Quote:
Originally Posted by bob47
If there was no backflow preventer in the water supply line, your water pressure would always remain at supply pressure. With a backflow preventer, as cold water expands when it is heated, it increases the pressure on the water heater and pipes in your house.
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Correct. Assume that you take several showers and use all of the hot water in the water heater, and then shut everything off in the house and leave for several hours. The water heater will come on and heat up the cold water in the tank causing thermal expansion and a significant increase in the water pressure throughout the house. But, if you have an expansion tank, the water can expand into the expansion tank and prevent the pressure increase. It only requires a small amount of thermal expansion of the water to create a significant pressure increase.