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Thread: hydronic radiant heating, wall area needed per sq.ft of floor space.

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    DIY Member rrekih's Avatar
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    Default hydronic radiant heating, wall area needed per sq.ft of floor space.

    I am toying with using a 2X6 wall for hydronic radiant heating some rooms.
    This wall runs down the middle of the house and will be used to heat both sides.
    This is in the basement with only 7ft head space, so underfloor is out.
    What amount of wall area is needed per sq.ft of floor space.
    I plan on using 2loops in each cavity.
    I am running 160deg. water temp.

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    In the trades Dana's Avatar
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    The heat load of the room is never calculated as function of it's square feet of floor area. In a basement it's a function of the square feet of exterior wall area above grade & R value, as well as the square feet of wall area below grade & R value, amount of air leakage, then the floor area and IT'S R value, etc. The heat loss/square foot of floor will vary widely- there is no magic ratio to use.

    Once you know how much heat you need to pump into the space, then you can work on how many square feet of radiant-wall it takes to put that amount of heat INTO the space. Radiant ceiling would be more comfortable though, and would give you a larger radating surface to deal with.

    Use an ACCA Manual-J type calc to come up with your heat load as a starting point.

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    DIY Member rrekih's Avatar
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    That's interesting about using the ceiling as the heat source.
    I have thought about that as well.
    Right now I am heating the upper floor with staple up radiant heat with a 1.5in to 2in gap to the reflective foil barrier.
    I plan to put roxul insulation under that, would it work to put another pipe system under the insulation for heating the space below from the ceiling?
    On either side of the middle wall the width is only 12ft. that is why I thought about using the wall space.
    The basement is pretty tight with spray foam on the foundation and box ends as well as batt insulation on the wall space.

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    In the trades Dana's Avatar
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    Rather than Roxul, 2" thick foil-faced iso board will give you ~R12-R13 separation between the first-floor staple-up and the basement ceiling radiant, which should give you plenty of room for spacing on the suspended-tube, and you don't have to mess around with more radiant barrier. (With less than R10 the zones would interact quite a bit.)

    It all starts with a heat loss calculation to actually design it, but if the first-floor staple up is keeping up with just the foil in place, a similar amount of suspended tube for the radiant ceiling will almost surely be enough with the basement load. They definitely need to be controlled as seperate zones however- it's all but impossible to balance first floor & basement radiation as a single zone since their heat loss characteristics are so different.

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    DIY Member rrekih's Avatar
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    Yes the foil only space is keeping up the heating needs, it takes awhile to come up to temp but once there it shuts down and keeps the temp for quite awhile before turning on again.
    Thanks for the idea of the Iso board, I will look into it.
    I will be separating the main floor from the basement, I have all the actuators for doing so.
    I am wondering about the fact that it is in the ceiling, will it radiate down properly?
    Or will it be hot at the ceiling and cold at the floor?

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    In the trades Dana's Avatar
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    Radiant ceilings are tried & true technology. If the floor slab is insulated, heat radiating from the ceiling will also raise the floor temp several degrees. (If it isn't, giving up an inch of headroom for even R2.5 1/2" XPS with 7/16" of OSB nailer deck above & a laminate/tile/carpet finish covering is worthwhile from a cruisin' in yer socks comfort point of view. If uninsulated, don't lay carpet down even with a vapor barrier, since the carpet itself is insulating enough that the bottom of the carpet is likely to be below the dew point of the room air much of the time.) With well-insulated floors (R10+) radiant ceiling rooms sometimes give the illusion of radiant floors. It's a very even and comfortable way to heat a space, since it doesn't induce convection currents in the room air the way radiators & radiant walls to. It's not as cushy in bare-feet as radiant floors, but it's not bad, not bad at all...

    With just the foil as insulation for the first floor radiant you are losing a LOT of heat to the room below. The foil has very low emissivity, true, but at the tubing temps you're talking more than half of the heat transfer is convective, not radiated. The foil is doing somewhat better than a sheet of poly or housewrap would, but not by huge margins. It's primary effect it blocking circulation of direct tubing & sub-floor contact with basement air currents. The performance difference is something like an R5-6 equivalent with the foil as opposed to ~ R3-4 equivalent with just an air barrier.

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