Vent pipe

to be honest I can't remember what i was taught and I just fitted them any ole way. I never got any snagged by any clerk of works or water board inspector, I just tried to look it up, I can't find a dimension and there is none on this,
Attachment Deleted

On the 7th page of this it says 450mm:

www.pearsonschoolsandfecolleges.co.uk/FEAndVocational/Construction/Plumbing/Levels-2-and-3-Diploma-in-Plumbing-Studies/Samples/FreesamplechapterPlumbing/Level2_PLUMB_SB.pdf

This is the paragraph:

Primary open safety vent

The purpose of the primary open safety vent is to:

• provide a safety outlet should the system overheat due to component

failure

• ensure that the system is kept safely at atmospheric pressure.

The minimum diameter of the safety vent is 22 mm, and the pipe should never

be valved. In a fully pumped system, the primary open safety vent should

usually rise to a minimum height of 450 mm above the water level in the F&E

cistern. This allows for any pressure surge e ects created by the pump. The

open safety vent also aids the removal of any air that the system might collect,

particularly on commissioning or re lling (see Figure 8.05 on page 316).

I was taught 450mm min

I've always just looped it over the tank giving it roughly 4" - 9" without giving it much thought.
If the tanks in a cupboard then you often don't get the chance got much height on it at all


On the last HEATAS course I did I can remember battles 150mm + 40mm being a new element that the Tutor heavily worked on.
I wanted to ask the question why? 
However,  these courses are so crammed in and move on so fast that you never get the chance.
!t's all about giving you the knowledge (short term) to pass a test at the end of the course!
But I am remembering something about something about this from my days way back at college.

A few weeks later It's come to me!
We all know that water finds it's own level here on planet earth (100% correct).
However, when you get a larger surface concentration of water (in a tank) then the same water close by in a column (like a vent pipe) the level actually settles slightly higher in the vent pipe and the more height that you have in that column (system) the higher that water will be in that column (vent pipe). Battles 150mm + 40mm formula does make sense here.

I think what HEATAS scientists are saying is that if the expansion pipe is only just hooked over the tank by 2" the water could (if there is enough height in the system) naturally make a water fountain into the expansion tank (this is without any pump)!
Perhaps this 240mm regulation is just thinking about preventing a back-syphon from the tank


I've never seen this happen in all my life as a plumber and it all sounds like poppycock to me.
Maybe if you lived in the empire state building with a tank on the top floor and your flat on the bottom if might just happen!

It'd be interesting to get other views on this.


 

Under the Water Bylaws at one time here in Jockoland the Open Vent/Expansion Pipe from a Vented HW cylinder was terminated through the roof.  We made Lead slate pieces & firmed copper swan necks. 
On unregulated jobs we were always told minimum 450mm.

Another regulation on council houses was the CH expansion pipe went through the roof also, so we made double expansion pipe Lead slates.  Still see some on 40-50 YO Council houses.