One toilet's flush removes water from other toilet bowl

Hi Guys,

I have an ongoing issue with my house where one toilet is flushed and the water from the toilet in the bathroom a few feet away gets sucked out. I have checked that the vent is open through the roof and suspect it is a piping issue. I am currently remodeling of the bathrooms and I have gained access to more of the piping and I have attached a drawing below in hopes that someone will be able to point me in the right direction to correct this.

Thanks in advance for your advice,

Brian

You have a siphoning issue.

The vent should have been between the 2 continuing up.

So If i could tie in the vent between the two toilets rather than the way it is currently installed that would resolve the issue.

I would believe it should, when 2's water drops vertical it creates suction. I would think by breaking that siphon it should solve the problem.

I will give that a shot. Thanks.

Before you go adding vents or anything like that, lets ask a few more questions. Is this a new problem or something that has always been there? If it's a new problem, then you probably have a clog in the drain line somewhere (possibly the stack). If you have access and could take a photo we would have a better idea as to what the cause could be if it's not a clog. I would consider it a rare occurrence if a single flush on a 4" stack would pull a trap on another toilet, but anything is possible.