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

Discussion in 'HVAC Heating & Cooling' started by rrekih, Jan 31, 2010.

  1. rrekih

    rrekih New Member

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    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.
  2. Dana

    Dana In the trades

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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.
  3. rrekih

    rrekih New Member

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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.
  4. Dana

    Dana In the trades

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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.
  5. rrekih

    rrekih New Member

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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?
  6. Dana

    Dana In the trades

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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.
  7. rrekih

    rrekih New Member

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    Location:
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    Thanks, lots of good info.
    I was thinking of putting down DriCore floor but I think your idea is better.
    Will probably use 1in of XPS and the OSB over top.
    The foil is for a radiant barrier that will be backed with Roxal insulation.
    So what I am thinking is having a sandwich type of thing within the joist space.
    They say a picture is worth a 1000 words, I don't feel like doing that much typing so I have attached a diagram.
    Let me know what you think.

    Attached Files:

  8. Dana

    Dana In the trades

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    Looks right. The space between the tubing and the foil should be at least 1cm (2 is better), and the R-value of the Roxul R12 minimum (R15+ is better)

    If the basement zone tubing is stapled directly to the joists the thermal bridging of the joist will increase the interaction between the zones. To minimize thes effect hang it with standoffs rather than staples to ensure at least some gap at suspension points, and rather than parallel straight runs in the joist bay have the tubing cross over between hangers.

    Without doing a heat loss analysis it's hard to say for sure, but you may be able to get away with 1 tube per bay on the basement zone, zig-zagged between the joists.
  9. rrekih

    rrekih New Member

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    Location:
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    That is one thought about snaking the tubing in the joist space.
    One thought I had was to run 2 pipes in one joist space but use only every second one.
    Good point about the thermal bridging though. I will probably come up with some kind of plastic standoff type of thing.
    I will do a heat loss calculation before doing anything.
    I know how to do the heat loss caluc. for the room but how do I figure how much output I will have with the pex running in the joist space?
  10. Dana

    Dana In the trades

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    There a professional design programs to run the numbers for you, but it'll be somewhat more heat than you're able to get through the sub-floor & finish floor above. The subfloor + finish floor puts about R1-R1.5 between the insulated cavity and room above, whereas with radiant ceiling you only have ~ R0.5-R0.75 between the warmed cavity and the room. Half the R-value delivers twice the heat flux when all else is equal...

    ...but it isn't quite equal.

    Reducing the amount of tubing by half doesn't cut the heat transfer by anything close to half, but the lower R-value of the gypsum is also somewhat offset by the lower amount of convective transfer at the gypsum-room interface as compared to what happens with a radiant floor, where the boundary layer of warmed air is constantly being displaced by cooler air sinking to the floor due to it's higher density. Most of this activity is happening at shin-level, though larger currents down glazed walls usually add some whole-room convection component.

    In very rough term half the tubing in a ceiling radiant suspended-tube would deliver at least 1/2 to 2/3 what the upper suspended tube is delivering to the first floor per square foot of ceiling or floor, and if there are rugs or furniture etc upthe radiant ceiling may exceed the total heat flux of the first floor. This isn't a simple thing to model with 3 equations in pencil on a napkin. But you probably don't need to.

    If the radiant floor above has been keeping up with it's calculated heat load at the water temp & flows you've been running, it has also been dumping as much as 1/3 of it's heat into the room below despite the foil insulation, so it's actually 30-50% or more overdesigned. Unless the basement room below has a much higher heat loss than the first floor room (not likely, but not unheard of) half the tubing in a ceiling radiant should be able to keep up. If your heat load calc comes in at something around half the BTU/square foot numbers (which it might, with minimal glazing and not much above-grade wall) it'll be at least as overdesigned as the radiant floor was. Odds are you have fewer obstructions to the ceiling too. (Have any furniture/rugs/cabinets/pets glued up there? :) )

    FWIW: You may already know this, but if you can lower the system water temps even 10F and still deliver the heat it'll save ~3% in fuel use. If you can lower them 20F it'll save ~5-6%. At 140F RETURN water from radiation and below you'd have to design in some return-water temperature safety if you have an oil boiler, but even down to 130F return water you can usually get away with it with gas cast iron, provided you have a condensing-tolerant flue liner. On cold-starts the heat exchangers will always be cold, but you want to ensure that oil HX reliably run above 140F, and gas above 130F for most of the burn to avoid damage to the boiler. 160F is/was a common design temp for baseboard radiation, but is on the high side for radiant in many/most applications (unless your outdoor design temp is -20F or something.) If you can get away with less, it'll usually pay off. Mixing valves, circulators & other controls that run a programmable temperature ramp based on outdoor temperature are available too, so you can run higher duty-cycle/lower temp water, which is easier on things in general, and more comfortable. (google "outdoor reset" )
  11. rrekih

    rrekih New Member

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    Location:
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    Just a quick reply.
    I will have to read your reply a few times to take it all in.
    I have dropped the water temp from 160deg to 140deg and it actually feels warmer overall. Let it run longer.
    I am not using a boiler but a tankless hot water system and a heating box connected to it. The tankless is fully modulating from 17,000 to 175,000btu's.
    I have found that the heat only over shoots the set thermostat temp by 1deg.
    Which I find acceptable.
  12. Dana

    Dana In the trades

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    The test will be whether it keeps up on VERY cold nighs at that temperature (which it might or might not.) But since it's easy to bump the water temps up/down with a tankless, manually adjusting it up or down on a monthly or seasonal basis isn't a big pain, just not fully automatic. Standard efficiency tankless will run anywhere from 80% to 85% combustion efficiency depending on the modulated level, but running longer fewer cycles at a 1% lower combustion efficiency is still more efficient multi-cycling it, and the room & floor temps are more constant, more comforatable.

    If this is a condensing tankless you'd do well to run the temps even lower. At 160F out, 140F return you're running no better than 85-86% efficiency with a condensing tankless, but if you can run it at 130F out with 110F return water you'll be in the low-mid 90s.:

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