Heat transfer plates, Stamped or extruded or none

Discussion in 'HVAC Heating & Cooling' started by Supervisor, Mar 14, 2012.

  1. Supervisor

    Supervisor Construction Supervisor

    Messages:
    7
    Location:
    Lunenburg ma
    I am researching "RADIANT HEATING" as much as I possibly can online, and I am just plain confused at this point, some schools of thought are you have to use extruded plates= thick fast heat transfer and Very expensive, some say use stamped=cheaper and it is not that much slower, I just got off of a link

    http://www.radiantdesigninstitute.com/page11.html

    from a reputable radiant designer that seem to state that they are not needed at all ( my uncle has one and doesnt seem to have any issues) I am looking for feedback, I have searched through previous threads and have not seen anything specifically asking this...if I just missed it please point me to that thread.

    thanks for your help,

    Chris
    Last edited: Mar 14, 2012
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    I'm assuming since you're talking about transfer plates that this will be in a wooden subfloor?

    Wood isn't a great conductor. You'll have hotter/cooler strips in the floor if you don't use the heat transfer plates and likely have to run higher temperatures. Cooler supply temps can mean more opportunity for condensing at the boiler which is a good thing, if it is designed for it, which is more efficient. Can you have radiant heat without it? Certainly. I've got mine embedded in the floor with a radiant foil sheet and air gap underneath. I think if I was going to do this again, I'd probably use the heat transfer plates, but mine works.

    This is all thinking about a low-mass floor. Now, if you're talking about radiant in a slab, that's a different thing...just the pex burried in the slab with adequate insulation beneath so you aren't trying to heat the earth works great. One room has tile, and I embedded the pex in slc. That bathroom, between two bedrooms, tends to keep both of them warm with no additional heat, and, at least in the winter, stepping out of the shower onto the nice warm floor is very pleasant!
  3. Supervisor

    Supervisor Construction Supervisor

    Messages:
    7
    Location:
    Lunenburg ma
    Thanks Jim,

    Yes this is for the portion of the floor that has a plywood sub floor and hardwood over it, I do have the Pex already in the basement slab, The cost of the extruded plates are 4-5K for my project and if is questionable whether or not they are relevant, I would like to understand that before the investment,

    Most of what I read is that the floor temps (120-130 F under floor) to acheive (85 F above) should be somewhat constant, not using a programmable t-stat to allow temps to range when using radiant, if that is the case doesn't the need for "rapid response" dimish if your only talking about a 1-2 degree swing?

    Thanks Again Chris
  4. Dana

    Dana In the trades

    Messages:
    2,974
    Location:
    01609
    You will not achieve 85F floor temps through 3/4 " subfloor and 120-130F AWT in suspended tubing or staple-up PEX, but you might get better than 75F with staple-up Onix (which I don't recommend, BTW- there's a long and mostly bad history of EPDM tubing in radiant systems.) For any staple-up you'd need considerable hotter water than that to hit 85F floor temps.

    But you don't NEED 85F floor temps either (and 85F floors are rather uncomfortable in the first place!) Start with a Manual-J heat loss calc for that room, and the number of square feet of available floor space that will have tubing underneath it. You get about 2BTU per square foot for every degree the floor is above room air temp. If your design condition heat loss in the room is say, 1500BTU/hr and you have 250 square feet of radiant floor, the max floor temp you'd need a floor temp of about (1500/250)/2= 6F above room temp, so at 70F room the floor would need to be 76F at peak temps, which is achievable with ~125F water using better-quality sheet-metal plates and 8" o.c. tubing, but not as a staple-up or suspended tubing without much higher water temps. If you need anything more than that you need to go with extrusions, or higher water temps.

    I've starred in the movie- I'm ripping out the thin plates under my lossy family room and am replacing them with extrusions to be able to keep up at design-condition, rather than boost the water temp above condensing range just for one room.

    The biggest heat-transfer problem in radiant is getting the heat out of the PEX with such little surface area. Extrusions grip the PEX very well, and because of the higher cross sectional area, present a lower temperature to the PEX/aluminum interface for higher heat transfer rates at any water temp. Thin plates don't grip as reliably and transfer less heat at the same delta-T. Staple-ups have very little contact between PEX & subfloor, which leads to noticeable temperature-striping on the floor when you crank up the water temps to be able to deliver much heat. Systems that use tubing in grooved material sandwiched between the subfloor and finish flooring (eg WarmBoard) with thin-metal plates work VERY well with low water temps and are very responsive- you can use overnight setbacks and get similar response as with baseboards or radiators, etc.

    At the water temperatures you're talking the radiated heat out of the PEX is actually pretty small, even in a staple-up, and just snugging up cheap low-density fiberglass batts works better than any kind of reflective goods. For 180F water and suspended tube, maybe, but its kinda academic at that point. With tubing in any kind of plate, extruded or sheet metal, the low emissivity of the aluminum renders radiant-barrier type goods useless at any water temperature.

    Without a careful heat loss calc, you'll have no idea what you really need, but unless you have a very low window/wall fraction and R19+7.5c.i. walls or better you're probably looking at sheet-metal heat spreaders at a minimum. If this is batt-insulated 2x4 construction with lots of U0.34 or higher windows you'll likely need extrusions. For reference: My family room is 2x6 R19 construction, no exterior foam, and a rather large glazing fraction- it doesn't quite cut it at sub 15F with just the thin plates, though it's still pretty comfortable sitting on the warm floor even when it's 62-63F in the room, which is about where it hit's when it's 0F outside. With extrusions it should keep up just fine without boosting water temps.
  5. Supervisor

    Supervisor Construction Supervisor

    Messages:
    7
    Location:
    Lunenburg ma
    Dana, great explanation, However now with that understanding, I would have to ask," does the AL pex have a better heat transmission rate than a Pex-a-b-c?"

    I thought that with all of the research that I have done it would be something that the manufacturer would have exploited if it were truly the case, so I assumed it was negligible at best?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,262
    Location:
    New England
    The metalic layer is (as I understand it) insulated from the surface so I don't think it would have any effect. It is there for an oxygen barrier.
  7. Dana

    Dana In the trades

    Messages:
    2,974
    Location:
    01609
    What Jim said- it's an O2 diffusion barrier, to keep atmospheric oxygen from ruining iron pumps & boilers, with no effect on the thermal properties of the tubing. The plastic walls are just as thick as other varieties, with identical R and emissivity values.
  8. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I would simply use MORE Uphonor pex; its cheap, and super insulate the bays. Perhaps a foil face foam just under the pipes, then whatever suits the depth. Your floor will get quite warm. I have done it.
  9. MT biker

    MT biker New Member

    Messages:
    1
    Location:
    Montana
    It looks as if this thread was made about a year ago. So I have a question for supervisor which method he choose? Plates or no plates and are you happy with it?
  10. Dave Collado

    Dave Collado New Member

    Messages:
    1
    Location:
    New York
    My family and I stamp the reflective radiant heat transfer plates used to increase your BTUs, but way cheaper than the big company brands out there. We sell them for $40 for 50 feet.

    www.pexplates.com

    Thanks again for the work you do.

    Dave Collado
    845 248 9040
    sales@pexplates.com
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