CH vented system - Vent pipe size question

[twiglet1]

Hi and thanks for providing such a helpful forum.

I'm working on a vented CH system (S plan). I've read on a couple of sites that the vent pipe should be a size up from the feed pipe. I'm sinking both the vent pipe and feed into a stone wall and would really like to use 15mm for both if possible.

Would using 15mm for both cause a problem or is it important to use 22mm for the vent (and if so, why)?

cheers,

 

[kirkgas]

the vent is a safety pipe and shouldnt be reduced, if under fault conditions the hot water overheats it needs to escape to the storage tank or outside as quickly as possible, reducing the size of the pipe can slow the water down

 

[twiglet1]

thanks for the prompt reply kirkgas