Air Admittance Valve placement

[henryk42]

I have a washing machine discharging into a 2†waste line that serves only my sinks and shower drainage. When the washing machine drains, I believe the force of the water causes the exit trap in the line to siphon out without retaining trap water. Septic gas then bleeds back into the house through the washing machine drain pipe. I think that installing an air admittance valve between the last trap and the septic tank would prevent the complete siphoning of the last trap. Am I correct, or should I look elsewhere to place the AAV, or for another solution?

 

[Jadnashua]

FWIW, EACH trap in the house should be vented. If they aren't, waste passing by the trap of any fixture can cause it to be siphoned dry. An AAV won't do much unless at least one atmospheric vent exists in the house, and you may need more than one.

TO perform as a trap, it must be located after the trap arm, before the waste turns down. Without seeing the layout, and seeing what's there, it's very difficult to give an accurate recommendation. AAV's are not always allowed by codes, and the inspector may reject its use if it could be properly atmospherically vented without major issues.

 

[Sponsor]

 

[Asktom]

Not enough information, how about a drawing of your system? (editorial comment: AAV is never the best answer)

 

[henryk42]

Thank you for your response. The piping is in an old, but updated, cabin. The piping was mostly there when we took over, and we only added the washing machine drain. All the sinks and the shower have their own traps, and there is the final one before the septic. I will now have to look for a vent stack for this line. I don't recall one being there, yet all was well until we added the washing machine drainline. A drawing of the final stage may be difficult to do. It's a handiman's effort.

 

[hj]

Where you install an AAV, assuming it would even cure the problem, is the most important factor and just "putting one in the line" somewhere may NOT be the answer.

 

[Jadnashua]

The issue may be that the WM just pumps water much faster than the other drains in the house...it's not just a trap that is required, but the vent to prevent it from being siphoned by the faster moving waste.