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I have a Baxi combi boiler and over the last 5 years it has had more problems than I can remember. We've had flow diverter valve failures about once a year, replaced both the hot water and main heat exchange plates, etc. It transpired this was due to 'sludge' in the system - the water was black - and we had the system professionally flushed (PowerFlush) but only 6 months later we are still getting problems.

My plumber has suggested there might be simply too much boiler damage and the whole thing needs replacing. But is this reasonable? I thought all the main parts could be replaced (and probably have been) so is it feasible the whole thing is simply 'worn out' due to systemic damage?

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Wow, you have a lot going here. I too have experienced multiple boiler problems as I have hydronic (in floor) heating. I have lousy water that has to go thru a water softener to be usable. That leaves a lot of salt in the water via the softening process. After cleaning/flushing the heating circuits several times, I finally brought in about 500 gallons of good water and flushed the system again. That was a couple of years ago and haven't had a problem since!

So, on to your question, you may have bad water, high in TDS (total dissolved solids), boilers don't like that. "Sludge" shouldn't be happening even with high TDS. You may have an anaerobic sulfur bacteria colony somewhere in your system. Nasty stuff. Running a diluted bleach solution thru the system, and letting it sit for a few hours, then flushing it might eradicate the problem for a while. You'll know if you have an infestation if the water comes out very dark or even black when you flush it.

I don't know how much your boiler is damaged, but I'd think it's OK, not sure, only you can determine that.

Have your water tested for TDS and sulfur. If it's bad, bring in some good water to fill your system. I presume you have some "make up" connections, but don't worry about that, a small amount of "bad" water won't hurt. Also consider boiler water treatments that could minimize your problems. Lots depends upon your actual boiler, your water quality, and any damage that might have occurred to the boiler.

Summarizing my thoughts:

  1. Test the water
  2. flush the system out with a bleach solution
  3. refill it with good water brought in somehow (I have access to a truck with a large tank, so it was easy for me)
  4. Based on your testing, develop an appropriate mix of treatments for the boiler water to prevent re-occurrence.

I'll put my money on you've got an anaerobic sulfur infestation that the diluted bleach solution will kill off and you'll get a ton of black water when you flush it.

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  • @mig Nice edit! appreciated! Really! Commented Nov 4, 2022 at 16:28
  • Makes it a bit easier to read :) Good post!
    – MiG
    Commented Nov 5, 2022 at 13:14
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I'm sure all the affected parts can be replaced, if parts are still available for that boiler. But there could be up to three heat exchangers that need replacing, and possibly other components such as vaves too.

If the original installer forgot to put in corrosion inhibitor, then other parts of the boiler could also be corroded internally, leading to leaks from all the joins.

It's then a question of whether it's cheaper to fix the boiler or replace it.

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  • Well the 'flush' should have prevented further corrosion but we still see issues. I think the plumber is frustrated he has to keep fixing the same parts but I am nervous a new boiler would be prone to the same issues if there is a problem in the system
    – Mr. Boy
    Commented Nov 5, 2022 at 0:59

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