Air Admittance valves vs stack
PAUL KUENN
| Posted in Green Building Techniques on
Anyone in the north (we’re on Washington Island, WI; NE above Door Cty) use air admittance valves. Only two baths directly above each other. Trying to keep this build at Passive build level. I just hate knowing we’ll have a 3″ tube of cold air right down the center of the build.
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You have to have at least one vent, you can't rely entirely on air admittance valves.
Right, I was thinking this was the case as well. You want to be able to release built up pressure of sewer gases should it occur.
I suspect the amount energy lost to the vent stack is pretty small. I don't think your visual of a long pipe of cold night air is a very close representation of reality. There would probably be some kind of convection current drawing the cold air down and sending warmed air out. Buried inside a wall, how much heat transfer is going to happen? If you're really concerned, you could insulate the stack pipe. If you did the math, you'd probably find out it's negligible.
This is the first time I've seen anyone ask about air admittance valves over a concern for energy loss. Usually it's a convenience thing or inability to connect to the stack.
Paul,
I don't think much has changed since you asked the question last summer: https://www.greenbuildingadvisor.com/question/anyone-using-air-admittance-valves
OOps, Thanks Malcolm! I thought that was on another site. Thank you all for the insight. Need to retire soon but I'm booked out 2 years at present... Gratitude, it feels good to be needed.
Paul,
The thermal losses in a PVC stack are likely very minimal even with the cascade effect of air in a straight run. If you can get the local inspector to allow a side wall vent in combo with the AAVs , you will save a potential leak point in your roofing and probably simplify the plumbing. Side vents are legal in the National Code if I am reading it correctly. You only need one "wet vent" as my plumber described the required open to outside air vent.
There is a different issue from heat loss I encountered some 20 years ago during a particularly brutal winter in Chicago. My vertical vent stack pipe frosted closed thanks to 4 bath loving residents. The warm, moist air rising up to the -20F outside pipe section created an icey cap. Due to the 9:12 pitch, the length of the stack beyond the roof line was taller than many, which added even more cold surface to freeze the vented air. The frost plug made itself known by causing the toilets to flush mostly oddly. I also had one instance where extreme snow fall covered over a stack so deeply that a similar problem with draining occurred. Washington Island is both cold and pretty snowy, so consider a side vent if allowed.
I have all AAVs and a side vent with no discernible downsides other than an occasional gaspy noise from one. My winter lows hit -12F enough to have shown a "fail" if it was going to happen. I put a 5" down leg on mine to keep the cold air from backing into the pipe too easily.
One caution. Make sure the vent run to the outside is sloped toward the interior vertical run. This pitch back will keep the condensate running to the warm side of things. This is normal practice whenever the vent stack is offset from the main vertical run. For AAVs that aren't under a sink, they make little box housings with removable grilles that fit into the wall to house the AAV. I have mine set into adjoining closets that share walls with the bath vent runs. The one we occasionally hear gasping had to be set on a hall wall.