A Megaflo system is a heating and hot water setup built around a Megaflo unvented hot water cylinder, which stores hot water fed directly from the mains, so every tap and shower gets hot water at mains pressure without a cold water tank in the loft. Megaflo is Heatrae Sadia’s brand, and it is so well known that many people, and many searches spelt “Megaflow”, use the name for any unvented cylinder.

How a Megaflo system works

An older, vented hot water system relies on gravity. A cold water tank in the loft feeds a copper cylinder, and the pressure at your taps depends on how high that tank sits above them. That is why showers on a tank-fed system are often weak without a pump.

An unvented cylinder like a Megaflo takes the tank out of the picture. Cold water comes straight from the mains, through a set of valves that control its pressure, into a sealed cylinder. The water is heated and stored there. When you open a hot tap, the mains pressure pushing in at the bottom pushes hot water out at the top, so the hot water arrives at close to mains pressure. Your cold taps are fed from the mains as well, so hot and cold are balanced.

The water can be heated in two ways:

• Indirect cylinders have a coil inside. Hot water from a boiler (or a heat pump, with a cylinder designed for it) flows through the coil and heats the stored water without mixing with it. Most also have an immersion heater as a backup.
• Direct cylinders are heated by electric immersion heaters alone, which suits homes without a gas boiler.

A typical Megaflo system in a home with a gas boiler means a system boiler (which has its pump and expansion vessel built in) or a regular boiler, a Megaflo cylinder, and a motorised valve or two that send the boiler’s heat to the radiators, the cylinder, or both.

Where does the expansion go?

Water expands as it heats. In a vented system, the extra volume goes back up to the loft tank. In a sealed cylinder it needs somewhere else to go. Many unvented cylinders use an external expansion vessel. Megaflo introduced an internal air gap with a floating baffle in 1988, and most of the current Megaflo Eco range still uses internal expansion, so it does not need a separate vessel. The air gap is checked and restored as part of a service.

The safety devices on an unvented cylinder

A cylinder full of hot water under mains pressure is safe when properly installed, because several independent devices stop the temperature or pressure going too high. The Building Regulations require precautions that stop stored water ever exceeding 100°C and that carry any discharge from the safety devices to somewhere it is visible but not dangerous.

Device	What it does
Cold water combination valve	Reduces and steadies the incoming mains pressure, and stops water flowing back into the mains
Expansion relief valve	Opens if the pressure in the cylinder rises too high, for example if the expansion space is lost
Control thermostat	Sets the normal stored water temperature
Thermal cut-out	A separate high-limit device that shuts off the heat source if the thermostat fails
Temperature and pressure relief valve	The last line of defence. Opens to release water if the temperature or pressure becomes dangerous
Tundish and discharge pipe	Lets you see if any valve is discharging, and carries the water safely outside

The Megaflo Eco range

Megaflo’s current range is the Megaflo Eco. Heatrae Sadia lists direct, indirect, slimline, solar, Solar PV Ready, SystemFit and SystemReady versions, most of them with 60mm of insulation, and a 30-year warranty on the stainless steel inner tank. Check the terms for your model, and register it, because warranty conditions apply.

Our unvented cylinder service covers installing, servicing and repairing unvented cylinders, including Megaflo, and we will size the cylinder to your household rather than to a brochure.

Megaflo system, combi boiler or vented cylinder?

	Megaflo (unvented) system	Combi boiler	Vented cylinder system
Hot water pressure	Close to mains pressure	Mains pressure	Depends on tank height, often low
Several outlets at once	Yes, if the mains flow is good	Struggles with more than one outlet	Yes, but pressure drops
Can hot water run out?	Yes, once the stored volume is used	No, heated as you use it	Yes
Loft tank needed	No	No	Yes
Space needed	Cylinder cupboard and a discharge pipe route	None beyond the boiler	Cylinder cupboard and loft tank
Best suited to	Two or more bathrooms, good mains supply	Smaller homes, one bathroom	Low mains pressure, simple systems

If you are comparing options for a smaller home, our guide to the best combi boilers covers the other route.

