Toilet Cistern Running or Leaking? Causes and Fixes
A cistern that keeps running may seem minor, but it wastes a lot of water and raises the bill without you noticing. Usually the cause is a part inside the cistern, like the float or the flush seal, and it can normally be fixed without replacing the toilet.
Common causes
Worn flush seal
If the seal that closes the flush valve is hard or worn, water leaks continuously from the cistern into the bowl, and you may hear the cistern refilling every so often.
Float or fill valve
If the float is set too high or the fill valve does not shut off, water keeps coming in and runs out through the overflow.
Flush button or lever
Sometimes the button sticks or the lever or chain catches, leaving the flush partly open.
External leak
Water around the base of the toilet or under the cistern may come from the fill hose or the seal between the toilet and the drain. This needs checking sooner because it can damage the floor.
A simple check you can do
- Carefully lift the cistern lid and see whether water reaches the overflow pipe.
- Press the flush button and make sure it returns fully.
- Add a few drops of food colouring to the cistern without flushing. If colour appears in the bowl after a few minutes, the flush seal is leaking.
- If the leak is external, close the small valve behind the toilet.
When do you need a plumber?
- Water keeps running after adjusting the float.
- A leak around the base of the toilet or a sewage smell.
- A concealed in-wall cistern that needs the button plate opened.
- The toilet rocks or is not secure on the floor.
A plumber identifies the worn part and replaces it, or re-fits the toilet and its seal when needed. See our bathroom plumbing and fixtures and our article on bathroom leaks.
Frequently asked questions
Does a running cistern really raise the bill?
Yes, because the leak runs all day even if it is small. It is worth fixing early.
Can a concealed in-wall cistern be repaired?
Usually yes, through the button plate opening without breaking, depending on the cistern type.
Does the whole toilet need replacing?
Rarely. Most problems are internal parts that are easy to replace. Replacement is for a cracked bowl or cistern.
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