So my mystery toilet problem update:

After two days of not working, the toilet spontaneously started working again. This is always the pattern. I think it is somehow related to the weather. I suspect that it is caused by the winds blowing from the northeast. I am going to keep track of it. Why do I think that?

Well, I do believe that all of those who commented on my previous post are correct in that it is a vent problem. Let me explain:

I live in a 4 bedroom, 2 bath house. The roof is a gabled roof, with the roof ridge running east to west across the house. This is important to my hypothesis.

There are two lines for sewage that are under my house. They both run north-south. One line is on the east side of the house and services the kitchen and laundry room. It has two vents: one vent pipe that runs up the wall and emerges from the northern slope of the roof, and the second is an under sink AAV valve that is in the island that my kitchen sink is mounted upon.

The second sewage line runs down the west side of the house from the master bath’s commode (which is on the southwest corner of the house), past the his and hers sinks, under the shower, and on through the guest bathroom. There is a vent that extends up the wall and emerges from the roof above the guest bathroom, again on the northern slope of the roof. The only other vent for this western line are two AAV valves located under the master bathroom’s sinks.

Both vents are clear. The sewage lines are clear. I know, it cost me a $500 plumbers visit, after which they told me nothing was wrong with the plumbing.

My hypothesis is this:
The vents are both on the northern slope of the roof. When the wind is blowing from a certain direction (probably a northern one), the wind is pressurizing the pipes through those northern vents. Once the sewage line is pressurized, this pressure prevents the toilet from flushing. The only thing that could destroy this hypothesis is the fact that no other fixtures, including the toilet in the guest bathroom, which is still on the other line, continues to work.

I don’t think it is malfunctioning AAV valves, because while the toilet was malfunctioning I removed the valves under the master bathroom sinks, and the toilet remained dysfunctional.

I am frustrated, the wife is getting aggravated, the plumbers are clueless (yet still billing me for visits), and I am out of ideas.

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Therefore · December 13, 2019 at 9:55 pm

Given the amount of frustration you are encountering, consider just replacing the toilet. You can pick up a wax ring and a new toilet for around $200. Turn off water, drain old, disconnect water supply, collect water that didn't get drained. Take tank off, undo bolts holding seat to wax ring to floor. Remove seat, remove ring. Install in the opposite order.

At $500 for a video of your pipes, this might be a lower cost eleminate potential problem.

I originally thought it was venting, not so sure any more. If the pipes are clear, could it be the toilet proper?

BC · December 13, 2019 at 10:22 pm

2nd on the remove the stool attempt. If the sink & shower drains in that bathroom work correctly even when the stool does not, then it sounds like the problem is most likely in the stool itself. This also lets you see right into the business end of the pipe. You can try dumping a bucket of water at a medium speed down the hole to see if it flows freely without the toilet installed.

Did the plumber remove the toilet to run the camera down the hole? If they did then they wouldn't have seen any blockage or snags in the toilet trap.

Divemedic · December 14, 2019 at 12:26 am

But if it was the toilet, wouldn't it happen all the time, and not just during the spring and fall, and not just for a couple of days and then clear up?

BC · December 14, 2019 at 5:59 am

So far you have:
-This toilet backs up
-Other fixtures on this side of the system are down stream and do not back up
-System is not pressurized because no other fixtures on the same run are showing symptoms
-System is not pressurized because removing the AAV did not release trapped pressure in this line
-Vent can't be plugged otherwise downstream drains would burp & vent when flushing

1 PSI is equal to 2.31 vertical feet of water column, you would get shower drains venting back into the home if there was 1 PSI of pressure generated by wind in the vents because the traps are less than 6" of water column.

There is a blockage someplace, because the toilet does not drain. It must be up-stream of the other drains on this line because the toilet does not even drain slowly when flushed when the lower flow of the sinks and showers still drain freely. This blockage must be after the visible toilet water and before the next drain Tee, otherwise you would get some sort of venting or back-flow at the other fixture on the line when you flush.

Did the plumber remove the toilet and run the camera from the toilet mount into the pipe or did they start someplace else?

Therefore · December 14, 2019 at 12:40 pm

When solving a problem like this, you want to eleminate places for failure. When we forced our plumber to check and clean or pipes, he pulled the stool for easy access to the big pipe.

Then he did the inspection.

For us it turned out to be two things. Artistic son who makes dumps the size of small houses and with a concrete like hardness. And a 1.5 inch dip in the unsupported PVC in the crawl space.

One and a half of those have been fixed.

In prior houses I have had stools that liked to clog. Replacing them fixed the problem.

Given your budget on this already, this might be a little cost eliminate a potential problem area.

Good luck

Don McCollor · December 15, 2019 at 12:47 am

(Don McCollor)…as for the vent hypothesis – try opening a window when the stool is failing…

Beans · December 15, 2019 at 3:20 am

Even weirder, if you believe it is your roof vents that are pressurizing the system, consult with a chimney designer. There are certain 'sweet' heights to chimneys to allow them to vent without having the wind blow back down them.

Weird, weird calculations take place, and then one needs to have the system working to make sure.

Got any civil engineer or architect friends, or have friends down at the local fireplace and woodstove store? That's where I'd go first, before changing the toilet.

Also consider dumping drain cleaner down all pipes and vents to get rid of any small accumulations that may be hampering the system.

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