25 years of unvented hot water

Heatrae Sadia, domestic hot water, DHW
Designed to exploit solar energy is the Megaflo eco Solar.

It can take a long time for practices in the UK market to change, and even longer for them to become commonplace and accepted as the norm. Heatrae Sadia’s Megaflo unvented hot-water system is one such product. Ken Sharpe traces the product’s development.

Building-services engineers much younger than their mid-forties will not remember the time when unvented hot-water systems were not allowed in the UK, even though they were the norm in continental Europe.

Despite the invention of unvented hot-water systems in the UK some 150 years ago, Building Regulations and Water Byelaws precluded their installation in this country. In contrast, unvented systems had become very common in continental Europe as a result of the post-war rebuilding programme.

The situation changed in the UK in 1985 with the revision of the Water Byelaws so they were no longer a barrier to trade. Within a couple of years in 1987 the precursor of Heatrae Sadia’s Megaflo unvented cylinder was launched by a company called Hotflo, which was acquired by Heatrae Sadia in 1993. So the Megaflo product is now 25 years old.

Since its launch, Megaflo has seen continuous development to improve its performance, exploit renewable energy and expand from the domestic sector into the commercial market. The complete Megaflo range has capacities from 70 to 2500 l.

Steve Rickards, the company’s commercial director, and who was also with Hotflo when the original unvented cylinder was first made, tells us that the latest version can deliver hot water at up to 72 l/min and that there is no significant fall off in performance if water is drawn off from more than one outlet. In contrast, he suggests that the sort of flow rate to be expected from a combi boiler is around 10 l/min.

The approach to delivering mains-pressure hot water is simple enough. Mains-pressure cold water enters a cylinder at the bottom to displace hot water. Special arrangements prevent turbulence so that cold water stays at the bottom of the cylinder.

Water is reheated by a stainless-steel coil-in-coil heat exchanger that can dissipate 25 kW, so recovery times are very short.

A significant difference between continental and UK practice in unvented systems is how the expansion of water as it is heated is handled. In Europe, water is simply allowed to go to waste. In Britain, the concept of not wasting water means that this expansion has to be absorbed. The expansion is only about 3%, so you can form your own opinion about whether the ‘waste’ is significant.

Domestic (left) and commercial versions of Megaflo – Megaflo eco (left) has a standing heat loss of just 70 W, and Megaflo Comerical is available with capacities of up to 2500 l.

Many unvented systems have external expansion vessels. The original Hotflo product used the same technique.

Megaflo, however, absorbs the expansion of water with an internal air gap and floating baffle system. This feature is said by Jon Cockburn, Heatrae Sadia’s head of marketing, to be unique in the marketplace.

The original product had a copper coil, but this was long ago replaced by stainless steel. Indeed, Megaflo products are made entirely from Duplex stainless steel, so they have a high resistance to stress corrosion, cracking, fatigue and erosion. In addition, there is no need for a protective coating or a sacrificial anode.

The inherent long life of Duplex stainless steel is enhanced by the production process. Each cylinder undergoes comprehensive, state-of-the-art weld treatments, and millions of pounds have been invested in these post-weld facilities.

Development work continues with innovations such as the pre-plumbed Systemfit option, wired to simplify installation and achieve consistent electrical and plumbing layout.

Renewable energy can be exploited by a solar-thermal version and a solar package that comprises collectors, cylinder, pump station. controls and roof fittings for slates and tiles. Solar versions can be used with flat-plate collectors or evacuated tubes.

From small beginnings 25 years ago, Jon Cockburn says, ‘The unvented market has seen considerable growth over the last 25 years, with consumer demand for powerful showers and fast-filling baths keeping sales buoyant. The market was still seeing double-figure annual growth in 2007 and into 2008, prior to the difficult economic climate and the subsequent reduction in construction projects.’

While Megaflo has been a successful market-leading product for the past 25 years, it needs to keep improving and evolving to meet changing installer and consumer requirements, as well as forthcoming legislation.’

So what else is 1987 famous for. Heatrae Sadia has done its research and come up with several memorable and significant things.

One is BBC weatherman Michael Fish famously declaring that there would not be a hurricane in the south of England.

It was also the year of the fire at King’s Cross Underground Station and the opening of the Docklands Light Railway in London.

Karaoke arrived from Japan, and The Pogues had a hit with ‘Fairytale of New York’.

Number students will observe that all the digits in 1987 are different, and immediately realise that the first year that this will happen again is 2013, continuing through to 2020.

Which begs the question, what further improvements will we see in Megaflo by then?

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