Toilet cistern constantly running into the bowl or overflowing outside? The usual causes are the fill valve, flush valve or overflow. Safe fixes and when to call us.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
A toilet that keeps running usually has a fill valve set too high, a worn seal in the flush valve, or an overflow set at the wrong height. Take the cistern lid off and watch what happens after a flush. Most of these can be adjusted, but a worn flush valve or fill valve usually needs replacing.
Water near electrics? Do not touch wet switches, sockets or light fittings. Switch off at the consumer unit only if you can reach it without standing in water. Water pouring in? Turn off the stopcock: find your stopcock →
What it could be and what to do
Fill valve set wrong You may be able to fix this
Likely cause: If the water level rises above the mark inside the cistern, it spills into the overflow and keeps running. HomeServe says the float on the fill valve controls the level.
What to do: Adjust the float or the screw on the fill valve so the water stops about 2 to 3 cm below the overflow. Flush and check again.
Worn flush valve seal Engineer needed
Likely cause: HomeServe says the flush mechanism at the bottom of the cistern can be damaged, worn or not connected properly, so water trickles into the bowl all the time.
What to do: Turn off the isolation valve on the pipe to the cistern, lift the lid and check the lever or button is connected. If the seal is worn, the valve or its washer needs replacing.
Overflow set too high You may be able to fix this
Likely cause: On toilets with an internal overflow tube, HomeServe says the tube can be set too high, so water spills into the bowl.
What to do: If the tube is telescopic, lower it. Water dripping from a pipe outside the house also means the cistern is overfilling.
Fill valve will not shut off Engineer needed
Likely cause: A worn washer or a bit of grit stops the fill valve closing.
What to do: Turn off the isolation valve and call us. The fill valve usually needs cleaning or replacing.
Safe checks you can do first
These are the checks we suggest before a plumber arrives. Stop if anything feels unsafe.
Stop the water: use the small isolation valve on the pipe to the fitting if there is one, or the main stopcock, usually under the kitchen sink. Stopcock finder →
Electrics: keep away from anything wet and electrical. Get an electrician to check before switching back on.
Is it just you? Ask neighbours. No water or brown water in the whole street is usually Scottish Water: 0800 0778 778, free, 24 hours.
Contain it: buckets, towels and moving belongings limit the damage. In a flat, warn the neighbour below.
Photos: take photos and video before you clear up, for your insurer or landlord.
Need a plumber now? Call us any time.
Our callout is from £75 in standard hours (out of hours £100–£180) for attendance and diagnosis. Any repair and parts are agreed with you before work starts.
Our emergency line is open 24 hours a day, 7 days a week.
Every property is different. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why does my toilet keep running?
Usually the fill valve is set too high, the flush valve seal is worn, or the overflow is set at the wrong height. Lift the cistern lid and watch where the water goes after a flush.
How do I stop my toilet running?
Turn off the small isolation valve on the pipe into the cistern with a flat screwdriver. If there is none, turn off the stopcock. Then adjust the float, or call a plumber to replace the worn part.
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