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Cistern keeps making hissing noise & trying to fill up even though it’s full.

I had an issue with my cistern where my toilet wouldn’t flush. I fixed this issue by changing the SIAMP DIAPHRAGM WASHER. The toilet now fills up and flushes fine. However since changing the diaphragm washer my toilet keeps making a hissing noise and keeps trying to fill itself up even although it’s full. The hissing noise stops and starts. It happens every 5-10 mins.
Any help would be appreciated

Thank you

4 Answers from MyBuilder Plumbers

Best Answer

There could be a couple of problems either the new outlet valve you put in is passing water into the toilet easy way to check this is wipe the toilet pan clean with toilet paper and then try again if gets wet then its passing n not sealing properly this sounds the problem . The other issue could be you're inlet valve going and not stopping the water coming into the toilet cistern hope this helps


Answered 13th Oct 2020

Hi Sarah. Allegedly, the diaphragm is poorly fitting thus constantly letting by... just a dribble, therefore, the inlet valve is constantly topping up the water... just a dribble. Check by holding some tissue against and inside the toilet bowl rim and you will likely find it's running water... just a dribble. Good luck Bob.


Answered 13th Oct 2020

Hi, this sounds like the inlet valve is faulty and may need replacing.
This is where the water enters the cistern it has a float on it and sometimes with time this can break and need replacing the hissing noise comes from the valve not completely closing and stopping the water flow. These are cheap enough to replace but the type is dependant on your toilet style, close coupled bottom entry, or sometimes on a concealed toilet it will be a side entrg version, the height and size of this is also important to ensure it fits within your cistern.


Answered 13th Oct 2020

You may have got some grit in your water inlet valve but unfortunately they are not as simple to maintain as the older models, you could try flushing the cistern a few times and wriggle the ball valve flushing arm up and down in order to try and release the grit, but it may be easier to replace the inlet valve, they cost about a £10
Best of luck


Answered 13th Oct 2020

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