Height of Vent Pipe above Cold Water Storage Cistern

[John Carroll]

Came across this in some old book from long ago where it states that  "The height of the vent above the CWSC (water level) should be 4% of the head from the bottom of the HW cylinder to the CWSC water level + 150mm"

 

This obviously refers to  water at 100C as the expansion is 4% (or very slightly higher) when heated from 15C to 100C.

If its easier, 40mm per 1M head can be used instead. (40/1000=4%)

My HW cylinder, like a lot of others is installed in the hot press on the landing with the CWSC in the attic, water level ~ 3M above the bottom of the HW cylinder. The theoretical height of the vent should be (3*40)+150, 270mm, if the cylinder was located downstairs with say a 5M head then the vent height should be (5*40)+150, 350mm

My vent is 350mm above the CWSC water level but I have seen several where the vent is resting on the CWSC top and ~ 100mm above the water level.

Someone asked me  years ago why the above vent height is required and I reall don't know.

Its  not a expansion allowance, my 135L CWSC measures 450mm from the bottom to its normal level so, 3.33mm/L, I have a 150L HW cylinder which would expand by 6L if heated from 15C to 100C, a rise of, 6*3.33, just under 20mm (vent pipe level will rise by the same amount), a long way from the overflow. even a 300L HW cylinder with a expansion of 12L at 100C would still not reach the overflow as the level would rise by, 12*3.33, 40mm, still leaving ~ 50mm to overflow. Of course, normally the HW cylinder never reaches these sort of temperatures.