Read a post here concerning the water line back flow preventer/expansion tank and am familiar with its operation and purpose when using city water. My question is, is this setup recommended or even needed on a well water system? My feeling is that the holding tank would take up any expansion. However, if the pump/tank shuts off and stays at its maximum (60 lbs) could the tank still absorb any expansion? My old water heater never released water via the T/P pipe and my new one hasn’t either, so if there is any expansion it is going somewhere. I installed the heater and all new copper piping so I could easily retro the preventer/tank if needed…. Thanks

Your pressure tank is likely rated for far more than the 60 psi cut off. As long as the water forced out of the tank is free to run back to it, it should be fine. That is the way it has always been. No problems until the municipal water systems decided to require back flow preventers. Suddenly the water had to have some place to go. The volume is very small. Very little water is forced out the relief valves. Certainly not enough to change the pressure much in a large pressure tank. Good question though.

Hey Shader, Back again I see.

Pump systems have always had a backflow preventer, (check valve) installed so the pressure tank retains pressure in the system and the pump doesn't lose its prime. Both you and Labman are correct when you assume the pressure/bladder tank compensates for the hot water expansion. Expansion tanks are needed only when back flow preventers are installled at the water meter. It's nice to see Labman back in action. He was missed by a lot of people here while he was on "vacation". Welcome back Lab! Tom

Yeah, I never left, HAHA... your responses and that of labman have been very helpful during my house's almost extreme makeover! I know a fair amount about plumbing but not all the real insider stuff you guys learned over the years...

A couple of details. Tom is the real plumber. I poke around the edges and help sometimes. A couple of years ago, it took us both when we started here on those expansion tanks. It seems now when we have an easy answer, we see fewer such questions.

Yes, well systems have back flow preventors too, as far as I know, always at the bottom of the well. The pressure tank works as an oversize expansion tank between the backflow preventor and the hot water tank, easily absorbing the small volume of water forced out as the water in the tank heats and expands. .