Unvented Hot Water Cylinder

Megaflow unvented hot water cylinders

Megaflow are renowned for quality unvented hot water cylinders and are manufactured by a company called Heatrae Sadia. Heatrae Sadia are the UK's leading manufacturer of hot water systems with their flagship brand ‘Megaflow‘ leading the way in unvented hot water cylinders. Megaflow are not the only brand (if perhaps the most well known) of unvented hot water cylinders we also install, maintain and service other cylinder makes such as Ariston and Ozo.


  • Internal air-gap with patented floating baffle
  • Strong, Duplex stainless steel construction
  • High performance 'coil-in-coil' heat exchanger
  • No sacrificial anode (eliminates need for annual inspection)
  • Appliance-like plastic-coated outer case
  • Patented cold water inlet diffuser minimises mixing of cold and hot water
  • Water inlet controls increase flow rates by up to 35%
  • Improved performance even at 1 bar
  • Every unit tested to 15 bar (217psi)
  • 25-year transferable cylinder guarantee
  • ½" temperature and pressure relief valve

The above features describe MegaFlow's new range of HE cylinders which use a ‘coil in coil’ heat exchanger to deliver more hot water quicker than previous models. Please also note that older models may differ in specification, always consult your manual.

Benefits of Megaflow HE system

  • Higher flow rates for efficient hot water delivery (Up to 72 l/min)
  • High performance showering –throughout the home
  • Fast-filling baths
  • Quieter mains pressure system (no noisy cistern in the loft)
  • Balanced water pressure - no surprises
  • Fast recover rates (eg. 15 minutes for CL125HE)
  • Exceeds CHeSS Best Practice for heat recovery and insulation
  • CFC free insulation for maximum heat retention
  • No anode to check or replace
  • Mains feed hygienic hot water
  • Appliance quality, easy-clean finish
  • Long-life Duplex stainless steel interior
  • 25-year transferable cylinder guarantee
  • Patented internal expansion system
  • No external expansion vessel
  • Complete with mains water isolating valve
  • All units are fully factory tested to ensure reliability
  • Neater and quicker installation

The installation of an unvented hot water system must comply with both the Water Regulations and the Building Regulations. You must also notify your local Building Control Office, who will want to ascertain who will be installing the system. The system must be installed by a competent person, in other words someone who is qualified for the installation of unvented hot water cylinders.

In the UK this means training and assessment by a recognised training body with cards being issued by organisations such as the Institute of Plumbing (IoP) or the Construction Industry Training Board (CITB). This qualification is not part of Gas Safe and so please make sure that if someone is installing an unvented system they hold the relevant qualification - we hold a CITB card and so are qualified to install, service and maintain your unvented hot water system.

Unvented cylinder- how it works


Unvented cylinder

This picture shows an indirect mains fed hot water cylinder. Water is heated by an primary heat exchanger within the vessel supplied by your boiler.


On this example there is also a provision for a immersion heater as back up.

Special measures are necessary to accommodate expansion as the high pressure water heats up, to ensure there is no dangerous build-up of pressure within the unvented storage vessel, and to discharge hot water safely away from the unit if necessary.

  • If the vessel fails, an expansion release valve protects the system by releasing water via the discharge pipe.
  • A normal thermostat maintains the temperature of the water below 65°C.
  • A second thermostat switches off the immersion heater and shuts off water from the boiler should the main thermostat fail and the temperature reaches 90°C.
  • If both thermostats fail, and the water reaches 95°C, a temperature release valve discharges all water to the outside.

We can offer our services to install, service or maintain your unvented hot water system. Throughout this page we have mainly been featuring Megaflow cylinders but don't forget we also attend to all other unvented cylinder manufacturers such as Ariston or Ozo. We do believe that Megaflow offer some of the best cylinders available but we do and indeed have installed cylinders from other manufacturer

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