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We all know that in an indirect (closed loop) hydronic system, an expansion tank is needed ,to deal with thermal expansion of the circulating water, that can be either open expansion or closed expansion tank however I've mostly seen indirect solar water heating systems with closed expansion tank. Can we use open expansion tanks in this type of system?

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  • $\begingroup$ Can you accept evaporation? $\endgroup$
    – Solar Mike
    Mar 5 '20 at 11:46
  • $\begingroup$ evaporation is not an issue, the question is about system pressure and temperature. I mean does it have to be pressurized (closed expansion tank) or low pressure (open expansion tank) and how to choose the type and size of the expansion tank accordingly. $\endgroup$
    – moeinSj
    Mar 5 '20 at 13:28
  • $\begingroup$ Our solar water heating system uses a closed tank, because water pressure from the entire water system would push water out of an open one. Are you making allowances for ordinary line pressure? $\endgroup$
    – fred_dot_u
    Mar 5 '20 at 15:01
  • $\begingroup$ @fred_dot_u I'm not sure if i understood your comment correctly, what if you have used an bigger open expansion tank that could accept all the expanded water without any spill? $\endgroup$
    – moeinSj
    Mar 5 '20 at 19:23
  • $\begingroup$ Is your water source of a limited amount? My water source is a drinking water well, with a pump triggered on demand. If I have an open "expansion" tank, it's the same as having an open spigot. AH! You are dealing with a closed/limited resource, whereas mine is for other purposes. Disregard my error. $\endgroup$
    – fred_dot_u
    Mar 5 '20 at 20:02
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You can, however you cannot reach a temperature higher than the static head of the expansion tank. So you are probably limited to around 100C. For high efficiency collectors you might run into problems during stagnation. Corrosion is also a topic to think about.

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