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Hopefully I can convey my question properly. My townhome is about 20 yrs old (relatively new). It is two stories tall. I have 2 toilets upstairs and one toilet downstairs. This issue just started happening a few days ago. When I flush one of the upstairs toilets, it causes my downstairs toilet to "flush". I put flush in quotes because the water from the tank doesn't come down in this case but it seems the outlet from the bowl bubbles and the water leaves the bowl. The downstairs bowl is empty whenever the upstairs toilets are flushed.

I've also tried to flush the downstairs toilet by itself and even though things get flushed into the sewer, it doesn't fill the bowl entirely with water afterwards, even if I hold the handle down to let more water get into the bowl. It seems to be filled 60-70% of normal when the downstairs toilet is flushed by itself.

It doesn't seem like a blockage since nothing thankfully is coming back up. I don't know how flushing upstairs is causing the downstairs toilet to lose water from the bowl and causing it to not fill to its old level.

What could be causing this problem and would this be something I could fix myself or is this something more complex like re-piping the whole toilet plumbing system in the house?

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Most houses have a 'stink pipe' - a 6" pipe that discreetly goes up the side of the house and ends either under the eaves or pokes above the roof. It's main job is to vent smells from the drains system. However it also has a secondary job:

When you flush the toilet, the water rushing down the pipe can create a vacuum in the drains system if there is no pressure outlet to to maintain a pressure equilibrium in the system. This is a second and most important purpose of the 'stink pipe' - it ensures the there is no build of pressure in the drains. If this pipe becomes blocked or otherwise stops working however, you can find that flushing one toilet can affect the pressure inside the system and you will get funny things happening to other toilets / drains such as them emptying or overflowing as you've described.

First thing I would do is locate this pipe, and make sure nothing has happened to it - have you had any external works done on your home recently?

Secondly, I would locate where this pipe joins the system - there should be a drain cover, or at least a rodding point. Remove this cover and flush the toilet as before. If the downstairs toilet is not affected this time then it's highly likely that the stink pipe has been blocked somehow.

If this is the case, I would rod it from the top to ensure it's not blocked (birds nest, animal litter, leaves etc). You'll need a good ladder and some patience. If it's blocked, clear it and then get a filter cap for the top to ensure it doesn't happen again.

If that doesn't fix it, I would try rodding all my drains in both directions. Make sure any blockages are cleared. And if that doesn't work I'd get advice from a plumber on what to do! Good luck.

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    The usual term I've heard is "vent" pipe. It's usually buried in the walls and exits through the roof, though on older construction, where it wasn't built-in in the first place, it can come out the side down low and run up toward the roof - our 1890s house has its vent plumbed in exactly that way. Don't know that I've heard the term "stink pipe" before, but I like it! – FreeMan Dec 18 '20 at 11:51
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    LOL. I never heard the term either! But my first though was a blocked vent stack. Maybe some goop or critters. – George Anderson Dec 18 '20 at 14:53
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    😁 glad you like it - guess I should have called it a vent pipe for accuracy but I just scribbled this quickly and couldn't think of the right name at the time... we've always called it a stink pipe, not sure why! – 5Diraptor Dec 18 '20 at 15:43
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So the plumber came and was about to go on the roof to check on the vent pipe like @5Diraptor suggested. However, before climbing up, he checked on the clean out pipe and saw it was backed up with standing sewage water and toilet paper floating.

Eventually, he found the main sewer pipe in the complex was blocked, when then led to us finding the main sewer pipe on the street was blocked so they city had to come in to clear it. After the city cleared everything in the main pipe, the complex and thus my house's plumbing was back to normal.

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