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Strange claims about heat transfer

  1. Feb 17, 2014 #1
    I'm trying to evaluate floor heating systems for installation in my new studio, and I have to admit that my thermodynamics is a bit rusty... But I came across the following, and I could use a sanity-check.

    Basically, it's an in-slab heating element that claims to be 2.5x "more efficient" (and thus cheaper to run) than the resistive cable type elements that are widely used. Link is here. The page will spit a pdf at you, but don't freak out. Direct link to pdf here.

    Q1: I understand that more surface area would lead to a more efficient & even transfer of heat to the slab. But iirc, resistive conversion of electricity to heat is nearly 100% efficent, and since the element is completely contained, there's nowhere else for the heat to go but into the slab. So if your building is losing X heat per hour to the environment, you'll end up putting X heat / hour back in to maintain a given temp. How could an element of one kind or another make any difference?

    Q2: More generally, afaik the point of radiant heating is that it doesn't "waste" energy heating the air, but instead radiates it directly to objects (e.g. people) in the room, allowing you to have lower interior temps. But for a slab-on-grade building, I'd think the higher temp differential between the slab and the earth would mean greater energy loss through the slab, negating the savings - no? (The pdf above weirdly doesn't even mention radiant transfer.)

    Please don't let the clumsy advertising copy on that webpage bias you, I'm genuinely interested in the physics behind this. (The company in question has been around for like 30 years, and has a 30,000sf factory entirely heated by their own system, so I don't doubt it's as good as any other in-slab solution. Just possibly not better.)
     
  2. jcsd
  3. Feb 17, 2014 #2

    russ_watters

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    Staff: Mentor

    Welcome to PF!

    Your knowledge/instinct is correct: This is BS. In my own words:

    1. An electric resistance heater is always exactly 100% efficient.
    2. A radiant floor should be well insulated, so all heaters should perform about the same: they should send about the same amount of heat into the room and waste the same amount of heat dissipated below the slab.
    3. A gas boiler is 80-97% efficient, but who cares that it is below 100% if the energy costs 1/5 what electricity costs?

    The only place where you might be a little wrong (but only a very little) is in part of #2. Delivering heat directly to your feet means that you can feel warmer when the room is cooler, so you save energy versus a system that heats the air directly. But this benefit applies to all radiant floor heating systems, not just to this specific one. The only potential pitfall (as you say) is you are also delivering some of the heat directly to the ground and lowing it. So in order to really be as efficient overall as forced-air, you need good insulation between floor and slab.

    BTW, is clumsy advertising copy any more effective if printed in a 30 year old brochure vs printed on a website? Do you really think good engineering sells products better than good advertising? Please note, I am an engineer and I'm still asking the question...
     
  4. Feb 18, 2014 #3
    What the company's claim is, is that more finely spaced heating elements in the slab can run at a lower temperature, to obtain the same amount of heating into the room as more coarsely laid heating elements, due to less heat loss going into the ground below as a result of the lower temperature with the finer spaced elements.
    Is it 2.5 times as efficient? That would depend upon the insulation laid under the slab. For a poorly insulated slab, it could be. For a reasonably insulated slab, the savings from less heat loss should not be as great.
     
  5. Feb 18, 2014 #4
    Hey guys - thanks for the responses. In thinking a bit about their "downward transmission" argument, it still doesn't sit 100% comfortably with me. The pdf seems to argue that the heated area is wider for their element, giving faster transmission to the air in the room. Surely this argument applies for transmission to the (lower) slab as well, though? Smaller area (cable) -> slower transmission to air -> higher temp needed, but smaller area -> slower transmission to sub-slab, and it all cancels out, since the terms are all linear. (Well, actually an integral across the area, with the cable-heated area having a hotter core and cooler edges, but I'm not going to try and figure that out.)

    I should say that there are other things I like about this system: it's wired in parallel, unlike cable systems, and you can cut or drill through the elements without knocking out the whole system. But it is more expensive, and requires several large 240-24VAC step-down transformers.

    w.r.t. radiant floor systems in general: as I understand it, the big win is that heat is radiated/transmitted directly to objects (you, and your feet), allowing you to lower the air temp and achieve the same subjective "warmness". Though I'd think that having a heated slab actually counteracts this to some extent, since concrete is a worse insulator even than glass, and certainly than a well insulated ceiling.
     
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