Advantages and disadvantages

Advantages

• Strong showers and fast-filling baths without a pump
• Several showers and taps can run at the same time
• No cold water storage tank in the loft for your taps and showers, which frees up space
• Works with gas boilers, heat pumps, solar and immersion heaters
• A stainless steel inner tank with a long manufacturer warranty

Disadvantages

• Only as good as your mains supply. Weak mains pressure or flow means weak hot water
• The stored hot water can run out, and then needs time to reheat
• Needs space for the cylinder and a safe route for the discharge pipe
• Must be installed by a qualified installer and should be serviced every year
• Some heat is lost from the cylinder even with good insulation

Is a Megaflo right for your home?

Before recommending one, a good installer will check:

1. Mains pressure and flow. Measured at the property. An unvented cylinder cannot improve a poor supply. If flow is the problem, see our guide to low water pressure in the whole house.
2. How much hot water you use. The number of bathrooms and people decides the cylinder size.
3. Where it will go. The cylinder, its valves and the discharge pipe all need space, and the discharge pipe must run to a safe, visible outlet.
4. The heat source. Existing boiler, a new system boiler, or a heat pump, which needs a cylinder designed for it.
5. Water hardness. Hard water, common across Hertfordshire, scales up coils and immersion heaters over time. A water softener protects the cylinder and everything downstream.

If you are replacing the boiler at the same time, our boiler installation team can design the whole system together.

Installation and servicing rules

Because unvented systems work under pressure, Building Regulations guidance says they must be installed by a competent person, typically an installer with an unvented hot water qualification, and the installation must be notified to building control. Megaflo recommends that its products are serviced annually. A service checks the valves, the expansion space, the thermostats and cut-out, and the discharge pipe.

Common problems

What you notice	Likely cause
Cold water dripping into the tundish, often when heating up	Expansion space lost or the expansion relief valve letting by
Hot water discharging from the tundish	Overheating or high pressure. Switch off the heat sources and call an engineer
No hot water	Thermostat, a tripped thermal cut-out, a motorised valve, the programmer or the immersion heater
Poor pressure at all outlets	A blocked strainer in the inlet valve, a pressure reducing valve fault or a poor mains supply
Hot water runs out quickly	The cylinder is too small for demand, or the coil is scaled or not heating fully

How we price it

Installations are quoted as a fixed price after a free survey, so the price you agree is the price you pay.

For repairs, there is no call-out fee in normal working hours. After the first hour, time is charged in half-hour steps, and parts are quoted before we fit them. Evening, weekend and overnight visits are charged at the out-of-hours rates. Every rate is published on our prices page, so you know what you will pay before we arrive.

Frequently asked questions

Is a Megaflo better than a combi boiler?

It depends on the house. A Megaflo suits homes with two or more bathrooms and a good mains supply, where several people use hot water at once. A combi suits smaller homes and never runs out of hot water, but struggles to supply more than one outlet at a time.

Why is it called Megaflow?

The brand is spelt Megaflo, and it belongs to Heatrae Sadia. “Megaflow” is a common misspelling, and the name is widely used for any unvented cylinder, whatever the make.

Does a Megaflo need servicing?

Yes. Megaflo recommends an annual service for all its products. The safety valves, expansion space, thermostats and discharge pipe all need checking to keep the cylinder safe and working properly.

Can a Megaflo work with a heat pump?

A heat pump needs a cylinder designed for it, with a larger coil because heat pumps run at lower temperatures. Ask for a cylinder matched to the heat pump rather than reusing one designed for a gas boiler.

Why is my Megaflo tundish dripping?

A drip of cold water usually means the internal air gap needs restoring or the expansion relief valve is letting by, both of which an engineer can sort out. Hot water running from the tundish is more serious: switch off the heat sources and call an engineer.

Help in Hertfordshire, Bedfordshire and Cambridgeshire

Whether you are thinking about a Megaflo or your existing cylinder is dripping, losing pressure or not heating, our engineers can help. See our unvented cylinder service or call 01462 506374, answered 24 hours a day, 7 days a week.