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Why does central heating water go black?

Discussion in 'Plumbing and Central Heating' started by pcsparky, 20 Dec 2008.

  1. pcsparky

    pcsparky

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    We has a new Vaillant combi boiler fitted 3 years ago and I watched the plumber completely flush the whole system until the water was running clear.

    I have just removed a radiator and emptied the contents noticing that the water was completely black but without sludge.

    So why does the water go black?

    And is there something that all householders should do on a regular basis to maintain their central heating system (other than an annual boiler service)?
     
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  3. swbjackson

    swbjackson

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    Corrosion. The black is magnetite suspended in the water. It is due to the flush not clearing all of the muck out, further corrosion or a combination of both.

    It doesn't necessarily mean that the flush wasn't done properly though. I recently spent 6 hours flushing a ten rad system and had it crystal clear from the flushing pump and at all drain offs. All radiators heated evenly and everything seemed fine. Two months later the customer bled a radiator and the water was dark brown again. When I returned to look at the job, if I hadn't flushed it myself, I would have doubted that the system had ever been flushed. I can only assume that the sludge was trapped in a pipe somewhere, possibly a deadleg. I drained it , fitted a magnaclean and re dosed it with cleaner. I'm due back there again on Tuesday to flush it again.

    I would suggest that the system is flushed again and a magnaclean fitted.

    Mike
     
  4. bengasman

    bengasman

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    As for the WHY, because the installer did a poor job, and whoever has done the servicing since, was not much better.
     
  5. namsag

    namsag

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    explain why ben on both counts?
     
  6. GPlumb

    GPlumb

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    Its possible that the installer did a fine job, added inhibitor and over those three years the inhibitor has been diluted by topping up. Maybe it was drained to fit another radiator in that time?

    Solution would be to run it with some x400 and then drain again 3-4 week later then refill with some inhibitor like x100.

    Magnaclean would be a good idea too.

    The black stuff is bits of your corroding radiators.
     
  7. bengasman

    bengasman

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    Why? simple.
    Installer clearly did not do the clean/add inhibitor routine properly.
    Any decent RGI should be aware how many cowboys out there don't clean systems properly and the problems this creates.
    New client for service: check for common cowboy problems.
    Litmus/dirty water check/ cold spot check is not that big a job or difficult to perform. I do it with every new client and also do basic ventilation, gasrate, electrical safety and that sort of little things that are often wrong, not to mention mechanical soundness.
    If you brought your car to the garage for a steering problem, and 2 months later your brakes would fail because they did not check them, you would not be impressed would you?
    My garage do a basic safety check with a service.
     
  8. gremlin16

    gremlin16

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    :eek: What on your heating system :LOL: :LOL: :LOL: :LOL:

    ;)
     
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  10. DP

    DP

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    If inhibitor was added, water would be brown in colour.

    If no inhibitor, water would go black because of oxygen content in fresh water. Constant topping up of system would make matters worst.

    I have seen well inhibited systems go from straw colour to dark coffee colour over time. Inhibitor concentrations being well, it still happens.

    Regardless of how well a system was flushed/ power flushed/ chemically cleansed, you will never remove all traces of magnetite from the system. Some magnetite always lurks in out of reach areas as base of rads and above the bottom rib that a powerflusher will not reach (check it yourself-cold water just runs along the base rib). Regardless of how many times a fill and drain is carried out, water will never be the same clarity as the tap water.

    Not every fitter/ installer or heating engineer is a cowboy. I have seen plenty so called pros making a hash of it. A DIYer at least does his/ her best.

    Those that like to bang their own drum, it must be great to be perfect and make no mistakes.

    Lately see plenty wanting to replace perfectly good boilers.
     
  11. Softus

    Softus

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    Severe corrosion, even when it happens fast, occurs over the course of several months.

    During that period, particles are sticking to every willing surface of every radiator, pipe, pump, valve, and fitting, so you can't hope to flush it all out during a single session lasting no more than a day.

    The only way to remove persistent Magnetite is to install a Magnaclean (or similar product), and the only way to remove persistent rust is to install a Magnaclean TwinTech (or similar).
     
  12. pcsparky

    pcsparky

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    Thanks for all the advice so far.

    A few weeks ago Vaillant had to replace the Heat Exchanger, which wasn't working correctly. I wonder whether this was due to magnetite. They have offered to come and check the water for free, which I will certainly take up. I've also had to add water to repressurise the system from time to time.

    This Magnaclean looks useful. I need to get the plumber to fit a new radiator in the bathroom and will ask him about fitting one as well as flushing the system again.

    How often should inhibitors be added to the system and which ones?
     
  13. Softus

    Softus

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    Whichever is recommended or approved by the boiler manufacturer.

    Fernox and Sentinel are the most commonly used brands.

    You don't need to add it routinely to a fault-free system, but many house-holders are unaware, or forget, that if water is repeatedly added to a pressurised system then it eventually dilutes the dose and negates its protective effect.
     
  14. bengasman

    bengasman

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    yep, i try to get the old boiler to bring the car to the garage when i am busy :D
     
  15. gremlin16

    gremlin16

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