Why a Toilet Would Flush Properly Every Other Time

4 min readJan 9, 2018


Or, the Alternating Flush Phenomenon Explained

— — — — — — — — — — -

(Dear Medium readers, this is my most successful Medium post. Funny, but then again, not. After all, it’s a matter that has a target audience of ~5 billion people. If it correctly explains your problem, please consider coming back, after your corrective measures of course, and posting a comment to that effect, so that others know that its popularity is commensurate with its plumbing profundity. -Thanks)

— — — — — — — — — — -

I know what you’re thinking: why would someone be using Medium.com to write about toilets? Because I love a good mystery, and if you can think of a better mystery than why a toilet would flush properly every other time, then good for you. Send me a link to your story.

But the real reason is because I tried posting this information to the pertinent plumbers’ forums — where no one actually explained the mystery — and the mystery is so old that most of the pages are locked. Anyway, Medium is more likely to get the answer to people, so here we are in the toilet.

I have solved this actual problem at our house. People answering the question on forums guess at a lot of causes, but only two posts that I found implicate a low water level in the toilet bowl (not tank). And none can explain why this results in the alternating behavior. In my case the low refill of the bowl was sort of intentional, let’s say self-inflicted, caused by the modern type of tank valve that includes a little throttle valve on the refill tube itself. Notice the green lever in this photo:

The purpose of throttling the refill tube is to save water, by making sure that the bowl [not the tank] only refills to its max and stops, without any water unnecessarily sloshing over the s-trap and down the drain. A few weeks ago, I thought my bowl might be overfilling a bit so I arbitrarily backed off the green valve a smidgeon. Oops. It wasn’t until weeks later that we noticed it was flushing poorly. It’s a mudroom bathroom so it doesn’t get, um, “intensive” use. By that time, my little ad hoc adjustment was a long-lost memory. Hell, I barely remember what question I’m trying to answer here.

Anyway, my wife noticed the every-other-flush pattern. (Brilliant!) I get on the web and find mentions of:
- Clogs (of course)
- Mismatched flapper to tank size
- Clogged vent holes
- Bowl water level
- Vent stack clogged

Mystery Solved

All seem reasonable as causes of slow flushing, but the bowl water level seems to be the key to alternate flush/no-flush. Here’s how it causes poor flushes only on alternate flushes. When the toilet flushes successfully, suction/siphon action draws the bowl level BELOW the full level; ‘full’ would mean level with the top of the S-trap). If the refill tube isn’t doing it’s full job, the bowl gets underfilled. Now you flush it and it doesn’t flush strongly because not enough water starts through the S-trap to create strong enough siphon action to draw the whole mess along on its merry way. BUT, BUT, BUT… because it flushed poorly, the bowl is now full (!) and you get a good flush the following time. Lather, rinse, repeat!

So, any problem which causes the bowl to underfill can logically result in the alternating good/bad behavior. I’m not a plumber, but someone who is could tell us which problems, in addition to the under-performing refill tube, could result in not filling the bowl. And any of those would be potential root causes.

Now you know.

Quickly Proving If this Is Your Problem

A Medium commenter had a great suggestion. Hold your toilet tank’s flush handle just slightly open — enough to hear water going from the tank to the bowl, but not enough to cause a flush — for long enough to let water fill the bowl. Then release it. Then flush. If it flushes properly like this twice in a row, you’ve proven the cause: a low level of water in the bowl. Figure out why it’s low and you’ve solved your problem.




Usability and other stuff