radiant floor heat??? Yes No Maybe So???

Discussion in 'Remodel Forum & Blog' started by nocry, Jan 25, 2007.

  1. nocry

    nocry New Member

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    So what is the consensus out there about radiant floor heat???

    I am doing a gut rehab of a full 7' x 8' main bath in a townhome and would be able to install a 2' x 8' run of floor heating coils...

    is it worth it??? what are the pros??? the cons???

    if it is worth the investment, what brand would you guys recommend???

    thanks!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Radiant heat is nice in a tiled room, especially a bathroom where you'll be barefoot. But, about the most you can get out of it is around 15W/sq ft, so don't expect to really heat the room with it, only take the chill off of the floor. If you want more heat, then maybe hydronic heat. But, as a result of somewhat limited heat, you can run it in the summer, too, and not create a huge load on the a/c. My unprofessional opinion...

    A heated towel bar is a really nice touch, too! Again, unless you go hydronic, they only use around 150W or so.
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    Honest question: Why would a hydronic floor-heating system heat a room where an electric system would not?

    Once a given floor area has been warmed to a desired or tolerable temperature, and assuming either system is capable of maintaining that temperature, I cannot see how the source of the heat would affect how well or how much a warmed floor might radiate heat into the space either above or below it.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You can circulate a lot more heat through the tubing than you can tolerate creating in the electrical resistance wiring. From what I've read, you can get a max of 15W per square foot from electrical resistance heating in a floor - enough to take the chill off of the floor, but not likely enough to heat the room. That's not much. Lets say you can circulate 2 gallons of water through the tubing per minute - that's about 16 pounds of water, and lets say that coming in, it is in the order of 130 and drops to 90 on the way out, that's 40 degrees * 16 pound, or 640 BTU/minute. That's equal to 11.25KWh. You'll never get that from resistance wiring. Water can hold a lot of energy more easily than a hot wire.
  5. leejosepho

    leejosepho DIY scratch-pad engineer

    I definitely understand that, and maybe my problem here stems from an incorrect belief that I can maintain an 80-degree (or whatever number) floor temperature with 15 watts per square foot of heating wire. But if I can, then the matter of heating any space above the floor is one of heat exchange and loss, and not one of how the heat is actually supplied. As compared to the hard work of heating wire, only a small amount of tubed hot water might be required to heat a square foot of floor to a given temperature, but then the question is whether or not that square foot of floor (a heat exchanger) can release the supplied heat fast enough to keep the space above it warm.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Without knowing what you heat loss is from the room, you'll never know unless you try it, and then its too late to change...
  7. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, and that is why I next intend to try to find out how to make the floor as efficient as possible in the exchange of heat!

    Do you know of a typical or usual and comfortable floor temperature used by any kind of floor-heating system being used to warm a room? I am assuming a surface temperature of 80-85f degrees would be about the highest that would be comfortable, at least for bare feet.
  8. Randyj

    Randyj Master Plumber

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    One thing most people do not understand is that there is a very big difference in the physics of radiant heat v.s. conditioned air heat. The heat travels to your body so to speak while the room air itself may not be warm. Yet, it is much more comfortable heat. To be able to design and utilize radiant heat it really helps to study what radiant heat is. E-foil a.k.a. double bubble uses the principle to give it the insulating quality it has. In explaining how this "foil" works it is usually stressed that it is totally different than insulation in that insulation impedes the flow of heat whereas the foil reflects the heat therefore keeping it inside the structure...or out of the structure. Heated walls and floors will radiate heat. If you heat the air then it is absorbed into the walls and runs right out the doors, windows, etc. Much of it is principle & theory but understanding it helps planning & design.
  9. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, and I am slowly catching on to that! I purchased some "double bubble" on my misunderstanding that it would do well under the concrete board in my bathroom, but when I read its application instructions, I began learning about what you are saying here. Not only would it be ineffective when not being used to confine a given volume of air, but my concrete board would have been applied on top of something soft! Bad idea all around.

    So, I now plan to simply install the concrete board and build up from there (including the heating mat), and to then use the double-bubble to close in the bottoms of the floor joists, thereby making a bit of a "heat box" through which I will run a few loops of copper supply line to my water heater. Even if I only get a few degrees out of that, it will be "free heat" to give the water heater a jump start, eh?!
  10. non-concrete underlayment for heated tiles

    use an underlayment made of cork or synthetic under the tiles, as a heat break. Prevents thermal bridge. Puts the heat cables energy into the tiles, not into the floor underneath.

    david
  11. leejosepho

    leejosepho DIY scratch-pad engineer

    That had been my first thought, and I would certainly agree as to that being a good idea in many, or even in most situations. However, and since the heating wire I have was specified for "deep heating" a thick concrete slab during off-peak hours, I am more inclined to at least give it an opportunity to heat more than a 1"-or-so tile floor.

    Bringing these thoughts over from another thread:

    What I was saying is that placing the wires too close together would be the same as placing hydronic tubes side-by-side, thereby leaving insufficient "breathing room" for the heat to dissipate as quickly as supplied.

    Yes, understood.

