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Thread: boiler manifold

  1. #1
    In the Trades mattbee24's Avatar
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    Default boiler manifold

    I was just wondering if you guys have a preference on manifolds used for floor heating systems. We usually just make up manifolds, but in this case, the customer has 17 supply and return loops all on one zone. All the loops are approx. 300ft runs of 1/2" pex pipe in a concrete floor. I would like to set him up with an 1-1/4" feed with 1/2" branches. Also, I would like to have balancing valves and shut-off valves on each loop. I have axcess to Watts, Vanguard, Sioux Chief and B&G brand items, but I am willing to look elsewhere if you guys think there is a better mousetrap out there. Any info would be greatly appreciated.

  2. #2
    In the Trades Bob NH's Avatar
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    You should determine how much water is flowing in the zone and in each PEX run, the head loss in the PEX, and the temperature of the outgoing and return fluid.

    There will be a large variation in heating in the zone if the temperature of the return water is very much less than the outgoing water. 300 ft is the maximum recommended length for 1/2" PEX in a slab heating system.

    If the runs of PEX are approximately equal, the pressure loss in the PEX will be so much greater than anything else in the system that the flows will be pretty well balanced as installed.

    How many BTUs per hour to be delivered?
    What temperature difference between outgoing and return fluid?
    Then calculate the required GPM in the zone and in each loop.

    From the total GPM in the zone, calculate size of manifold. I would keep the velocity in the manifold not more than about 5 feet per second. The velocity head in both manifolds shouldn't exceed 10% of the head loss in the PEX to keep it from affecting uniformity of distribution.

    From the flow in the PEX runs calculate the head loss in the PEX. The head loss in each run will be the same; they don't add.

    Then add the head loss through the boiler and manifolds to determine the required head for the pump. Add 25 percent if you really want to try to balance each loop, which shouldn't be necessary if the loops are all about the same length and the loss in the PEX is more than about 60% of the total head loss in the loop.

  3. #3
    In the Trades mattbee24's Avatar
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    Thanks Bob,
    I guess I should have asked this in a different manner. I am just looking for recommended brands of manifolds. Ones that the different plumbers here have used and were happy with. But it is good to know all the different factors involved. I have never got that in-depth with the systems themselves. I am not a plumber, just a salesman at a wholesale warehouse. I was always under the assumption that if the loops were balanced, and the delta t was 10-20 degrees, that everything was fine.

  4. #4
    In the Trades Bob NH's Avatar
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    Quote Originally Posted by mattbee24
    I was always under the assumption that if the loops were balanced, and the delta t was 10-20 degrees, that everything was fine.
    That's correct. The other part is what must be done to get them balanced and actually get 10 to 20 degree delta-temp.

    For example, to deliver 100,000 BTUs per hour through the system at 15 degrees delta-temp, you need 14 GPM of water flow (a little more if glycol). Therefore, the pump needs enough head to put 14/17=0.82 GPM through each 300 ft loop of 1/2" PEX + all of the losses in the boiler, manifolds, and valves in each part of the line.

  5. #5
    In the Trades mattbee24's Avatar
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    great info!
    Is there a chart somewhere online that I could get all the formulas?

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