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"Flushing/Descaling" a Bosch Greenstar Combi 151 boiler (zwb 42-3)

GeorgeLI Member Posts: 3
Hi All,

My original installer sent two different techs to my home and offered an "annual routine flush/descale" of our boiler system for a lot of money. The problem is I got two different stories from the two techs that came out (at different times)

One tech said they only flush the water supply lines (circled in yellow, #1 & #5 in the diagram), but another tech said they also flush the heat supply lines (circled in red, #2 & #4 in the diagram).

Any input would be greatly appreciated. I do not want to spend many hundreds and have only half the job done, or worse, have the wrong lines flushed.

NOTE: this unit provides both hot water AND heat to the home. I think most systems have only hot water, and therefore only need that flushed. Mine has two systems, hence my question.

Thank you!


  • Leon82
    Leon82 Member Posts: 684
    Usually it just the domestic hot water that is flushed. Unless the heating side got plugged, but that would raise concern about a problem with the system
  • GeorgeLI
    GeorgeLI Member Posts: 3
    No, heating side is fine, their service is for just routine flushing. So thank you SO MUCH for the input Leon!

    Any other opinions?
  • SuperJ
    SuperJ Member Posts: 605
    edited December 2018
    You're better off to insist on documented water quality (TotalDissolvedSolids/conductivity, PH, etc.) and documented combustion analysis/adjustment than flushing and refilling the heating water side of the boiler. Adding new water to a system filled with good water something to avoid.
    The DHW side needs routine maintenance to avoid scaling up with minerals.
    The burner could be cleaned though.
  • GeorgeLI
    GeorgeLI Member Posts: 3
    OK, great. Then only a flush of the DHW then (for routine maintenance)

    Thanks for all your input Guys!
  • DZoro
    DZoro Member Posts: 1,048
    I would assume they cleaned the burner side of the system?? Flue analysis, and left the results with the boiler :)
  • hot_rod
    hot_rod Member Posts: 15,064
    The owners manual does give you guidelines for service interval and procedure. The warranty may be dependent on services being performed and documented
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream