Venting the dry return?

cn30

Would there be any problem with putting a main vent at the end of the dry return? I have a one-pipe system serving two apartments (duplex), and each apartment has a short main and a long main. The long ones are only about 30 feet, and the short ones about 12-15 feet. The two long mains feed into the same dry return at the end of the run, and the two short mains both feed into a second dry return at the end of their runs. The two dry returns join together right at the drip line into a single wet return near the boiler.



The reason I want to put the vent there is that there is a place to put it—there’s a short capped pipe coming off the dry return, and if I can get the cap off I can set it up. The short mains are covered in asbestos, and the ends of long mains are uncovered but would have to be cut. One of the long mains does have a non-functioning main vent at the very end of the run (not 15 inches back) which is maybe why it doesn’t work.



I know that one vent is probably not enough, but it’s better than what I have. Other than maybe wasting some steam, would it cause any problems to vent the dry return?

furnacefigher15

Yes

You can vent a dry return without causing problems.



But, if the vent is not away from the riser drop, it will water hammer the vent, and kill it in short order.



If there's not another place to put it, you may be able to put it on a long nipple, and maybe even add an offset, to help keep the water from the vent.



I would look into having mainline vents added. This would help dramatically.



Ideally, each main would get its own vent.

Abracadabra

should be fine

Should be fine.