Have a PRV and wanted to know if I also should have an expansion tank. A plumber that was going to replace my hot water heater wants to sell me an expansion tank also for 175-225.00. Is this necessary? Thank you, Julie

Every hot water heater has a PRV that SHOULD open with over pressure or excessive temperature. However, they are known to clog with hard water and also open and flood basements-that's why they are vented outside or into a floor drain--but I have seen them jury rigged to vent inside. If your local water supplier has installed a backflow preventer on the water supply line or plans to do so, you need an expansion tank which has a bladder inside and equalizes the house pressure. If you are on a well you don't need one. The backflow preventer is a safety device to prevent possibly contaminated water from your house entering a public system if the line pressure should drop below your house pressure. The price you quote is not out of line with buying the tank, fittings and installation.

Hello Ma0641,

FYI, The pressure regulator valve (PRV) are installed to control the water pressure to your house, the temperature and Pressure valve (T&P) are installed at the water heater as a safety device to prevent the water heaters from explosion in case the pressure or the temperature exceeds inside the water heater, Thanks

John

Some PRV's don't require an expansion tank. Backflow preventer's do. The expansion tank with a PRV makes sense.
Here is an idea about parts cost: PlumbingSupply.com - Water heater thermal expansion tanks