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flushing a Bosch ZBR 28-3A

dmauger Member Posts: 2
edited February 23 in Gas Heating

This post is regarding a natural gas boiler feeding a radiant floor heating system and an indirect fired water heater. It's 12 years old and given me no trouble at all. The only service done is simple cleaning (condensate trap and inside the heat exchanger) and replacing the ignitor and pressure valve as preventive maintenance. I was recently advised that it would be good to flush the boiler and was given a pricey estimate due to the way it was installed.

As shown in the picture, the installer made it relatively easy to isolate the boiler in case of major repair or replacement, but they didn't install drain valves that would make routine flushing easy. My internet reading leads me to believe that there are mixed opinions as to whether flushing the boiler is appropriate preventive maintenance. Makes me wonder if the installer intentionally omitted drain valves to discourage routine flushing. I note that flushing is not mentioned in the service manual section on regular maintenance.

In any case, flushing it now would necessitate one of three options: 1) use existing drain valves elsewhere in the system not near the boiler, or 2) install new drain valves adjacent to the boiler, or 3) isolate/disconnect the boiler and then flush through the disconnected ports. Option 1 results in replacing a lot more water from the system than is necessary. Option 2 is expensive but would make regular flushing easier in the future. Option 3 is cheap compared to Option 2 and would minimize the amount of water replaced compared to Option 1, but it wouldn't make sense to do regularly. My sense is, if I am going to flush it at all, that option 2 would be best, followed by option 3, while option 1 is the worst.

I'm looking for advice whether I should flush it now and, if so, should I flush it regularly in the future. The estimate for Option 2 is $, which I would be willing to pay if I could expect it to increase the lifespan by at least 4 years over what I can expect without flushing. The manual is silent on flushing (doesn't say that you or shouldn't) and my instinct is not to mess with a (mostly) closed system that has been working for 12 years.

Thanks very much!


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    edited February 23
    There are 3 valves that need to be closed to isolate the boiler for "Flushing" the heat exchanger wet side. I dont believe that this is necessary, however if you close those 3 valves and add three valves, there is a preminant fix for future service if it is deemed necessary.

    All I see is the three valves you need to close (you probably already knew that). And three valves to add.
    Then on the first flush, monitor what comes out of the boiler from this first flush after over 10 years. You will most likely see very little from inside the boiler. maybe just some grayish color but hardly any scale of crud of mud, that you would expect from a DHW wall hung boiler on an open system. That is the system that need flushing on a regular basis. Not the closed system that you have.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • dmauger
    dmauger Member Posts: 2
    hi, Ed
    Thanks for the feedback! Just to make sure I understand your bottom line. I get that you are saying I don't need regular flushing. Are you also saying that I shouldn't just leave it alone; that it would be worth investing the $500 for the upgrades to flush it now and then again every couple years in the future?
  • GGross
    GGross Member Posts: 1,043
    Generally it is not a good idea to add fresh water to a closed loop boiler system. The heat exchanger inside of these is a bit different than a typical mod/con so the manufacturer may have a different service plan, but generally you don't flush out the water side of a closed loop boiler because you will inadvertently add more minerals and possibly air to the system. Having drain points for filling and purging is a good idea in general though.