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Sizing an Internal Expansion vessel???

Tomorrow I'm installing a Bosch ZBR 35 combi boiler for the first time. I'm just reading through the manual and Its says that that Internal Expansion vessel pre-charge pressure must equal the static system height of the boiler.

How exactly do I calculate whether or not  I would need to add an on another expansion tank?   I believe I need to find out how many gallons the system holds. Now this being a retrofit that might be tricky. Can anyone point me in the right direction here?


  • kcoppkcopp Posts: 1,843Member ✭✭✭
    edited July 2013
    If you doing....

    a simple system w/ a copper finn tube set up you are prob going to be fine. I might leave a threaded tee on the return to make a way in case you need it.

    You can add a bit of air to the extrol inside if you have abnormal height.
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