Hello all
I have a washroom off my bedroom which contains a tub,sink and a toilet. I also have a main washroom with the same. I'll call this one the primary. I'll try to explain the layout. My secondary tub is back to back with the primary, there is a 1 1/2" vent between them. This then travels horizontal about 8 feet to the toilet which then continues to travel hortizontal to the center of the house where it dumps into the main line. The secondary toilet drain has both sinks attached to it with NO vent and this travels horizontal to the main drain as well. It enters down stream about 3 feet from the primary drain. When the primary toilet is flushed a bubble comes up in the secondary toilet every time. Is it a clog some where or poor venting? I have total access to the pipes under the floor, and I have plans to move the primary toilet to the other side of the washroom which gives me an oppurtunity to give it a dedicated line and it's own vent. I do not see any other vents as the main drain on the up stream side has a kitchen sink and a laundry tub attached to it.

Your advise will be greatly apprieciated!

That secondary drain line is the problem because it has no vent. You have to get a vent line for that drain branch up to the main vent stack or out the roof on it's own. An AAV (air admittance valve) will not correct this one because it only breaks a vacuum and your problem is caused by positive pressure.

EPM

EP nailed it!

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The secondary toilet drain has both sinks attached to it with NO vent and this travels horizontal to the main drain as well

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Each sink should be vented. Code dictates that EVERY fixture that has a trap MUST be vented.

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When the primary toilet is flushed a bubble comes up in the secondary toilet every time.

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Bubbles indicate back pressure caused by a partial clog. This is the opposite of a blocked vent witch will cause gurgles. I would snake the toilet drain line and see if that didn't stop the bubble. Good luck, Tom