I can easily understand the purpose of the overflow tube itself, but why is the little black hose that comes from the fill valve placed down into the overflow tube? On my toilet, there is a strong stream of water that comes from the black hose, and it seems like going right into the overflow tube is a complete waste of water.

I know I must be missing something obvious, but I can't see it.

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7 Answers 7


That tube provides water to refill the bowl. Without it, you'd only refill the tank.

  • 1
    During the flush, the siphoning action through the trap pulls more water out of the bowl than is necessary to maintain a water seal, the water is replenished by the bowl refill. Plus dry bowls = sticking contents = partial flush = multiple flushes = wasting more water. Jun 29, 2014 at 15:05
  • 4
    More to the point, the water in the bowl is responsible for blocking sewer gases in the drain line from reaching the room. If the bowl were left empty after a flush, the room would likely get very stinky.
    – supercat
    Oct 18, 2014 at 12:46
  • 3
    That makes sense, but then if the water going down the overflow is refilling the bowl, how is the tank itself being refilled? Is there some kind of splitter valve, down in the base of the overflow pipe, such that some of the water goes into the bowl, but some gets diverted into the tank?
    – tkp
    Jan 18, 2019 at 5:24
  • 1
    @tkp Exactly. Water fills the tank from the very base of that water intake tube. I flushed my toilet and stuck my hand around that big tube where the water comes from and could feel the water coming in at the very base (in addition to the refill tube spraying into the overflow tube). I see this base-water-source labelled "water supply valve" on one diagram but it is missing from almost all googled diagrams. Apr 17, 2020 at 5:59
  • For part 2 of the question: I have high pressure supply and it is a waste of water - the instructions weren't clear on this. I bought a regular Fluid Master 400h and it has a Fill Valve which I had to turn down all the way. It uses less water and is much quieter as well.
    – Richard
    Jun 12, 2022 at 22:34

The refill tube goes into the overflow tube and that water fills the bowl after each flush.

However, the original poster is correct that a lot of water is wasted because once the bowl is filled with water to a certain line and if the tank isn't filled yet, the refill tube will keep flowing and the additional water added to the bowl via the overflow tube simply trickles goes down the siphon.

I guess they could add a feature to restrict the refill tube flow but that would add costs to the mechanism and make it more complicated.

What you can do is simply position the refill tube over the overflow tube so that some will go into the refill tube and some will go into the tank.


The overflow tube leads to the bowl. It prevents the tank from overflowing, by directing the extra water to the bowl.

Nothing prevents the bowl from overflowing, other than the fact that once the water level reaches a certain height the toilet flushes. If the toilet is clogged, the bowl can overflow.

If that tube didn't spray water in the overflow tube, there may or may not be enough water left after a flush to seal the passage in the bowl and prevent gas in the pipe from being released into the room.


FluidMaster sells a kit to replace a flush valve. One of the components that I had not seen before was a roller valve you slide onto the black tube that allows you to control the amount of water entering the bowl during the flush. I have high water pressure where I live and after flushing, water would drain out of the bowl for 15 seconds to get down to the proper level. To adjust the valve, you flush the toilet, wait till the excess water drains out of the bowl, use a marker to place a small line to mark the optimal water level on the bowl, then use the flow control valve on the (black bowl refill tube), to reduce the flow as needed to only fill the bowl to the optimal level. Fluidmaster sells this tube and valve separately at Home Depot Model # 215 (Store SKU #427596) for $2.99.


To add to some of the other answers, yes, the tube is designed to drip into the overflow tube.

However, if the spray tube goes too far down into the overflow tube, it can siphon water from the tank. It took a long time to realize this is what was happening to mine, as all components were new and functional. I just had to back the spray tube further up the pipe so that it would still drop properly. Fixed the siphoning and therefore eliminated the wasted filling.

  • I definitely need to learn this the hard way too. I wasn’t sure how the overflow tube worked. But after shortening the spray tube, it was perfect.
    – josephnvu
    Aug 9, 2021 at 14:56

1 water goes to the overflow tube thru the refill hose to refill the toilet after flushing , this is to keep sewer gas from entering the room and to prepare for the next flush. 2 At the same time water comes from a different part of the fill valve to refill the tank , all water flow stops when the float level is reached. Yes sometimes the refill tube adds more water than the bowl needs to reach its flushing point which is a waste of water, i would like to have a small valve on the refill hose so i could adjust the flow.

  • 1
    That's the point that hadn't occurred to me: there are TWO paths for the water to fill things. One for the bowl and one for the tank. I couldn't understand how the tank ever fills with the water going down the overflow into the bowl!
    – user98837
    Apr 12, 2020 at 19:11

After each flush the water replaces itself already in the syphon constructed down in the bowl, a so called "swan neck", so there is no need at all to put extra water in there. Or do you need an additional tube under your zink in the kitchen to prevent odor from the sewer in your kitchen?.... no! In my eyes its totaly sinless and designed to waste water. Not to mention the additional costs of the syphons and the rubber flaps which need to be replaced every so many years (2-4yrs), designed for ripoff. Measure once the amount of water what you waste putting in the sewer, you will be amazed. There are some manufacturers who sell to their new syphons for the tank little valves which you put on the rubber hose to reduce the waterflow in the overflow tube. I prefer putting a cablebinder over the looped hose to prevent wasting water or to put a 90 degree angle hose so that the water coming out of the hose goes into the tank. In several countries i`ve been you still find the traditional floater ball in the tank where there is no tube going nowhere in the overflow tube.

  • 1
    Hello, and welcome to Stack Exchange. This doesn't really make sense: a toilet's siphon is very different from a sink's P-trap, and the rubber flaps are for the flush action, not the refilling of the bowl. Sep 2, 2016 at 20:40
  • I have a toilet that's in the wall and I didn't put my refill tube into the overflow tube. It makes no sense for me to do so, there's always water in the toilet after the flush. Perhaps toilets in other countries need to refill the bowl with the refill tube? No idea why people downvote you, your answer is valid and makes sense to me. Nov 22, 2022 at 18:25

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