Quote:
Originally Posted by retiredguy123
Having low water pressure has very little to do with needing an expansion tank. Here is why you need one. Suppose you use all of the hot water in the water heater, taking showers, washing clothes, etc. Then, you close off all faucets, toilets, etc. in the house and leave the house for a few hours. Now, you have a closed system and the water heater is full of cold water. The water heater will come on to heat up the water in the tank. As it heats up, the water will expand, but it has nowhere to go, so the pressure can greatly increase due to the thermal expansion. The function of the expansion tank is to prevent this rapid rise in water pressure by allowing the water to expand the diaphragm in the expansion tank and reduce the water pressure.
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It's only a closed system if there's a backflow preventer between the municipal supply and your house. Ours does not have one. Ours only becomes a closed system when we shut the main supply valve in the garage.