How to use anti-vacuum valves

Anti-vacuum valves prevent hot water cylinders from imploding

Venturing into the unknown is part and parcel of being a heating and plumbing engineer, and the majority of installers are adept at dealing with any curveballs that come their way.

Being cautious, especially when it comes to hot water cylinders that may not have been installed correctly, is vital. If a system is being worked on for the first time by an installer, for example carrying out simple maintenance work such as changing a shower valve, they must take care. A vacuum can occur if the hot water cylinder has not been fitted with an anti-vacuum valve, which can result in a collapsed cylinder. This is because the greater atmospheric pressure outside causes the vessel to implode.

Anti-vacuum valves should be installed to prevent a vacuum being formed in installations where the draw-off rate becomes greater than the inlet flow rate. This might be where the cylinder is located in a loft or on the floors above in a tall dwelling with the draw-offs below.

Anti-vacuum valves have the ability to thwart damage to properties, prevent serious injuries and save installers’ reputations. However, it has become increasingly apparent that there is some confusion as to when and where they should be installed.

The science

A vacuum is often referred to as negative pressure, which occurs when the pressure drops below atmospheric pressure. Atmospheric pressure is all around us and is recorded at sea level at 101.325Pa (Pascals) or 14.696psi (pounds per square inch).

We typically feel the difference in atmospheric air pressure with changing weather patterns or when air pressure is adjusted when taking off and landing an aircraft. It is the same principle with draining down a cylinder (sealed vessel), however no air is getting in. If the cylinder is installed above the draw-offs, typically in a loft, and no air is introduced by opening taps or occasionally twisting the temperature pressure release (TPR), then the atmospheric pressure is far greater outside and the risk of implosion is high.

Of course, these systems will potentially have very hot water within them and should they implode then that hot water can cascade down onto the area below, damaging the ceiling and other property as well as harming occupants and the installer.

Insurance companies might question the potential claim for subsequent damage as to how a cylinder could implode. If it’s because the hot water cylinder was incorrectly installed then they might be less inclined to pay out and the installer will be considered responsible.

How anti-vacuum valves work

Inside the valve is a check valve cartridge which, under normal conditions, is closed by the system pressure. When a vacuum occurs the suction on the valve will open and allow air in to flow through. This equalises the pressure within the hot water system to that of atmospheric pressure on the outside, therefore preventing collapse.

Getting the system design correct is paramount, so following the manufacturer’s instructions is vital. Not only is an imploding hot water cylinder extremely dangerous due to the risk of potentially life threatening scalding, but the damage can be very costly to repair too. This can also occur when an atmospheric system has a blocked vent pipe, for example, when it is frozen.

Ideal scenario

Hot water systems should have an anti-vacuum valve on the outlet when first installed if the cylinder is located above the draw-offs. There are many chat rooms on the internet in which consumers and plumbers ask questions after this type of failure, frequently resulting in completely wrong answers. This can cause problems with a customer who, having read these false diagnoses online, calls in a knowledgeable and competent installer to work on their hot water system but is unwilling to accept the right answer when it is presented to them.

This situation is becoming more common as systems are being used in areas that traditionally wouldn’t be used in the first instance. A roof void, for example, isn’t always an ideal place to install a hot water cylinder. Installers must factor in things like whether the water pressure will be enough for the additional height, weight and structure, as well as temperature in the colder months and getting the vessel up there in the first instance.

When a bathroom is converted and the cylinder is relocated into the loft or a loft conversion then the installation might require experience and knowledge from the installer and a thorough reading of the manufacturer’s instructions.

Training and its curriculums are also potentially lacking in the industry as behaviours change and consumers change their ideals. The siting of the cylinder is down to the design and planning of the installer – manufacturers can’t be expected to list every scenario. The responsibility lies with the competent installer in the first place and then moves on to the person carrying out alterations, so it’s important that all heating and plumbing engineers understand the power of atmospheric pressure.

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