I used a low flow toilet in an apartment that had been vacant for some time, and a toilet that had not seen much use. The bath in this area was never used. My recollection was that I flushed the toilet, and stayed around the unit for another 30 mins. without hearing or noticing an overflow. After leaving to the gym for 2 hours, I came back to find the apartment totally flooded to the extent that it flooded( or seeped to 2 I had heard gurgling in the past upon flushing. units below me). Only yesterday in my lease did I see mention that low flow toilets can overflow. Please provide me an explanation for this, and causes. Thank you.

  • I've revised your title to reflect what you're actually asking. However, there's nothing inherent in a low-flow toilet that makes it more likely to overflow. Something has clogged or malfunctioned.
    – isherwood
    Oct 31 '17 at 14:16

I have seen something similar happen in an area with hard water.

Over a period of time limescale builds up around the water inlet into the toilet cistern. This in turn leads to the water flow not shutting off completely as the ballcock rises. The limescale keeps the valve fractionally open, enough for water to keep flowing slowly.

Usually this water should just go down the overflow and not cause a flood.

If this is your problem, it is easily cleared by breaking up the limescale in the cistern.

  • Unless the tank is leaking or the main drain is clogged, a running toilet should drain and not overflow.
    – JPhi1618
    Oct 31 '17 at 15:12

One has nothing to do with the other.

For a toilet to overflow, either the operator has to flush more than the drain can take (presumably because it is blocked)...

... Or a double failure must occur: 1) the drain must be blocked. And 2) the inlet valve or flush valve on the toilet must be leaking at a higher rate than the drain can handle. One way is for the flush valve to be stuck, and the flow would be quite high and audible.

Low-flows tend to have trouble flushing, and that only makes work easier for the underlying plumbing. It's possible a toilet could have a blockage still within the trap of the toilet proper -- but that would be from recent immediate business, and my suggestion would be Do Not flush that type of object down toilets, or else your lo-flow is a terrible model.

If the flush valve has reseated properly, and the toilet fill valve has reached "full" level and shut off, then water cannot enter the tank and it is impossible for an overflow to continue after you have left. So one of those things must be defective. This being a separate problem from the pipe blockage.

Unfortunately people can be fairly oblivious to the functioning of their thrones, and do not detect slow leakage even when the signs are obvious when you look. The requisite parts are under $10 and will be paid for in a few months' water bills.

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