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hot-water radiators on steam system

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tomwc
tomwc Member Posts: 6
in Strictly Steam
I have a steam system that the previous owner installed a hot-water loop for cast iron baseboard and toe-kick heaters on. Every time the hot water loop turns on (separate thermostat) the water level in the steam boiler drops and the boiler shuts down. I have read Don Holohan's discussion on piping a hot-water loop without a heat exchanger: the "water in the straw theory". The existing piping is not done as Don describes (not bypass line, and valves not in the right locations). I was going to have my plumber add piping to match the diagrams from Don, BUT THE QUESTION IS: I have 1/2" copper piping running to and from the boiler to the hot-water radiator loop - do I need 3/4" as Don illistrates, or will this system work with 1/2" copper piping? ANY help would be very appreciated. THANKS

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  • SWEI
    SWEI Member Posts: 7,356
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    Sounds like the loop could be draining when the pump shuts off?

    1/2" is pretty small. How many BTUs does the zone need? What are the supply and return temps when it's operating?
  • tomwc
    tomwc Member Posts: 6
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    Yes, the loop drains when the circulator stops.
    I'm guessing that the zones need between 40-46,000Btu's. Not sure of the temperatures of the supply & returns. The supply piping is currently off the 'mud' leg of the boiler.
  • SWEI
    SWEI Member Posts: 7,356
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    46k at a 20˚F ΔT is 4.6 GPM. 1/2" pipe is not big enough.
  • tomwc
    tomwc Member Posts: 6
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    The 1/2" piping is what I'm stuck with, in the existing construction. If I figure 20 deg. delta, then I'm down to +/19K. Will the 1/2" piping hold the water in the piping under atmospheric pressure, so it does not drain down into the boiler when the circulator stops?
  • SWEI
    SWEI Member Posts: 7,356
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    Is this a series loop? Any chance you can split the loop?

    I need to defer to one of our steam experts on the water level question.
  • tomwc
    tomwc Member Posts: 6
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    The loop is separate from the steam, but the radiator loop is a series.
    rrwitherspoon
  • SWEI
    SWEI Member Posts: 7,356
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    Can you break into the loop somewhere in the middle and run a 3/4" line back to the boiler?
  • tomwc
    tomwc Member Posts: 6
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    In order to make it 2 loops, and install another circulator?
  • SWEI
    SWEI Member Posts: 7,356
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    Split the loop, feed from the middle and return both ends in parallel. Look here and scroll down about half way.
  • tomwc
    tomwc Member Posts: 6
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    THANKS

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