Toilet runs when tub or shower on

[mmax]

I am having a strange problem with my Toto toilet. If I flush the toilet and then turn on the shower/tub or sink faucet, the toilet will run until I turn off the other faucet. When I turn off the other faucet, the toilet immediately stops filling. The tank is filling and going in the overflow tube, not into the bowl. The toilet has to be running when I turn on the other faucet. If the toilet is not running, it will not start just by turning on the other faucet.

It happens whether I am running a shower, or filling the tub so doesn't appear to be related to the drain.

The toilet and sink are on old galvanized pipes, while the shower is on new pex lines.

My water pressure is high - there is no PRV. A couple of years ago I added a thermostatic mixing valve on the water heater in order to turn the temp way up.

Is it likely just a bad fill valve? Replacing it is the first thing I plan to try, but very curious why it would behave this way.

 

[Reach4]

My water pressure is high

How high?

 

[Sponsor]

 

[mmax]

How high?

80 psi

 

[Reach4]

80 psi

Is it likely just a bad fill valve?

I think so. Some are repairable. I am not a plumber.

When replacing the fill valve, people often replace the supply line too. They also review whether to upgrade the stop valve at the wall.

 

[Jadnashua]

Galvanized lines rust out from the inside. Depending, that can cause some rust particles to sneak places that don't do anything any favors. It may be that the seal inside of the toilet has some caught in/on those sealing surfaces, and thus leaks. Operating other valves in the home can mess with things.

But, if your static pressure is 80psi when you check it, it's possible that it could be going up quite a bit higher at night and early morning when the utility is refilling their water tanks and few people are using water. Most toilet fill valves should be able to handle 80psi, but that is the maximum code allows, and, when the seals start to wear and/or rust particles mess with them, they may start to leak at a lower value.

Changing the valve's seal, depending on the type, may be a cheaper, quicker option than replacing the whole thing, assuming it allows for that option. Otherwise, swapping it for a new one should help, at least temporarily. Most Korky and FluidMaster valves can quickly be taken apart and their seals replaced without tools in minutes. A picture of what's in your tank would help identify it. Water rising to the overflow tube is a classic fill valve leaking indication. That's either excessive pressure, or bad seal (or both) almost all the time. Sometimes, that takes a new valve to fix, sometimes, just a seal.

In addition to repiping the rest of the house, you'll probably want to seriously consider adding a PRV and an expansion tank. The new pipes will help dramatically with the flow, especially on the hot side, which, in galvanized, tends to rust quicker than the cold side.