Where should the expansion tank be installed? On the primary side or on the secondary side? Both sides perhaps?

  • x1 ?
  • x2 ?
  • x3 ?
  • or x4 ?

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  • As I understand expansion tanks, they are for hot water tank pressure expansion, so close/on the tank. Your #1.
    – crip659
    Feb 10, 2023 at 0:54
  • @crip659 My understanding is that they buffer back-pressure on the cold water input to the heater, so would go in position 4; that also prevents accelerated failure of the rubber membrane inside the expansion tank due to direct hot water exposure.
    – Armand
    Feb 10, 2023 at 2:37
  • 2
    Two different applications, though in point of fact the one crip659 is thinking of is also better served by having it on the cold side of the (domestic out of the taps) water heater tank (so long as there's no valve between the water heater and the expansion tank.)
    – Ecnerwal
    Feb 10, 2023 at 3:20

1 Answer 1


If this somewhat confusing due to local terminology not disambiguated question is about a circulating heating loop, the expansion tank goes as close to the inlet of the (unseen) circulating pump as is practical. That makes the risk of cavitation less, and cavitation is Not Good for pumps.

Assuming the circulating pump is contained in the white and black unlabeled thing that's presumably a boiler with streamlined housing, because that's a terribly useful thing for a boiler to have, as they often travel at speeds of 100 kilometers an hour while sitting in your house, x4

  • yes, you are right. black&white box contains a pump inside located after x4 point. the box also contains an expansion tank (also after x4 point and before the pump) but this expansion tank is small/insufficient (based on the calculation) so that's why I need an additional one - just wasn't sure where to put it
    – servant0
    Feb 10, 2023 at 1:21

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