Boiler system for hydronic heat - where to put check valves?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by ceiiinosssttuu, May 5, 2014.

  1. ceiiinosssttuu

    ceiiinosssttuu New Member

    Messages:
    11
    Location:
    Idaho
    I have a wood boiler with (2) 300 gallon hot water storage tanks. I use the hot water in the tanks to heat my radiant floor when the boiler is not firing. The boiler system has a loading unit (pump that circulates water from the boiler into the tanks and controls the return water temperature into the boiler), and a circulator (supply) pump to supply the hydronic manifold, which has a mixing valve to control the temperature of the water going into the floor.

    Here is the problem: (see photo below)
    When the supply pump is off but the boiler is firing and the loading unit is circulating hot water to the tanks, I notice hot water "backing up" into the return line of my manifold, through the mixing valve. Foolishly, I did not put a check valve in here when I built this system. My question is: where should I put check valves, what kind, and how many, to prevent hot water from backing up into the manifold instead of all going into the tanks? Ideally, I would put some kind of spring check in the supply line just upstream of the mixing valve, but would this stop the hot water that the loading unit is circulating, since it is going in the same direction? What about a gravity check - I didn't think this would be an option because it would be upside-down in this case. Is there a mixing valve the opens when it is "pulled" instead of "pushed"? Do I need to re-plumb my mixing valve altogether some other way?

    IMG_34309.jpg
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,255
    Location:
    Cave Creek, Arizona
    W do not need a photo of the manifolds, even if they look very nice. We need a photo of the piping to the tank. A "pumped" system is like a racetrack. The water ONLY goes where the loop takes it, so if it is going into the return line, SOMETHING is forcing it to go there. A check valve may cure the "symptom" but not the "disease".
  3. ceiiinosssttuu

    ceiiinosssttuu New Member

    Messages:
    11
    Location:
    Idaho
    Here is a plumbing schematic of the "as-built" system. Does anything jump out at you? Drewien Plumbing Schematic As-Built.jpg
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,255
    Location:
    Cave Creek, Arizona
    I would have to be there to actually test the piping, but IF I am correct, you are forcing the flow through the mixing valve to get to that tee. The mixing valve may be "modulating" allowing the pump to draw water from the lower side but that water would have to come by reverse flow through the manifolds, thus heating the return manifold. If so, a check valve between the manifold and mixing valve should work. The heating pump should be circulating through the mixing valve, NOT the system's hot water.
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