Upstairs Saniflo bathroom in attic

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okcRental

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I am adding a room in my attic and would like to add a bathroom upstairs. I can get water supply there but the drain is the difficult part. I have an air vent from my downstairs laundry nearby I could tie the saniflo into but I worry about negative pressure issues on the ptraps below. I don't really have a way to add an air vent for the downstairs fixtures without some major demo. I am thinking of several options.

1. will it really cause negative pressure? it will not be a bathroom used often, it will only be a 3/4 inch line tying into a 1 and 1/2 inch air vent?

2. I could add an AAV to the downstairs sink (no room to add for the washing machine drain due to studs in the way) and thereby relieve some of the negative pressure risks.

3. I think I could manage to get a 3/4 inch pipe down the inside of the wall and tie into the vent close to where the sink and dryer are tied into the stack and as the Saniflo will have its own air vent about 8 feet away I think that may solve the issue?

Any advise would be appreciated. I am pulling my hair out trying to find a solution that does not involved a total demo of my laundry room.

2.7 - SEWER PIPE (COPPER, CAST IRON, PVC, ABS) 1.Install an appropriate wye fitting into the sewer pipe, (with  1 1/2" branch hub).
 

Terry

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The 3/4" drain for the Saniflo will need to tie into a drain line below any of it's venting. Fishing a line that small to a floor below shouldn't be that hard.
At least you have a vent to tie into for the Saniflo vent.

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okcRental

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so you dont see an issue with tying in the upstairs saniflo into the vent stack down on the first floor but just above the washing machine and sink drains on the first floor? would this help avoid any risk of negative pressure in your opinion?

I do think I will need to vent the saniflo pump itself separately as it is in another room about 10 feet away and it would seem that it is best to go straight out the roof instead of over 10 feet and then up.

I have included a rough drawing of what i have in mind. There is a downstairs laundry room and looking to add an upstairs bathroom with shower, sink and toilet.

The vent is 1 and 1/2 and then goes up to 2 inch right below the washer connection. pretty difficult to connect below that as the home is concrete slab.
 

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