Discussion in 'Plumbers' Talk' started by strappy, Jul 15, 2013.

  1. strappy

    strappy New Member

    Evening all, many years ago I was a heating engineer,in the last couple of years I have been asked to sort out a few problems for mates at work and have now been asked to sort out the big boss mans heating system,basically he had a fully pumped system and had a woodburner boiler and neutraliser fitted, the 28mm gravity pipework on the woodburner is like nothing you've ever seen and has never worked, I'm ripping it all out and starting again.
    As per the rules/regs I am going to fit a Galvanised f&e tank with a copper float but what about electrolysis, from memory the only thing I can remember is that if you had a Galv calorifier with a copper secondary flow and return you would get the electrolytic action and another thing I remember hearing was that as long as you had a brass fitting between the Galv and copper it would be alright, I've tried finding out but there's so much c-ap on the net I don't know what to believe, also the neutraliser is Galv so it's a bit late to worry,I'm not sure if inhibitor would do any good, any advice would be appreciated,thanks
  2. gsmann

    gsmann New Member

    we used to use fibre nipples between galvanised and copper or brass fittings.you need a joint that seperates the metals electrically.
    anything with zinc in it will fail due to electrolysis
  3. Onetap

    Onetap New Member

    Ignore all that.
    There are insulating unions available, they seem to be popular in the US ("dielectric unions"), but SFAIK the earth bonding will by-pass the insulators and make them ineffective.

    You usually cannot avoid a mix of metals in a heating system, the steel tank is no different to steel radiators. The zinc coating is there to prevent unsightly rusting and the zinc, being most anodic/least noble, will act as a sacrificial anode, inhibiting corrosion of the steel until all the zinc has gone.

    For galvanic corrosion to occur, you need two dissimilar metals and a suitable electrolyte, usually acidic.

      The best way of preventing galvanic corrosion is to make certain the water cannot act as an electrolyte.
    Flush out and neutralise all flux residues from active fluxes (or just use passive flux), treat the water with corrosion inhibitors and ensure it doesn't boil over or pump over.

    You can get sacrificial anodes (magnesium alloy) for use in steel water storage tanks, I don't know if they'd work in a heating system or whether the corrosion products would be detrimental to the system.
  4. Onetap

    Onetap New Member

    PS   Forgot to say.
    The dielectric union things are used on water heaters, water supply systems, to which you obviously cannot add inhibitors.
    In the UK steel or stainless water heaters used to have sacrificial anodes fitted that preserved the tank. these were (I beieve) banned by the Water Regulations because the corrosion products wound up in the potable water.
    Such water heaters usually now come with electric  corrosion prevention device (Pro-tech? or similar).

    On a closed heating system (i.e., not a water supply system), you don't need them because you can break the electric circuit by treating the water with corrosion inhibitors to make it non-conductive.
  5. teabreak

    teabreak Screwfix Select

    Just curious, do you know why the rules say a galvanised F&E tank?  Is there some possibility of it getting hotter than a normal set up perhaps?
  6. Onetap

    Onetap New Member

    Yes, a solid fuel boiler is more likely to boil over and discharge steam or boiling water into the F&E tank, because there's no over-heat thermostat that can cut off the fuel supply.

    The BS plastic F&E tanks will take boiling water if installed correctly, the few water storage tanks that collapsed (defective immersion heaters) hadn't been supported properly.
    I don't think the usual plastic valve floats will withstand boiling water.
    I'm not certain what the HETAS requirements unless I look it up.
  7. teabreak

    teabreak Screwfix Select

    Thanks Onetap:)

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