How does air get into system through vent pipe

[lordyjordy111]

Hi all

Can some one help??

I'm currently training as a heating engineer a recently come across a boiler that was drawing air into the system through the vent pipe but do not understand how this happens.

I understand the water level goes up and down as the water expands and contracts but how does air get drawn into it?

for example say the vent pipe is 4m long and is vertical to the primary flow and that when cool the vent will fill upto the level of the f&e tank

now say when the boiler is fired it pulls on the vent and reduces it to 3m vertically.

how does air get drawn in as there is still water in the vent???

surely it has to drop all the way to where it tees to the primary flow???

I did ask the chap training me but he said "because it just does" which didn't help much

 

[bob the plumber]

having your feed and expansion in the incorrect position in relation to your pump can cause either pump over or pull through the expansion. the pump will in effect pump back up the feed and replace with whats in the expansion including the air. you cant beat a seperate feed and expansion (not combined a lot of guys on here think its ok but in my opinion you always end up with problems)

expansion, feed (within 150mm of each other) , pump bypass(s plan) valves

 

[gaspastemania]

Pump can draw air through if incorrect placed , on rare occasions can draw tiny amounts of air into systems from i believe on fittings etc.

 

[rotra]

normally on a incorrectly positioned cold feed and vent pipe, the system will pump over (through vent pipe) and refill through cold fill which is fresh water and is aerated

 

[lordyjordy111]

Thanks lads I appreciate your help

 

[Best]

Air will also get absorbed into system on open systems at exp tank & pipes. Also if fresh water is being added a little due to leaks or evaporation at tank. As Bob the plumber said, it is better with both separate vent & a feed, if joined right. Obviously sealed system, on oil or gas, best job.

 

[diamondgas]

It's all about differential pressures as far as I remember ... A pump has to put negative one side to produce a positve movement the other! In theory everythin behind (upstream) umtil the feed, is negatively prressured compared to downstream to the vent, positive pressured ... The rads etc are therefore positively pressurise as is the open vent pipe. If you're getting suction down the vent then they've been configured incorrectly!

 

[FD Powerflow]

Diamond Gas is spot on, thats exactly what happens.