I'm installing a water softener and expansion tank similar to the picture below. Does the expansion tank get installed before the water softener, or immediately before the HWT?

enter image description here

  • you want to keep the calc to minimum inside the expansion tank for prolonged life
    – Traveler
    Commented Jan 10, 2023 at 19:23
  • External salt storage tank? Interesting!
    – FreeMan
    Commented Jan 10, 2023 at 19:43
  • @FreeMan yes, they are common on water softeners. Lowes (among others) sells salt for water softeners in 40 lb bags. When I "stock up" for mine, I usually get about 400 lbs (10 bags). lowes.com/… Commented Jan 11, 2023 at 9:55
  • Yes, @GeorgeAnderson, I'm quite familiar with buying salt & filling the softener -30+ years experience with that. My "salt storage tank" is the concrete ledge near the softener. This drawing appears to have a secondary tank into which salt can be poured from the bag then, it either automatically feeds the salt into the softener or, perhaps, the "salt storage" tanks is also used for creating brine for the flush process. Maybe I'm misinterpreting the drawing, but I would never even think of drawing in the ledge & pile of bags as part of my plumbing system, so that's how I took it.
    – FreeMan
    Commented Jan 11, 2023 at 14:02
  • @FreeMan sorry if I offended you, your first comment made it seem like you weren't familiar with them, apparently you are, my apologizes. In my system the salt storage bin not only stores lots of salt, but as you mentioned, it creates the brine for the flushing process. Commented Jan 11, 2023 at 16:20

1 Answer 1


You want the expansion tank immediately before the cold water inlet to the water heater. The water in the tank expands due to heating and the hot supply side doesn't leave much room for expansion. Putting the tank on the cold supply side gives that water a place to expand before it causes backflow into the utility.

In your case, it will also protect the softener from hot water backflow.

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