TECHNICAL 35
Hot water cylinder ( HWC ) installations explained using SANS 10254 ( Part 3 )
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 coldwater 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 .
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 . PA
Images supplied by Patrick Gordon
“ The vacuum breaker has only one purpose and that is to let air into the system .”
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June 2024 Volume 30 I Number 4 www . plumbingafrica . co . za