Installing New Toilets

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I'm installing 2 new toilets in bathrooms that are back-to-back. I've read about how the new toilets don't play well with double sanitary tees (although I don't think I have that issue). I have little information about what my plumbing looks like so I wanted to make sure I shouldn't have any siphon issues. Here is what I know:

1. The toilet in the master bath is NOT back to back with the toilet in the guest bathroom. The center of the flanges would be about 22-26 inches apart (although this is an estimate because there is a wall separating them).
2. The toilet flange in the master bath is just on the other side of the wall from the tub drain in the guest bathroom. I can see that the master bath toilet drain pipe flows in a direction that is parallel to the wall between the bathrooms (so NOT toward the tub drain) but I don't know where the tub drain goes (except for the info below).
3. Joists run parallel to the wall between the bathrooms.
4. I don't know where the bathroom drains go and where they meet up but they definitely flow horizontally because there is a kitchen below the bathrooms. I believe that eventually all of them connect horizontally and then turn vertical inside a wall that is about 8-12 feet from each fixture. I've seen the 3" pipe when I opened up the ceiling in my kitchen pantry. I could see the 3" pipe coming through the center of a joist and then turning to run parallel with the joists and then turning vertical inside the pantry wall.
5. Toilet drain pipes are 3" and guest tub drain is 1.5".
6. Tub drain in the master bath is 1.5" and is completely separate from all the other bathroom drains (I saw this drain pipe when I removed soffits in my kitchen).

I live in Illinois and the house was built in 1990. I haven't changed any plumbing so, assuming the house was built to code originally, is there any possible plumbing in the ceiling that would create a siphon issue with the new toilets or a siphon issue with a new toilet and the guest tub drain? I really don't want to open the ceiling just to look at the plumbing. I'm sure it's obvious but I don't know much about plumbing.


6th clinical trail chemotherapy 5/15/24
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Orlando, Florida
If the old toilets didn't have a problem, there should be no issue with the new ones.

Many times posted on Youtube and other articles make for some confusing information. I have found many times incorrect information when it comes to electrical or plumbing articles.
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