Combined Feed and vent pipe

[Bennyboy1974]

Hi, I was just doing a bit of repair work on my mother in laws system as the F&E tank was slightly overflowing, seems the ball valve was set far to high, so i adjusted it which naturally took the water level down to below the overflow, however, i noticed that there was no vent pipe over the F&E. It seems that it is a combined feed and vent pipe in 22mm. Could someone explain how this works as i thought that open vented systems meant exactly that and they were supposed to be open to atmosphere. I can understand the principle of the feed pipe but thought it would have made sense to tee from this pipe and vent over the F&E. Look forward to understanding how this works? Cheers in advance.

 

[Mike Jackson]

When filling any air will bubble up the combine pipe or vent out of the automatic air vent that should be fitted on a combined feed and vent. The advantage of combining it is there is no possibility of air being sucked down the open vent. Combined feed and vent systems should only be used on boilers with an overheat stat and not all boiler manufacturers approve of them so there can be warranty issues.

 

[Bennyboy1974]

Thanks Mike, I just thought it unusual as I have only ever seen a set up with a vent pipe. Thanks for the explanation though. It does indeed have an AAV installed.