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Pex Sizing Recommendations

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Mushroom Man, Sep 4, 2009.

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  1. Mushroom Man

    Mushroom Man Member

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    I am running Double - 3/4" PEX-AL PEX lines to the BARN. The ID of the 3/4" PEX-AL PEX lines is .807 (each) so the water carrying capacity of the double line is far in excess of my current needs. I am building with expansion in mind. I think we'll use spray foam insulation. The distance outside is roughly 200 ft (one way) but I'll have to go about 80 feet through the house. It's a long run. By the way, I chose Doubling the 3/4" Pex-AL Pex rather than 1.25" PEX because of a good sale price.

    Because my heat load is very low (10-11000 BTUs), due to some straw bale insulation, the flow doesn't have to be huge and therefore the head is manageable.

    Inside the barn, I plan to run pex piping along one wall in each of 2 rooms in a staple-up configuration. I am planning on using reflective bubble barrier behind the pex.

    What I need help with is this -
    What is the minimal size pex that I can get away with in the individual MushRooms ?
    Do I need another circ in the barn?
    Should I have a manifold in each room and run loops off the manifold or can I just loop back and forth in in one continuous loop in each room.
    The rooms measure 40'X12' and 16'X28'. The long wall in each room would support the staple up pex.

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  2. stee6043

    stee6043 Minister of Fire

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    Most folks use 1/2" Pex for staple up heating. Seems to be the standard (unless you have rather high demand compared to normal).

    I'd have a seperate circ in the house for the barn. Makes it easier to access in case of a problem (assuming the barn is a long walk in the middle of winter).

    You should design your loops to be witin 10% of the same length and most say a max of 300 feet each. Using these guilelines will tell you how many loops and how big of a manifold you'll need.
  3. stee6043

    stee6043 Minister of Fire

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    I just re-read your post. Are you suggesting you're going to heat these two rooms soley from pex tubing mounted on the walls? Hmmm....I'm not sure that's going to give you particularly good results. Others will likely chime in...
  4. pybyr

    pybyr Minister of Fire

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    lots of variables-- how well are the 'MushRooms" insulated (walls, ceilings, floor?)? How tall are the ceilings? What temperatures do you need to aim to maintain there?
  5. Mushroom Man

    Mushroom Man Member

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    Yes the idea is to mount the staple-up pex on the walls. Most people put it under the floor but I have read from credible sources that radiant works in walls, ceilings and floors.

    One MushRoom is very well insulated (R60 in ceiling, R52 in 2 walls and R22 in the other 2 walls). No windows. Steel insulated door.
    The other is a more conventional. R14 walls, R22 ceiling, no windows, wood door.
    Max temperature is 62*F and much of heat is produced by the decay process itself.
  6. in hot water

    in hot water New Member

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    radiant walls and ceilings are a great option. Actually, you can get more output from walls and ceilings as you can run them much warmer. no bare feet on ceilings and walls. here is a picture from I-dronics 6. It shows a good way to install the tube with aluminum transfer plates. sounds like you have plenty of r value behind it!

    Rule of thumb is .08 BTU/ hr per square foot for every degree difference between room temperature and wall temperature. So supplying 110F to the wall panel in a room of 70F would give you 32 btu/ sqft output. 110-70 X .08 = 32

    Ceilings work as well also, they warm very similar to radiant floors as radiant energy travels from warm to cold. It warms everything it can see, just like the sun. Except under tables :)

    Also notice the radiant under power viewed through an infrared camera.

    hr

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  7. stee6043

    stee6043 Minister of Fire

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    Huh...you learn something every day around here!
  8. Mushroom Man

    Mushroom Man Member

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    That is helpful "In hot Water". I expect to have 160* water at the barn. With a 62* room temperature, I should have 98* DeltaT.
    Using the formula you provided I'd expect 78 btus per square foot.

    If I cover the wall (less the door), I'd have (7'*24') or 168 sqft * 78 BTUs = 13Kbtus, well in excess of my requirement.

    Would it be reasonable to run fewer loops (i.e. cover less of the wall surface). For example maybe I'd start 1.5' above the floor and end 1.5' below the ceiling. That would provide 4'*24' or 96 sqft * 78 BTUs = 7.4 kbtus, much closer to my heat load but still 25% (roughly) leeway.
  9. in hot water

    in hot water New Member

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    some installers just do the bottom 4 feet of the wall, like shown. Plenty of output, it's inline with where you sit, and folks don't tend to hang pictures that low :)

    I think the sheetrock manufacturers like to see 140F as a max. temperature against sheet rock. Same with plywood and other sheet goods. Even with a 130F supply you should have plenty of output for that well insulated space. Poorly insulated, typical glassed, "average" homes have under 30 btu/ sq. ft./ hr. loads.

    Get a thermostatic mixing valve to reduce the supply temperature to the wall system, 120F may be plenty.

    But the aluminum transfer plates are critical, you need to conduct the heat from the tube wall and get it to the wall surface, so good conduction and a wide emitter works best.

    Some guys use the Roth panels or other brands as the tube holder and emitter plate, here is an example of the Roth 2X4 foot panels and end turns. It provided a solid aluminum layer under the sheetrock. Pex al Pex tube works nicely as it holds the shape better.

    hr

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