In our industry, there are few genuine innovations.  When you look behind the hype, very little is new.

The unvented mains pressure hot water cylinder existed for years on the continent, but in the UK it has only been permitted for domestic applications since 1986!  This is because storing significant quantities of pressurised hot water brings certain risks, the one people get excited about being explosion.

Unvented cylinders deliver mains direct hot and cold services to all outlets in the house, dispensing with the need for a water tank in the loft.  They are typically the next stage up in performance to a combination boiler –  subject to the water mains supply being of a suitable flow rate.

Because of the small jeopardy*, there are a set of regulations, known as G3, that a heating engineer or plumber, has to hold an exam based ticket for, if they are to legally service or install domestic unvented cylinders.

*if you are curious, the stored water flashing over to steam is the main concern.

When the water in a sealed vessel is heated, it expands, and the pressure inside therefore rises.  Water is not compressible, so when it is heated in a sealed vessel, something has to give.  We can’t have cylinders rupturing, so every unvented cylinder in the UK has to have a way of accommodating the expansion of the stored water inside, as it is heated.

This is normally achieved by using a diaphragm vessel, which is similar to a rubber tyre innertube inside a sealed metal vessel.  Air (or on better quality units, Nitrogen) is separated by the diaphragm from the heated hot water, and is compressed every time the water heats up.  Gases can be compressed easily, unlike liquids.

A typical expansion vessel (source of image:

The problem with a diaphragm expansion vessel is that it has a limited lifespan, because the air or nitrogen eventually escapes, just like in a car tyre.  This can typically happen every 5-7 years, and when it does, water pressure in the cylinder rises to a critical level and is discharged from one of the safety devices on the side of the cylinder, into the drain.

You then have to call out a G3 qualified repair person to replace the vessel, and sometimes, the safety valve too.

Some cylinders have an air gap trapped in the top of the cylinder instead of a separate diaphragm vessel;  these are known as ‘bubble top’ cylinders.  A typical manufacturer of these is OSO Hot Water.  The disadvantage with bubble tops is that because the air rides on top of the water, over time it is ultimately eroded due to absorption in the water.  Then the pressures goes up beyond safe limits and the safety valve opens….

In the mid 90s a new innovation was patented involving a ‘floating baffle’.  This design separated the water in the cylinder from the bubble of air, by a floating plastic disc, or baffle;  reducing the air absorption rate.  However, although these devices significantly delayed the absorption of air and depletion of the air buffer, and reduced the occurrence of the safety discharge operating, it didn’t completely remove the problem.  The licensee of the patent was Heatrae-Sadia Megaflo.  Their cylinder became the best selling unvented cylinder in the UK.

The UK market leading Megaflo unvented cylinder

The floating baffle patent has now timed out, so other manufacturers can employ the design without the risk of patent infringement claims, such as Joule UK, but the same issues remain.

2018 Joule Cyclone Air unvented cylinder with floating baffle

Personally, I am not happy going back to customers with failed diaphragm vessels, or depleted air gaps, and charging them for labour and/or parts.  I’d much prefer the unvented cylinders we specify and fit don’t need repairing or maintenance visits after 5 or 6 years.

This is why we’ve recently started specifying the Fabdec Excelsior 3S unvented range.

And the first one went into my mother in law’s summer home in Minehead, Somerset.  That was brave!

Fabdec have designed and applied for a patent for a venturi-based device that entrains a small quantity of air every time a hot water tap is opened.

Venturi device by Fabdec

In doing so, the air gap in their bubble top cylinder is constantly being topped up, so the most common cause of cylinder repairs is removed.

Our first Fabdec installation, in Minehead, Somerset.

Fabdec are no strangers to the unvented cylinder market;  they were the company who filed the unvented cylinder floating baffle patent.  Furthermore, in the initial years, Megaflo branded cylinders came out of the Fabdec factory in Ellesmere.

Although Fabdec are pioneers in unvented technology, their business is primarily commercial milking equipment, which demands large scale stainless steel engineering skills.

When other unvented cylinder manufacturers boast about experience or capacity, most won’t have experience designing and fabricating anything of the size below, or have been trading as long.

25,000 litre Fabdec milk tank

We are your local Fabdec installer, call us for details.