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Question: Why does my Toilet Fill up with Hot Water and is it bad for my Toilet?

Answer:

The main reason you may have hot water in your toilet is a bad flapper in the toilet tank or a bad fill valve. Either one of these can allow the water to seep into the tank. When you have a mixing valve on your toilet to prevent condensation, some hot water is introduced into the cold water line to warm the water just a little, but when the water just seeps a little at a time the hot water dominates the cold and you get a tank of hot water. It's not bad for your toilet, but it could be hazardous to your wallet. The hot water being heating takes gas and/or electricity and could run up your utility bill.