If I remember correctly, adding water to the back of the toilet storage tank will cause the level in the toilet to rise(after the next flush).

I have a dehumidifier now, and am wondering about pouring the water it collects into my toilet's storage tank. Not only would it save on water usage, but would reduce the stress on my septic tank. The amount of water it collects is probably worth a couple flushes a day. Admittedly it's pretty minor, but I can locate the dehumidifier near the bathroom and it becomes no more difficult to empty it into the toilet than it is to empty it directly into a drain.

I can either manually pour it into the tank, or setup a drain hose to automatically empty into the reservoir.

  • What will happen if I overfill the reservoir, either by pouring too much into it or via an automatic drain hose that might pump too much water?
  • Can I adjust the bulb in the toilet in such a way that it fills the reservoir significantly less, so that it uses less water after a flush, since I will be replacing some of that with grey water? Or is it not possible to make such an adjustment without screwing up the normal operation of the toilet?

Note there are two concerns to overfilling:

  1. Could the reservoir overflow and flood out of the top of the reservoir and onto the floor? (with a continuous drain hose from humidifier)
  2. Could the reservoir fill to a point where it does not overflow, but on the next flush it overflows the toilet? Sometimes my toilet clogs but it doesn't overflow as long as you only flush it once and clear the clog before flushing again. I want to be sure the reservoirs are designed such that it's impossible to fill them to a point where the next flush would overflow the toilet.

I think one of the vertical pipes in the toilet is open on top to allow overflow to drain out. Will this prevent both of the potential scenarios?

Note, this information would be useful to others who might want to create a DIY grey water reuse scenario that is similar to the continuous drain scenario. Perhaps having the drain from a second story sink feed into the reservoir of the first story toilet.

  • What kind of toilet do you have? Is the reservoir in a box directly behind or build into the wall?
    – Julian
    Commented Apr 30, 2015 at 7:02
  • Directly behind
    – AaronLS
    Commented May 4, 2015 at 15:22

2 Answers 2


Normally, the reservoir has an overflow pipe that would prevent both your concerns. The reservoir is set up so it normally fills to just below the level of the overflow, so you would have to adjust the bulb to get enough room for your dehumidifier water.
The adjustment range depends on the exact design of the reservoir.


I found it hard to find a good picture of a reservoir. This one from wikipedia isn't great but it will suit.

Toilet reservoir source

Most toilet reservoirs look similar to this. The actual parts may vary but the function should be the same.

You have a swimmer (1) which will "tell" the toilet when it's full, by floating on the water, releasing the pressure knob 11 when the favoured water level (10) is reached and thus stopping the filing of the toilet.

If you drain the toilet by flushing (8) the stopper 6 opens to let the water urn out. With this, the swimmer goes don, pressing on the button (11) and refilling the toilet. If you put extra water into the reservoir it will fill, until it reaches the opening next 5. Any more water will drain through 7 into the toilet.

Therefor in a standard toilet, there is no risk in overfilling the reservoir.

BUT: Please note, that water tank from different sources can be filled with a lot of minerals (not in the case of a dehumidifier, since it's nearly distilled water) or microbiological contaminations. These can grow in your reservoir, producing a biofilm which will start smelling/ be unhygienic etc. So please have an eye on the cleaning schedule.

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