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gregrehn Member Posts: 9
In the book EDR Dan explains the relationship between the temperature differential of the average water temp and the btu output per square foot of radiation. In his chart the minimum water temp is 150deg. Can we assume the same projected numbers of water temp to btu output? I.E. 145 deg water and 100 btu/h per sqft and 100deg water and 10btu/h per sqft? Although this relationship holds true for the temperatures from 150 to 215 deg f. I do not know if this would be the case for the lower temperatures.

In other words does this equation hold true for the temperatures between 150 and 95 deg f. (average water temp)

BTU = 2(delta T) - 50

where BTU is BTUs/hr/sq'

delta T is temp defference between average water
and room temp.

145deg > delta T > 20deg


  • Greg,

    Not really sure I understand your question, but is this what you are looking for?

  • gregrehn
    gregrehn Member Posts: 9
    more on edr

    Thanks Dave for that PDF, where did you locate that source? The information on the table you provided gave the exact numbers the equation i generated did. So therefore those 2 sources of data overlap. As nice as that is it only brings about more questions. With the data Dave and I were looking at one will see that at 25 deg delta T there is 0 btu output. I fail to see how this could be possible. When we size for a low temp floor radiant panel we use the equation 2(delta t) = btus. The slope of this function is the same as that for the radiators, i.e. 2. Yet the point in wich each line intersects the Y axis differs by 50deg. Because we can compare the data for a radiator and a radiant floor and because the data does overlap I raise the question of is this function truely not a linear relationship but a curve? The comon ground between the two graphs would support this point.
  • Greg,

    Oh Man,,I`m just a tradesman, looking for thought & theory on this matter?, hail Brad White(the "Walls" resident engineer), as he can explain-it better than me!

  • gregrehn
    gregrehn Member Posts: 9

    does any on know how to get brad white on this thread... or any one else who might understand?
  • Bruce Stevens
    Bruce Stevens Member Posts: 133
    Make a post

    Title it Calling Brad White, then cut and paste your questions there, I am sure he will respond
  • Brad White
    Brad White Member Posts: 2,398
    Interesting discussion

    It is an interesting situation, is it not?

    I really had to think about this (or "re-think" a bit) and this is what came of that, rightly or wrongly and maybe rambling a bit too much before coffee....

    My thinking took me into micro-terms near the radiator, the coldest air in the room versus the coldest water in the system as the temperatures approach. Then there is the radiant portion versus the convective effects....then to laminar flow within the radiator (assumed cast iron radiators for this discussion).

    This is all raw thinking today, open for discussion.

    The relationship is really a curve especially at the low end, I guess we can call that "starting torque". It is understood that the temperatures cited are "average" water temperatures, mid-way between your supply and return. Half is above, half is below. The lower end of that scale could be very close to room temperature but would never be below room average temperature of course. At best or worst, it would be neutral.

    I see what you say, that extrapolating that "70-degree room data" downward (120F=50 BTU's, 110F=30 BTU's, 90F=10 BTU's...) so when you get to 85 degrees (15 degrees above the room) you theoretically have bupkes.

    Not possible as you say... but reality does step in and this is where the "wedge" comes into play. (Wedge is my term, not official by any means). I would also call it a "soft landing". I think it means that there are limitations to such tables or rather our ability to extrapolate beyond them.

    Say I have an 85 degree average water temperature in a radiator in a 70 degree room, my entering water might be say, 95 degrees and my leaving at 75 (20=degree delta-T which would require a TRV or other means of flow control).

    The leaving 75F water is only 5 degrees above the room temperature so that has to be worth something, ten degrees if by the ratio.

    We know, as you stated that there has to be some heat transfer. If the "2 BTU per degree" rule of thumb holds, the 85F average water to 70F room spread would dictate 30 BTU's per SF, -which corroborates pretty much what radiant floor design tells us.

    I would say that, by definition, most of the heat transferred at that point, where the radiant surface and air surface nearly intersect, is radiant versus convective. Sort of like free points!

    Within the radiator itself we also have potentially 65 degree air entering it near the floor (baseboard is so rated, even for a 70 degree room which says that stratification is acceptable, how so radiant becomes another thread...). This becomes a possible way to further reduce the leaving water temperature closer to actual room temperature.

    But the point is, there is cooler air within that room, nearer the floor, and the delta-T is increased at that point. Similarly, the air near the top of the radiator is approaching or even above the average water temperature. There is a crossing at some point where the air passing over the radiator has an average in the same range as the water passing through the radiator, albeit offset (air being cooler, water being warmer of course. Say air in at 65 and air out at 85 which could be measured, I suppose.)

    Then within the radiator itself, we have water perilously but delightfully close to average room temperature, maybe barely below where the coldest air in the room air meets it.

    But at these low flows and narrow temperature ranges, there is doubtless laminar flow within the radiator. A surface film of water at low velocity clings to the inner surface, -an exclusive club to which the hotter water in the center of the radiator is denied entry....

    This got me thinking about radiant floors and tubing in general. Break up the flow into smaller but velocity-driven circuits to keep laminar flow at bay....how much more effective and pure that is.

    Sorry to ramble but had to revisit thermodynamics versus the realities of the radiator and the room. Not sure if this helped but I am reminded of what a professor once told me:

    "Data is often wrong."


    The bumblebee is aerodynamically incapable of flight, yet he merrily buzzes along in his ignorance..."

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • gregrehn
    gregrehn Member Posts: 9
    thanks Brad White

    Brad I appreciate you taking the time to respond in such detail to my questions. I found your thoughts interesting and conclusive. After reading you response I did have a couple of questions. The fist was about you comment of 2deg delta t rule of thumb. The second was your statements on laminar flow and surface film and their effects on heat transfer. I admitedly have little knowledgd in both these areas. Could you reference some works so that I may try to catch up. Thanks again for your time Brad.

    Greg Rehn
  • jp_2
    jp_2 Member Posts: 1,935
    proud ignorance :)

    brad I always like those statements that some "thing" doesn't follow the mathematics of such and such....

    we forget that "WE" are not following the mathematic of the bee!!!

    the BEE came before we.

    I've always understood heat transfer to be nonlinear, so my assumption is that using linear equations are close enough and easier to use and understand. being in a narrow range.
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