heating system, copper to cast iron

Discussion in 'HVAC Heating & Cooling' started by mnickson, May 5, 2009.

  1. mnickson

    mnickson New Member

    Apr 14, 2009
    I'm installing a cast iron air scoop on my boiler system, but the piping is copper. Do I need a dielectric union between the two? If so, can black iron be used between the union and the scoop? thanks
  2. Bill Arden

    Bill Arden Computer Programmer

    Sep 30, 2006
    computer programmer
    MN, USA
    That depends on a lot of other factors.

    There are cases where adding a dielectric union can make a problem worse.

    Here's some variables
    1. what's going to be in water that's in the pipe?
    The more conductive and the more acidic the water, more likely you will get "electro plating" type corrosion.

    2. Temperature difference between the copper to steel from the steel to copper joints? The two joints have to be looked at together since it's essentially a thermocouple that can create a voltage due to what's called the "work function"

    The problem is that just adding a dielectric union can leave a piece floating electrically and that can cause even more voltage across the dielectric union, which can then cause even more corrosion.

    more theory for those that are curious about how thermocouples and thermopiles(multiple thermocouples in series) work.
    Last edited: May 6, 2009
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  4. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    Heating systems typically, do not have electrolysis or corrosion problems which is why black steel is usually used and can be mixed with copper when necessary.
  5. plumbingskool

    plumbingskool Retired Tradesman

    Feb 25, 2008
    Not Sure ;-)

    I used to just screw the male adapters into the air scoops, we also used Teflon tape on the threads of the male adapters.

    Sweat the male adapters onto the copper pipe first and then let cool and then you can screw the male adapters into the air scoop.

    If you Google "boiler pictures" you will see TONS and TONS of boilers set up this way.

    That's how I always did it here in the north,

    Hope that helps.
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