Dwight Barnett answers home-improvement questions. This week's topic is on a fixing a leaky toilet.

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Q: When we flush the toilet and the water fills up the tank, it makes a hissing noise; it also takes a long time for the water to fill up the tank. This is totally different from our other bathroom’s toilet, which works fine. What is the problem? How do we correct it?

A: The hissing noise is most likely a defective refill valve.

There are two valves inside the tank on the back of the toilet bowl. One is the flapper valve, located at the bottom of the tank, which allows water to enter the bowl. The other is a refill valve that automatically refills the tank each time the toilet is flushed.

The refill valve may be an older-style ball cock with a stem and a floating ball, a modern tower valve with the float attached to a plastic tower or even a tower with no moving visible parts.

In any case, the refill mechanism is blocked, creating a higher-than-normal water pressure inside the refill tube. As the high-pressure water passes through a restricted opening, it creates vibrations that you hear as hissing.

To replace the valve, turn off the water supply to the toilet either at a wall-mounted shut-off valve or at the house main. Flush the toilet to drain the tank, and then remove any excess water with a sponge.

Under the outside of the tank, you will find a large metal or plastic nut that secures the water-supply pipe to the toilet. Remove the nut, but do not damage the water pipe. There will be a second nut that secures the refill tower to the tank. Remove the second nut and the refill tower.

While you’re at it, remove the flapper valve and take both valves with you to the home store so you can find matching or similar parts.

After you have reinstalled the refill tower and flapper valve, reconnect the water-supply pipe. Now turn on the water supply, but do this slowly and check for leaks before turning the water to full force.

Another easy repair to consider is replacing the two screws and washers that secure the tank to the bowl. Over a period of time, the washers will fail and the tank will start to leak, so replacing these is preventive maintenance.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies.