Puzzling toilet leak

December 31, 2018 9:37 AM

Our toilet is leaking from the tank, at the flush valve seal, but only when the fill line faucet is on...

Our toilet is leaking from the tank, at the flush valve seal, but only when the fill line faucet is on...
I would think this seal would leak WHENEVER the tank is full of water, but no - it happens only if the tank is full of water AND if the fill line faucet is turned on EVEN IF the fill valve is closed due to the full tank of water.
I would understand this if it was leaking from the fill line connector - but it isn't: the leak spreads across the bottom of the tank from the flush valve seal, then runs down the fill line, before dripping onto the floor.
I seem to be faced with an impossible combination of facts. What am I missing?
Any thoughts as to why I see these symptoms, but no leak when the tank is simply full?

posted by blue_wardrobe to Home & Garden (9 answers total)

Best answer: Nothing comes to mind if the symptoms are exactly as you describe them; it would be believable if the toilet leaked from the flush valve when water was flowing through the fill valve, because water comes out of the valve down at the bottom of the tank (in most designs) and could be impinging on the seal, or otherwise creating turbulence. But if no water is entering the tank (and you're sure of it—perhaps the fill valve is leaking and letting water past?), that theory doesn't… hold water (sorry).

The other thing, which it sounds like you've considered, is that the water is actually coming from the incoming water supply somehow. And when you turn the fill line valve on, pressurizing the line and the fill valve itself, it would be reasonable if you started to see leakage.

It is possible, I suppose, that perhaps there is something going on in the porcelain of the tank itself. Perhaps the glazing has separated from the underlying ceramic, creating a thin channel from the fill valve area towards the flush valve, bypassing the fill valve. Or maybe the water has just saturated and is percolating through small fissures in the ceramic itself, under pressure. The more I think about this, the more I like it as a theory! However, most modern fill valves seal using a soft gasket on the inside of the tank, with a nut on the bottom/outside that just applies pressure to the seal which is on the inside/top, so the tank wall shouldn't be exposed to water pressure along the hole where the water enters. But if you have some less-common arrangement with the gasket/seal on the outside, it could perhaps happen.

Anyway, the steps I'd take would be to first replace the fill valve, because they are cheap and easy to do—no disassembly of the toilet required. Make sure you get one that seals from the top. I have recently done most of the toilets in my house, and would recommend the Korky "QuietFill Platinum", which costs a whopping ten bucks ($3 more than the cheap ones) and didn't leak at all. The Fluidmaster ones, which I'd used previously, seemed to start misbehaving after 5 years or so, which is more often than I want to replace the damn things.

If that doesn't do the job, then of course you have to replace the flush valve seal, which is a much more involved and obnoxious job. I think there are some products that basically "replace" the seal from the inside, without removing the toilet tank, by somehow putting a sleeve inside it... but I haven't used and can't vouch for them. Probably the right thing to do is remove the tank and replace all the seals and tank attachment bolts (and their gaskets/seals, if present) all in one go.
posted by Kadin2048 at 9:55 AM on December 31, 2018 [1 favorite]

I'd say it actually IS the fill line leaking, and it only appears that it's leaking from the flush valve. You haven't explained why you're convinced it's otherwise.
posted by humboldt32 at 10:10 AM on December 31, 2018

Agree with Kadin2048 on the Korky fill values. They are worth the extra $
posted by walkinginsunshine at 10:20 AM on December 31, 2018 [1 favorite]

I wonder whether it could be condensation on the bottom of the tank which then runs down the fill line.

The reason this would happen only when the fill valve is open under this scenario, is that you do have a leak inside the tank in the valve which is closed as the floater rises, which causes the water in the tank to be continually renewed, which in turn causes it to stay cold enough for perpetual condensation. My tank is unglazed on the bottom surface, which facilitates this kind of condensation.
posted by jamjam at 10:23 AM on December 31, 2018

Is the tank filling too much, over topping the overflow tube, and leaking as it drains to the tank? This would be sustained perpetually as long as the water is on, but once the water is off, it would stop as soon as the tank drains down below the top of the overflow tube. If the shutoff level is just exactly right (wrong?), you can get it so the water sort of sheets over the top of the overflow tube and constantly refills without ever fully shutting off, but isn't really noticeable at first. If the tank fill stops well below the top of the overflow tube, this is unlikely.

When you turn off the water, does the level in the tank remain constant over time, or do you have a slow leak through the flush valve?
posted by yuwtze at 10:40 AM on December 31, 2018

In editing, I confused myself and obscured the thrust of my last thought. I meant to ask if the level remains exactly the same after you shut the water off, or if it lowers even somewhat. If there's a slight change, that indicates the slow leak and perpetual refill scenario, even if we dont know where the leak is yet. If the level holds steady, that tends toward the idea that the leak is actually from the fill valve somehow (or that the leak is exactly at the water line).
posted by yuwtze at 11:03 AM on December 31, 2018 [1 favorite]

Response by poster: I am leaning towards the condensation idea. The reason I thought the leak is from the flush valve seal, is that it is near there where I first feel wetness after it has been dried completely. I will investigate the fill level which is suspiciously near the overflow lip at the moment.
posted by blue_wardrobe at 11:07 AM on December 31, 2018

Response by poster: AAANNDDD... a fresh check this morning, and it's the fill valve seal that is leaking. There must have been condensation forming near the flush valve.
I'll take the advice regarding the Korky Quietfill Platinum. Thanks!
posted by blue_wardrobe at 11:22 AM on December 31, 2018

Response by poster: Korky Quietfill Platinum fitted. Had to also change the fill connector which was copper, and it was slightly too long to let the valve seal seat properly. Swapped it for a flex hose.
So far, so good!
Thanks folks, and Happy New Year!
posted by blue_wardrobe at 4:27 PM on December 31, 2018 [1 favorite]