Sink vent

I think (don't quote me) vent pipes must be at minimum, 1/2 the diameter of the pipe they service.
A 3" main drain would need a 1 1/2" vent. I don't know if this would apply to a RV because of the short runs of pipe.
Hopefully someone will correct me if this is wrong. :)

There’s several reasons the manufacturer doesn’t use a ‘real’ vent pipe..

First and foremost every perforation in the roof is a potential leak, but also an open vent means you could get precipitation down the pipe that could introduce water into an already winterized system.

JaxDad wrote:
...........an open vent means you could get precipitation down the pipe that could introduce water into an already winterized system.

Interesting. I never thought about that.

I would be venting out the old propane refrigerator roof vent. It's covered so rain isn't an issue. I'm also sealing that vent up with foam and can run 3/4 pipe up there, then seal around the pipe. I don't have room in the wall for 1 1/2" pipe.

AAVs are downstream of 'P' Traps so they do NOT get sucked dry from negative pressure created in sink drain line when DUMPING waste tank

Don't have room for 1 1/2" vent line to roof.....then upgrade and use a better AAV

Volume/weight of water being drained WILL overwhelm a small vent line and could collapse waste tank.

Fresh water tank can use smaller vent lines due to relatively slow water removal

Old-Biscuit wrote:
AAVs are downstream of 'P' Traps so they do NOT get sucked dry from negative pressure created in sink drain line when DUMPING waste tank

Don't have room for 1 1/2" vent line to roof.....then upgrade and use a better AAV

Volume/weight of water being drained WILL overwhelm a small vent line and could collapse waste tank.

Fresh water tank can use smaller vent lines due to relatively slow water removal

I'm not planing on venting the tank with this 3/4 pipe, just the kitchen sink. The tank has a 1 1/2" factory installed vent out the roof already. I just was wanting to do away with the "air admitance" valve that was under the sink. Anytime anyone has a smell, that's the first thing everyone says to check. They go bad. I was just thinking I could do away with a "future problem" by venting the sink externalally out the old fridge vent. I have to go on the roof and seal that old vent off anyway. I was thinking about dropping a 3/4" flexable hose down behind the fridge, connecting it to the sink Tee with a adapter and using it to vent the sink. I have a residential refrigerator now so the roof fridge ent needs sealed up anyway.

Install a "quality" Air Admittance valve and you won't have any more problems. There's $5 ones and $15 ones. The cheap black plastic ones like they use on RVs have a rubber flapper inside that goes bad. The higher quality residential type like in the link below have much better internals.

STUDOR REDI-VENT

Old Biscuit is trying to tell you that small vents will cause p-traps to be sucked dry when the dump valve is opened and a lot of water leaves the tank quickly.

True, but they can also be sucked dry when a sink with a lot of water is drained and the drain line between the sink and holding tank becomes full of water. The water creates its own vacuum and if not enough air can mix with the slug of water in the pipe it will pull all the water out of the p-trap too. It's not for only when the tank is drained.

Seems like alot of work (and potential leaks) for a problem that "may" occur "someday", and that can be solved (IF it ever does occur) easily and relatively cheaply ($15). I've been RVing for 30 years, 3 different rigs...never had an AAV go bad. Just sayin'.