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electric radiant

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JimGPE_3
JimGPE_3 Member Posts: 240
Hot Rod - With electric heat the btu output WILL be the wattage of the installed element times 3.41. With hydronics, if you wrap the heating element with insulation you get no heat. With electric elements you wrap the element with insulation you get glowing elements, but they will put out the rated heat.

To me the important question is how much heating element was installed and what the floor area is. That with your rule of thumb will yield the floor surface temp. and then you can decide whether the floor will be comfortable.

With electric heat the only variable in the heat transfer equation Q = U x A x (temp diff) is the temp diff. The heat output and the heat transfer coefficient are not going to change.

So, how much heating capacity WAS installed? How many feet of element at how many watts/ft?

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  • Phil_6
    Phil_6 Member Posts: 210
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    Electric radiant

    Hi everyone.
    I'm doing a job where when I asked about the heating in the kitchen the contractor said they were doing electric radiant. It's a contractor I never worked for before and he said they use it occasionally in bathrooms and little areas and since this place has a steam system it seemed like the way to go. Okay, sounds good to me, someone elses headache and all, so we did the plumbing and moved on.

    Now he calls and says that they need a steam radiator also because the according to several manufacturers the floor won't give enough heat. Didn't seem right to me so I looked into it and picked a website at random, "NuHeat", which claims an output up to 42 btu/sq ft. According to the heat loss I did that should be plenty, even deducting the cabinets and all from the floor area.

    So my question is this...why are the companies telling the contractor that the floor won't heat the room? I never was involved with an electric radiant job before, but I don't have a problem with it, and from what I've read here it's pretty widely used. I just don't want them to waste money on an un-needed steam radiator. But we don't want them to be cold either. What do you think?
  • hr
    hr Member Posts: 6,106
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    42 BTU/ sq. ft, IF

    you like 65° ambient and roomm temperatures :)

    Keep in mind the output of any radiant floor is pretty much based on the floor surface temperature to air temperature delta t

    Knowing that we would not be comfortable, in a residential setting, with hard floor surfaces over 84- 85, then use the 2 btu/ foot for every degree difference, as a fairly accurate rule of thumb. So with a 68° air space and a 85° degree floor surface you get a difference of 17 times 2 = 35 BTU/ sq ft.

    For me if the load cannot be met with 35 btu/ ft from the floor, look into supplemental heat.

    Driving that surface temperature beyond 85, to meet a higher load, usually gets upset customers.

    Bottom line, as ALWAYS, what did the heat load calc show for required load, in BTU/ sq. ft?

    Doesn't matter if the floor is heated with hydronic, electric, or campfires underneath (Roman style :) the floor is only going to give you so much heat!

    Unless the campfire gets outta hand.

    hot rod

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  • Phil_6
    Phil_6 Member Posts: 210
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    42 btu/sq ft

    Well we don't need that much output. That's my point, really. I think it worked out to around 34 or 35 with my software, which has always seemed to estimate high in my opinion. I'm just trying to figure out why the manufacturers are discouraging the customer from using the electric radiant for the only heat source. This is a pretty simple job, a little kitchen with heat above and below, no crazy glass walls or anything. If we were doing a hydronic system I wouldn't have even considered adding supplemental heating. I guess we'll go with our instincts and skip the radiator. Thanks
  • Phil_6
    Phil_6 Member Posts: 210
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    electric radiant

    Nothing's been installed at all yet. That's why we're still talking about it ;-) Not being versed in electric radiant I was some what suprised that several manufacturers were so negative about the performance of their product. From our calculations the radiant should heat the room just fine.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    Many of the mfgrs. consider such as a "floor warming" product, not necessarily "floor heating". They generally recommend supplemental heat. Believe it has a lot to do with who installs (often a flooring as opposed to heating contractor) and a good measure of CYA.
  • jerry scharf_3
    jerry scharf_3 Member Posts: 419
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    I think it's CYA

    If they say you can't do it, then they are off the hook if it doesn't work. If they say you can do it, then they have to field the calls when it doesn't work. It's easier in today's world to say no (thanks to the lawyers and the people who hire them.)

    I bought some engineered wood that said it wasn't approved for radiant. Before I bought it I called up the tech support and asked what the problem was. He said there was no problem, just they haven't tested it so they say it's not approved. It just means they can void the warranty. I liked the product so I took the risk and went with it.

    jerry
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