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Recently, I got an exceptionally high bill for water consumption. I went to check my water meter, and saw that it rotates in a pace of about 200 cc per minute, even though all the faucets in my home are closed. So, I concluded that there is probably a hidden leakage somewhere in my home.

But then I did the following experiment: I closed the main faucet (the one just next to the water meter). The water meter immediately stopped rotating. I waited for 1 minute, and then opened the main faucet again. At that second, the water meter rotated very fast - it made about 400 cc in a second! After that second, it returned to its previous pace of 200 cc per minute.

Is there a physical explanation for this phoenomenon?

(Some details that may be relevant: I live in the 2nd floor in a 3-floor house. We get our water from the urban pool, located at the top of the hill in the center of town).

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up vote 5 down vote accepted

Yes, you have a leak. The fact that there is no obvious pool of water somewhere means that the leaking water is going down the drain.

The most likely culprit is the plunger at the bottom of the toilet tank. If it leaked a little, the float valve would be open just a bit on average. In effect, the toilet bowls would be constantly "filling", but doing it so slowly that you might not notice and so there wouldn't be any gratuitous flushes. This also explains the fast water flow after having been off for a while. While the main valve was off, the toilet tank got drained a bit. When the water got turned back on again, the float valve was well open because the tank level was low.

To test this theory, close the shutoff valve for the toilet and see if the slow drain shown by the meter stops.

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+ That also explains the transient high flow when turning the main valve back on. –  Mike Dunlavey Sep 9 '13 at 0:42
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The above explanation is also the reason that newer float valves do not open "slightly" to quietly waste water. Rather, they are designed to come on, full blast, when the water in the tank drops significantly, and go off when the thank is refilled, thus alerting people to the leak. –  User58220 Sep 9 '13 at 0:50
    
Thanks Olin, you were absolutely right about the leak in the toilet tank! I replaced the leaking parts, and now the water meter does not rotate at all. But... the strange phoenomenon I described in my question has not gone away. Now, when I close the main faucet for a minute, and open it again, the water meter rotates about 100 cc in a second, then stops completely. How can this be? –  Erel Segal Halevi Sep 9 '13 at 16:51
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@Erel: 100 ml is a very small volume compared to the volume of water in the entire supply side of your plumbing system. Something isn't perfectly rigid; it doesn't take much. When you shut off the feed, the pressure drains slightly somewhere, which allows something (a rubber hose?) to contract. The 100 ml is what it takes to get your system back up to pressure. –  Olin Lathrop Sep 9 '13 at 22:21
    
@OlinLathrop: I don't understand. Can you please elaborate, how the 100 ml effects the pressure? (Note that this happens over and over again - whenever I shut off the feed for a minute, 100 ml is wasted). –  Erel Segal Halevi Sep 10 '13 at 17:20

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