toilet cistern overfills when bath tap on All aspects of plumbing questions and answers, help, tips and information

[f1steven]

i,m very confused ... my toilet cistern fills up to required level, but when i run the cold tap in the bath or have washing machine on it fills up more and overflows !!!!

[htg engineer]

what type of cistern is it ?

standard handle with ball valve ?
Or dual flush with float valve ?

Old or new ?

the flow of water when the taps are on is probably enough to force water through the valve to fill the toilet. When the water is standing the washer is probably doing it's job.

Hope this makes sense - get similar problems with combi boilers. When the cold water is running the diverter diaphragm moves slightly.

Try replacing ball valve washer/diaphragm in cold water feed part of the toilet cistern.

[f1steven]

cistern is 6 months old, dual flush type with a float valve on side of entry valve . Not sure how to get to washer on this type of valve !! surely it cant have worn already can it !! when bath or washer is on will pressure at toilet increase or decrease .

dont really wanna change all the inlet valve just yet . only as a last resort