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Old 01-11-2011, 05:29 AM   #1
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Shutting of running water make toilets run

Hi All,

Wow, I just joined here and it seems like I've asked a million questions already that I couldn't find answers for in a search. Hopefully I'll be able to put my newfound knowledge to use to help newer members in the future.

I have two low flow (6L/flush) toilets that both sometimes run for a few seconds after water has been running and shut off. It's like there is a momentary spike in water pressure that pushes past the fill valve or something.

It happens sometimes when turning off a tap quickly and all the time with the washer or the shower. It started happening when we replaced our upstairs toilet that constantly ran on so theoretically that would have tightened up the system I guess. There's no water hammer noise at all.

How can I stop this?



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Old 01-11-2011, 06:00 AM   #2
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What is your water pressure??? Inexpensive meters available at home/hardware stores. Pressure could be too high. Another possibility, toilet flapper valve is leaking lowering the tank water level, so that a slight surge is enough to cause the fill valve to open.


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Old 01-11-2011, 08:36 AM   #3
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i noticed this happening to me after i upgraded my lines through the house to 3/4" and then branched off to 1/2" for each fixture. if another fixture is on when the toilet is flushed and then after is turned off, the toilet appears to "top" itself off. the toilets in my house are at the end of the run, so not sure if that is why..

if you wanted to try to remedy this, you could adjust the float to be a little bit lower, this will shut it off sooner and possibly not need that last little top up.

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Old 01-11-2011, 09:04 AM   #4
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I'm far from an expert, but when I suspect a toilet is leaking down, I put a little cake coloring in the tank and watch to see if it makes its way into the bowl just sitting idle. If it does the flapper and or flapper seat have a defect of some sort. Another thing to look at is to see if the chain from the flush lever is too short or is catching on something and not allowing the flapper to seat tightly.
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Old 01-11-2011, 06:25 PM   #5
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Do the toilets run for a few seconds anyway every once in awhile when no one is using either the toilets or other faucets? That would mean that the flapper is leaking slowly to the bowl.

It is also possible that the flapper is not leaking but on the last flush, the fill valve shuts off completely but not all that tightly. A surge in the cold line forces open the fill valve and this time it shuts off a little more tightly.

The dye test mentioned further above would indicate which of the two scenarios just above is the case.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.
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Old 01-12-2011, 09:29 AM   #6
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As a few other posters have alluded but not said explicitly, this is a water hammer problem. In fact several posters here have had water hammer problems. One poster said his problems seemed to get worse when he upgraded to 3/4 inch pipe. That's pretty classic, if you know what you're looking for. Whereas previously friction through the long runs of 1/2" pipe (I presume) limited the flow to sinks and washers just slightly, now with 3/4" trunk pipe the flow is no longer limited by the pipes. More water is driven through the shower and washing machine valves. Although the water velocity through the 3/4" pipe may be lower, ultimately net force equals mass times acceleration. The mass is greater than before, and so too is the deceleration to which the mass is subject now that the valves are fully "in play". Upgrades like this frequently bring inadequate or non-existent water hammer prevention measures to light.

Although the mechanical advantage of the toilet float is great, there is nevertheless a small force exerted against it which is proportional to water pressure and which tends to keep the float more submerged that it otherwise would be without this force. Water hammer hits the float valve and "bounces" the float down for a moment: the valve opens. There is some hysterysis in the system, to keep this from happening due to very small variations in pressure: otherwise all of our toilets would do this all of the time. But this hysteresis also expresses itself as the toilet tank filling for a short time after the major disturbance, which is why the toilet tank does not stop filling instantaneously, despite the fact that the force to which the float valve was subjected is relatively instantaneous. Eventually, a new equalibrium is established: when the tank is so "overfilled" that the float regains the advantage over the pressure pulses, insulating the (toilet) system from further insults of the same magnitude. The annoying behavior stops until the toilet is flushed again. I bet if you sit there at the bathroom sink and bang the cold water faucet open and closed, eventually you will reach a point where the toilet no longer responds.

Unless, of course, the situation is so bad that tank water starts flowing into the overflow tube. If that's the case, the toilet may respond to high pressure insults indefinitely.

In general, cranking the float down won't help unless water is in fact spilling into the overflow tube. Regardless of the float's absolute position at the end of a flush, it is still caught in the balance of the same forces. It may work in some band-edge cases, by "tuning" the system ever so slightly that the symptoms disappear. But generally the toilet is not capable of influencing the forces allied against it.

Suggestions for water hammer arresting abound, including "whole house" arresters and arresters at each fixture. Ultimately you should pursue these types of solutions for the benefit of all of the appliances and plumbing system.

One thing I've never seen mentioned is the idea of an arrester at the toilet. This is because the flow through the float valve is limited in general, and in particular probably slows to a trickle immediately before stopping completely. Thus it is not a common cause of water hammer. However, I would be very curious about the possibility of limiting the insult to the float valve by installing an arrester at the toilet. If the system appears more or less properly "arrested" and the toilet is the last remaining nuisance, you might try an arrester at the toilet. I'd be very interested to know the results of such an experiment.

BTW, the reason I figured this all out some years ago was that the hot water valve in my washing machine eventually gave out and wouldn't pass water. The price of a replacement valve was quite shocking, and I got the impression it was a common problem. So I grabbed a highly regarded industrial solenoid valve from my inventory as a professional mad scientist. The valve was huge in comparison to the original, so I included a ball valve to limit the flow. One day some months later I started hearing water hammer. I figured my home-made arresters had finally filled with water. When I had installed them, I was clever enough to allow them to be easily and completely drained through the washer service valve. I was pretty incredulous when draining the arresters didn't solve the problem. Eventually I figured out that one of my renters wasn't happy with how long it was taking to fill the washer, and had opened my ball valve. I was tipped off by occasional overflows of the soap compartment. Once I cut the flow rate about in half, the water hammer dissapeared. It still seems strange to me, but despite the fact that the arrester was oversized and ideally placed, it was ultimately overwhelmed by the snap action of that electric valve when the water flow and hence velocity exceeded a certain threshold. I had always thought of water hammer as having a broad spectrum: present to some extent in all systems, but not troublesome in a well designed system. But my experience that day suggested this is not necessarily true. It indicated to me water hammer is a NOTICEABLE problem only if a certain threshold is exceeded somewhere in the system. You either have a noticeable problem with water hammer, or you don't. There's no in between.

I'd be interested in hearing thoughts and experiences related to this issue.

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Old 01-12-2011, 08:24 PM   #7
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Thanks for the help everyone. This wife doesn't have any food colouring right now so I'll do the "fill to the overflow and shut the supply off leak test" before looking at water hammer arrestors near both toilets.
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Old 01-12-2011, 11:07 PM   #8
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Pete, as long as there is no wasted water, the toilet that runs for a few seconds during a pressure spike is a good thing. Kind of like an automatic hammer arrestor. Mine does it. It's better than a bursted pipe.


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