By John Felgate, Head of Technical, STIEBEL ELTRON UK
Most small water heaters sold and fitted in the UK and Ireland are of the unvented variety, requiring additional safety valves and expansion vessels compared to the vented type. However, in many cases, installers may choose an unvented heater based on a misunderstanding of how vented heaters perform.
A significant number of unvented heaters are selected for applications such as student accommodation, where space is limited and aesthetic considerations—such as the broader range of taps available for unvented systems—are secondary to cost and installation speed. Despite this, consultants and installers continue to specify an option that is more complex, time-consuming to install, and occupies more valuable space.
An outdated misconception associates vented water heaters with poor water pressure at the tap. This belief stems from older whole-house vented systems with loft-based header tanks that relied on gravity for water pressure.
For small vented water heaters serving a single outlet, this is not an issue. They are connected to the mains, meaning the pressure at the tap is the same as the mains water pressure. When the tap is opened, cold mains water displaces the heated water in the tank, forcing it out at the same pressure.
In essence, both vented and unvented small water heaters supply mains pressure at the tap. However, due to the misconception that vented systems inherently provide lower water pressure, there is a tendency to assume an unvented unit is the better choice—even when a vented unit would be simpler and more practical to install.
Vented units do not require bulky expansion vessels or pressure relief valves, saving space. They are also quicker to install and more affordable due to fewer components, particularly as recent regulations requiring additional relief valves have increased the cost of compliant unvented units.
With models available for both over- and undersink installation and featuring innovations such as STIEBEL ELTRON’s Thermostop and anti-drip devices, vented heaters are now more efficient and reliable than ever.
Of course, there are valid reasons to choose unvented systems as well. If multiple outlets need to be supplied, it is often more practical to install a single unvented heater rather than multiple vented units. Additionally, if a client prefers a particular tap for aesthetic reasons and no vented equivalent is available, and space is not a constraint, an unvented system is the right choice.
With recent regulatory changes making unvented heaters more expensive, vented heaters may be experiencing a resurgence. Given these factors, industry professionals may wish to reconsider their options for future projects.
You can check out STIEBEL ELTRON’s full range of stored water heaters at: https://www.stiebel-eltron.co.uk/stored
