By Patrick Gordon
The next items in the water supply to the hot water cylinder (HWC) are the vacuum breakers.
Yes, you are not seeing double there are two vacuum breakers, one on the hot and one on the cold-water sides of the HWC. This is what the South African National Standards says about it:
Vacuum control valves
In closed systems and in valve-operated vented systems, vacuum control valves shall be installed on both hot and cold-water pipes to and from the heater to ensure that both lines and the water heater are vented and that siphonage is prevented. When installed at a horizontal distance of more than 700mm from the outlet of a water heater, the vacuum control valve may be teed direct into the hot water supply line unless the manufacturer’s installation instructions or the local authority bylaws require the vacuum breakers to be mounted on 300mm risers.
According to the SANS Standard we must have a vacuum control valve (we know it as a vacuum breaker), on both the hot and cold[1]water sides of the HWC. If the installation does not allow for a 300mm riser, you can take the connection 700mm away from the outlet and then tee it directly onto the line. However, some municipal by-laws do not allow for this provision. You would also have to confirm with the manufacturer of the vacuum breaker if their vacuum breaker may be installed directly on the pipe without the 300mm riser.
Supplied by Patrick Gordon
The other question is, what is the pressure rating of a vacuum breaker? There is no pressure rating for a vacuum breaker as it should only open when the system pressure drops below the atmospheric pressure – i.e. a vacuum is starting to form in the system.
There are manufacturing specifications (SANS 198) that strictly control the first opening pressure of a vacuum breaker.
So, where the other safety control valves are rated for letting pressure out, the vacuum breaker has only one purpose and that is to let air into the system. The only functions that the vacuum breaker fills, (anti syphon protection, cross fixture contamination protection, etc.) are all achieved by the valve letting air into the system.
All things considered, the vacuum breaker forms a very important component of the HWC installation.
If you have enjoyed this article, look out for the next article. We will be discussing the temperature and pressure safety valve.
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