The unvented solution

Stuart Doggrell, unvented internal expansion heaters, unvented systems, stainless-steel cylinder, unvented water heaters, Venturi effect, Fabdec
Stuart Doggrell

Stuart Doggrell highlights the growing popularity of unvented internal expansion heaters and their impact on the UK’s changing property landscape.

The UK property sector is growing quickly, with 80,855 private rented sector (PRS) homes completed or planned and increased housebuilding promised following the government’s pledge to invest £2bn in more affordable housing.

In order to provide the high volumes and quick delivery in this market, developers are turning to the unvented water heater as a way to service compact flats with reliable heating and hot water as well as strong energy-saving credentials.

The latest unvented systems are well-suited for small spaces, foregoing the requirement of a cold-water storage tank and providing external fittings that can be recessed into the body of the tank, saving space and simplifying the installation process.

By incorporating the latest stainless-steel cylinder materials, the tanks are lightweight without compromising on internal insulation, strength or performance. This provides developers the flexibility to site the cylinder wherever is optimal and provides a vessel that will not become hot to the touch or require regular maintenance.

Modern unvented water heaters also provide advantages in flow rate over conventional vented water cylinders with loft installed feed and expansion tanks, with the latter limited in the hot water pressure that they can deliver without the use of booster pumps. Industry-leading water pressure and heating consistency across a development can provide a unique selling point.

Renewables ready

The latest vessels also incorporate a twin coil arrangement that accommodates a primary and secondary heat supply. This integration can be used to siphon energy produced from solar panels or ground and air source heat pumps, providing opportunities for environmentally friendly and cost-effective heating.

In fact, the latest state-of-the-art unvented systems now provide performance levels that exceed the CHESS best practice guidelines produced by the Building Research Establishment – as well as being constructed from recyclable materials.

Innovation has always been a major driver of the popularity of the unvented water heater. For example, Fabdec introduced of the floating baffle that removed the need for external expansion vessels and has more recently developed patented automatic internal expansion systems that overcome the shortcomings of the floating baffle.

By installing a device on the inlet valve, unvented heaters are able to harness the natural phenomenon of the Venturi effect – named after the Italian physicist Giovanni Battista Venturi.

Stuart Doggrell, unvented internal expansion heaters, unvented systems, stainless-steel cylinder, unvented water heaters, Venturi effect, Fabdec
Unvented systems: Useful technology in today’s property market

By mixing liquid and air, or inducting air bubbles into a liquid, systems can now maintain the internal air gap as hot water is demanded further upstream throughout the course of typical usage. This technology means a unit can replenish its internal expansion device permanently without the need for recharging or regular maintenance.

In fluid dynamics, a fluid’s velocity must increase as it passes through a constriction in accordance with the principle of continuity, while its static pressure must decrease in accordance with the principle of conservation of mechanical energy. Thus, any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is balanced by a drop-in pressure.

A Venturi system can also be used to mix a liquid with a gas or air. If a pump or mains water pressure forces the liquid through a pipe connected to a system consisting of a Venturi valve to increase the liquid velocity, bisected by an inlet tube at the narrowest point, and then returning to its original velocity, the air will be drawn in through the inlet tube due to a differential in ambient pressure.


It’s because of this induction of air bubbles mixed with the flowing liquid that units are named selfsustaining systems or the ‘3S technology ‘– as they allow for the automatic replenishment of the internal air gap when hot water is drawn off further upstream in typical usage without the requirement for a floating baffle or any external expansion vessels.

Self-sustaining internal expansion heaters also provide the benefit of removing all internal, moving parts from the cylinder, meaning that once the unit is in place there are no additional parts required. The development of the selfsustaining internal expansion water heater has also streamlined the process of ensuring that units are fully compliant with section G3 of UK building regulations. Not to mention the potential environmental gains in the reduction in risk of waste water passing through neglect and lack of attention.

It’s this compliance, high performance, cost-efficiency and sustainability that positions unvented water heaters as the go-to water heating solution for the continued development of the UK’s property market.

Stuart Doggrell is Water Heating general manager at Fabdec.

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