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How to run radiant heat zone off boiler's tankless coil

wolfcry911
wolfcry911 Member Posts: 5
in THE MAIN WALL
I have a small addition that I'd like to heat with under floor radiant heating - probably two loops approximately 250' each. My oil fired boiler has a tankless coil once used for domestic hot water, but that's being eliminated. Can I efficiently run the radiant off this tankless coil? And if so, how? TIA

Comments

  • HVACNUT
    HVACNUT Member Posts: 6,053
    Why in the world would you want to do that? Is it a steam boiler?
  • wolfcry911
    wolfcry911 Member Posts: 5
    no, it's forced hot water. I just thought it would be easier to have the radiant (lower temp) separate from the other zone.
  • HVACNUT
    HVACNUT Member Posts: 6,053
    Can you post pics of the boiler and piping? It shouldn't be too difficult to add a zone. The lower temperature would be from the mixing valve setting. The tankless coil temperature would be the same as the existing zone temperature. 
    Others know better than me on radiant design, and you didn't say what size loops, but 500 feet through a tankless that gives you 5 GPM, I don't know. 
  • MikeAmann
    MikeAmann Member Posts: 998
    From standard pipe tables 1/2" Copper type M Pipe: .0132 gal/ft. or 76 ft./gal.
    500 feet of pipe (you didn't say what size) holds 6.5+ gallons. And you plan to supply heat to this loop with a tankless coil of what capacity?
    I'm no expert either, but I don't think the math adds up to do this.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,092
    @wolfcry911

    Just take the new radiant zone off the boiler. You will need a mixing valve and a circulator as well as some controls.

    There is no benefit to using the tankless coil other than another part to fail and making things more complicated
    Zman
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,567
    edited November 2021
    If you do the zone off of a tankless coil, you will need to treat that a a separate closed boiler system. It will need a relief valve, expansion tank, feed valve, air purger, circulator, and a mixing valve to regulate the temperature to the lower water temperature.

    If you just add a zone to the existing closed system, all you need is a circulator with IFC, and mixing valve. Less stuff needed. The water feed, relief valve, air purger, and expansion tank are already there

    And what @HVACNUT, @EBEBRATT-Ed, and @MikeAmann said

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    MikeAmannZman
  • wolfcry911
    wolfcry911 Member Posts: 5
    Thanks for the advice guys. Looks like I'll abandon the tankless coil idea.

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