We all have experienced the toilet with a clogged drain that countered gravity---until it overflowed onto the floor.
Diving behind the toilet to get the water shut off as quickly as possible has happened to us all—especially if you have kids that are too young to deal with the crisis in any other way to scream. I have never known screaming to help in the slightest—except in getting dad or mom on the scene pronto.
All plumbing drains rely on gravity to function. This is a good use of gravity. The quicker “out-of-sight-out-of-mind” the better, when it comes to the toilets function. The rules of drain pipe installation are oriented toward assisting gravity as best we can. The violation of these installation rules with improper piping, slope of pipes or types of fittings, can promote clogging and end up working against gravity.
This first picture shows a drain from a toilet that is flowing up hill. Now we all know that under “normal” circumstances water doesn’t flow up hill, so what is really happening?
In this next picture I have done an overlay on the drain to show where there is going to be standing water in the pipe after the toilet has been flushed.
All of this water must be pushed out of the pipe every time the toilet is flushed and the toilet will not flush properly. A shortcut taken by not calling a real plumber that understands the rules of gravity as well as plumbing pipe and fittings, has resulted in a toilet that does not flush properly.
When you flush your toilet, it should make siphoning sounds consistent with good drainage. In other words--glug, glug, glug.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board