Mystery Vent

L'town Radiant_2

Went to go look at a job yesterday and while measuring the convectors, I came across this vent arrangement (pictured). It looks like a regular 67 vent with some kind of a chamber under it. Can anyone explain what this is and how it works?

Thanks in advance!

DanHolohan

Here you go:

http://www.pexsupply.com/Hoffman-401476-550-1-4-Straight-Air-Chamber-11735000-p

L'town Radiant_2

Thanks for

the input Dan. Though, I must say that I don't really understand why it is necessary. Doesn't the 67 vent serve as a chamber in itself?

DanHolohan

Some think it's not large enough.

And the chamber keeps it from spitting. These have been around for longer than I have. 

L'town Radiant_2

OK,

Well,  thanks again!