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

GeorgeLIGeorgeLI Posts: 3Member
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!


  • Leon82Leon82 Posts: 615Member
    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
  • GeorgeLIGeorgeLI Posts: 3Member
    No, heating side is fine, their service is for just routine flushing. So thank you SO MUCH for the input Leon!

    Any other opinions?
  • SuperJSuperJ Posts: 478Member
    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.
  • GeorgeLIGeorgeLI Posts: 3Member
    OK, great. Then only a flush of the DHW then (for routine maintenance)

    Thanks for all your input Guys!
  • DZoroDZoro Posts: 760Member
    I would assume they cleaned the burner side of the system?? Flue analysis, and left the results with the boiler :)
  • hot_rodhot_rod Posts: 11,439Member
    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
    The magic is in hydronics, and hydronics is in me
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