaflo cylinders typically last 20–25 years. Annual servicing is the most important factor in achieving this lifespan. The expansion vessel inside the unit usually requires replacement every 5–10 years.
Can I install a Megaflo in a flat?
Yes, but it requires adequate mains water pressure (typically at least 1.5 bar static) and the installation must be carried out by a qualified engineer holding the relevant G3 unvented cylinder qualification. RARE's engineers hold all necessary qualifications.