PSA: Fixing Slow Low-Flow Toilets

We’d been having issues with our toilet for a while that it would not flush properly. The symptom which indicated the problem was that it would fill the bowl very slowly, at which point maybe it would flush, maybe it wouldn’t. It turns out that the problem was not that the drain was clogged but that the siphon jet had become clogged with mineral deposits. For this to make sense, we have to quickly describe how a toilet works (which is interesting, especially considering how old and barely-modified-since-then the design is).

To help with this, I’ve made a crude diagram of the important parts of a toilet (note: nothing is to scale):

The goal of a toilet is to use water to move everything in the bowl into the drain pipe that leads to the sewer system (in a modern toilet, with as little water as possible). Any sort of basic hose arrangement can do this, but for various reasons it is preferable to have standing water in the toilet bowl when not actively flushing. This introduces the problem of how do we pump the water out of the toilet bowl and into the drain pipe?

Obviously, a giant plunger that used enormous amounts of air pressure to force everything in would work, but it would be difficult and if one is not very careful, messy. There is another way to take advantage of air pressure to do this work for us, though: a siphon!

If you don’t remember, here’s a helpful illustration of a siphon from Wikipedia:

User:Tomia, CC BY-SA 3.0, via Wikimedia Commons

If you can get the tube to be filled completely with water, the pressure of the air will force all of the water in the higher container to go up through the tube and back down again, transferring the water into the lower container. The key to this siphon action is that the water falling down the tube has the effect of pulling water up the tube, and so the water flow is proportional to the distance between the height of the intake and the height of the output, with the height of the tube in the middle having negligible effect. For anyone who’s tried to use a siphon in practice, the trick is how you get the tube completely filled with liquid.

If you look at a toilet, you’ll notice that the output drain has a siphon shape. The tube goes up, higher than the normal water level in the bowl, and then down again. It is, in fact, a siphon, whose purpose is to pull all of the liquid out of the bowl and drain it into the sewer system. The tube is porcelain rather than rubber, but that doesn’t make a difference to the siphon action.

So, how do we get the siphon started, i.e. how do we fill the tube with water? You’ll notice that the top of the bowl is above the top of the siphon, so one way would be to just pour so much water into the bowl so quickly that the siphon tube can’t help but fill up. This works, but the downside is that it needs a very large amount of water to do it. This isn’t so helpful in places that don’t get much rain, but it’s also annoying in that it just takes a long time to transfer that large a volume of water, especially if done by gravity, and also means it would take a long time between flushes to refill the tank.

Enter the siphon jet. It’s an output port of water from the tank which is directly opposite to the intake port of the siphon. It’s far below the tank, so water pressure is at its highest, and it’s built so as to accelerate the water coming out of it. The result is that it shoves water up the siphon tube much faster that filling the bowl would, causing the siphon to form which then pulls the liquid out of the toilet bowl and into the sewer.

This is better in approximately every way since the toilet flushes faster, uses less water, and refills more quickly. The only problem is that it makes the siphon jet a point of failure. If mineral deposits build up on the siphon jet it can significantly reduce the speed of the water coming out of it. If the water coming out of the siphon jet is slow, it can’t shove the water up the siphon tube and the siphon doesn’t get formed. If the siphon doesn’t get formed, the contents of the toilet bowl will remain in the bowl.

The good news is that cleaning mineral deposits isn’t super difficult. Basically what you need is some sort of acid to bread down the mineral deposits. Common household vinegar is a weak acid but will work if you use enough of it, for long enough. Getting out as much water as possible from the bowl (a good start is to use a plunger to force as much of it up the siphon as possible, then you can use a dixie cup and a buck to remove most of the rest, and small amounts of water don’t matter) then pouring in enough vinegar to cover the siphon jet will do a lot toward removing mineral deposits.

This is as far as I’ve gotten and there’s been a significant improvement in my toilet that is having this problem. The mineral deposits are rather bad, though, so I’m planning to buy some hydrochloric acid (frequently sold as “muriatic acid”) at the hardware store to try. It’s a much stronger acid and also is sold in more concentrated forms, so it should remove the decade of mineral buildup much faster. At the current rate of improvement, I’d probably need a few gallons of vinegar per week to completely clear away the deposits that have built up.

Oh, I forgot to mention that the rim of the toilet also puts some water into the bowl so that all of the sides of the toilet get wet. It is important to make sure that these outlet ports (there are a lot of them) are unobstructed, but their primary purpose is to move everything in the bowl down towards the area in the bottom with the siphon jet so they’re not as critical to whether the toilet flushes.

I hope that this is useful and possibly interesting. Good luck with your toilets.

Update: I was asked if older, non-low-flow toilets also have siphon jets. My understanding is that yes, they do, but they were bigger and didn’t accelerate the water as much so they weren’t as subject to working badly because of mineral deposit buildup. It’s still worth keeping them cleaned off of mineral deposits, it’s just less critical. This is related to how Efficiency and Robustness Are Enemies.

2 thoughts on “PSA: Fixing Slow Low-Flow Toilets

    1. Thanks! It’s a problem I had misdiagnosed for a while until I did more research, so I figured I might possibly save someone else the trouble. Plus once I figured out how this old technology works, I thought it was interesting. I wonder if this obligates me to work it into a mystery as a clue…


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