missing vent pipe

[ben28]

I have a question and I am hoping someone can help - just purchased a new house and noticed a TRV was leaking tried tightening etc but came to the conclusion needs taking off and replacing - its a gravity fed system so was up in the loft about to plug the vent pipe and the cold feed - however there is no vent pipe.. I double checked to make sure that I wasn't being stupid but surely it should have one after all there is a feed and expansion tank that is definately connected to the system - its a not a sealed system or a combi there is no expansion vessels or pressure gauges anywhere.
just the cold feed coming down from the tank going into an air seperator - (4 conections the top tee off to the right is capped off, the tee below feeds down into the pump) should the capped off pipe be venting to the F/E tank? the system has obviously been working ok as the house is 7 years old and this system has been in since then. (although would the fact there is no vent cause leaks such as the one I found on the TRV?)

 

[ChrisR]

It's quite common to have a "combined feed and vent", which is OK as long as the pipe is 22mm, because water can go down while steam bubbles go up.
Often you see such a pipe split by the f/e tank so one pipe goes up and over, but I really can't see why it should make any difference! Mfrs Instrs tend to show it like that, but they aren't all that bright .

No it won't cause a leak. Handy for bunging the system, too!

 

[chrishutt]

You're up early/late today ChrisR!

I think the arrangement described is potentially dangerous. If the feed and expansion pipe were to block (it has been known) there would be no other route for thermal expansion to be accommodated with the result of rising pressure until something gives - possibly the blockage but possibly a joint somewhere.

Combined feed and vent pipes are at least 22mm, whereas I suspect this feed pipe is only 15mm, so more likely to block. Since the air separator is in situ and with a vent connection, why not extend this up to the feed tank?

 

[ben28]

Thanks for the help guys, and such a quick response.. I'll definately be back here again.
the feed pipe is 22mm and the air seperator device has all connections in 22mm also. It is some way from the F/E tank actually in the airing cupboard next to the cylinder / pump etc. Does this make any difference? or should I still considering extending the capped off section to vent into the tank?

 

[chrishutt]

You could attach a vent pipe to the feed pipe next to the feed tank, so air (and steam) can escape more easily without conflicting so much with the feed water flow.