    In my own case, "response time" is not a concern. It will take whatever time it takes to heat the floor to a pre-set temperature ... and what happens after that is what I am hoping to effect by constructing an as-efficient-as-possible "heat exchanger" that happens to look like a bathroom floor. It might take an hour or more to heat the floor, but my thoughts here are about where and how quickly the difference between the "get there" energy and the "temp maintenance" energy will go after that.

    According to the man who sold me a couple of thermostats a couple of days ago -- we are doing two bathrooms here -- the matter of 15 w/sf is about not being able to make the floor uncomfortably warm ... or along the line of safety you have mentioned, to not be able to melt a vinyl floor or cause spontaneous combustion within carpet! But if you have a high-efficiency "heat exchanger" for processing the supplied heat, whatever its source, then the same levels of comfort and safety could be maintained even at higher wattages per square foot.

    Thank you for the thought about the wax ring! I was planning to stay at least 3" away from anything, but now maybe more than that around the toilet.

    Yes, and I intend to take pictures of certain spots for that kind of reason ...

    At a large auto dealership here where I happen to live, the entire floor of its new building has waste-oil-fired hydronic heat. But after the concrete crew missed placing the bolts for the front-end alignment rack, that rack later had to be cemented to the floor to avoid the risk of drilling any holes for anchors!
  12. cork or foam to separate the CBU from the wood subfloor

    ok, it's obvious you have this all thought out.

    A thicker amount of concrete is OK, if it be your wish. The heat transfer to the room will not be faster, i.m.o., because it is just more mass to heat, more deadweight, like a flywheel evening out highs and lows a bit, which you don't need. It won't speed up heat transfer to the room. Besides, you are going to leave the floor warm as a regular state. It makes no difference that the cable is a 208Volt cable. Also probably no difference: that it was "designed" in some way to be embedded in slabs, which to me means that its outer sheath (a grounding insulator) will be designed to hold up well against caustic wet curing concrete and thus it has no bearing on the stable steady state that you will have once your installation is done.

    It is OK to put the cables much closer than 3" to walls. I really really doubt you will get the heat in the floor up to any level that anyone will deem uncomfortable, even if it might be higher than 85 Fahrenheit. People like having their toes toasty warm. Toilets can be sealed with non-wax rings now, so you could run no risk even with a heated toilet base, i.m.o. I would design a floor layout with a cable running as close as possible to the perimeter, not 3" from it. Before anyone reacts too strongly to what I just said in these last three sentences, read the next paragraph.

    The Aubé company makes good thermostats with good features; it will do the job that a flywheel would, so you don't need more thermal mass. Thermal mass is important in a hydronic system (slow to react, high water temperatures), and not in an electrical cable system. The thermostat is what keeps your floor at the temperature you desire, regardless of whether the cables are real close together, or right against the wall or the toilet, or whether your floor has high or low thermal mass. Let the thermostat bring the floor temperature up to the level you feel comfortable with, and DON'T ever worry about the floor potentially being too hot to handle -- that is what the thermostat is regulating; how can I say that more clearly? . BTW, Lee, you didn't mention buying a probe, but i guess that came with the thermostat, right?.

    So based on all the bove, I hope to have made a clear case why it is a good idea not to attempt to re-create a slab even tho' the cables were initially designed for another application. Let the thermostat do the thinking; it trickles out the right current to suit the heat needed. Ask Aube, for confirmation. Just focus on isolating the heat from losses. There is likely to be an external wall and a subfloor connected to the outside wall structure... and this is a small permanent invisible heat loss. A heat bridge.

    I need to repeat the importance of a heat insulator if you want to ensure you get the most efficient heat in the room and the highest temperatures. You could use 1/2" Wedi board. Or cork. Or XPS foam. Or a sound proofing membrane, a felt-like type.

    Heat loss in the room will depend on air transfer (minor air currents due to convection) and on the heat bridging in your subfloor. If the floor heat is not in contact with a heat-conducting subfloor, if doors are closed, if there are no air leaks, and if you have a generally well-insulated house, then the room will get very warm. That may be overkill. If you install the cables on CBU directly on top of a wood subfloor, the room will still get warm. How tightly your building is insulated is also a factor, since the heat loss (gradient) of the whole building is a factor in the room's heat loss.
    http://en.wikipedia.org/wiki/Thermal_insulation
    http://en.wikipedia.org/wiki/Thermal_bridge
    http://en.wikipedia.org/wiki/Building_insulation


    david
    Last edited: Jan 31, 2007
  13. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, we are thinking alike there. And when I talk about heat transfer, I am talking about looking into any "heat-exchanger" differences between qualities of ceramic tiles.

    We again agree.

    Okay, I likely have enough cable to go closer!

    Why so?

    Yes, and I bought an extra one for each system in case an original ever fails. Also, the stat man told me he had just read some literature from a company providing a short piece of some kind of tubing that can be used for installing sensors in removeable ways.
  14. i edited while you were responding

    so some of the answers are now in the first post,,

    About the best tiles to ensure heat transsfer: once you get into the range known as Porcelain (not Ceramic) you have gotten the best for heat transfer. There is not much difference anyway.

    david
    Last edited: Jan 31, 2007
  15. leejosepho

    leejosepho DIY scratch-pad engineer

    Thermal mass is necessary if one is storing heat for later, but thermal mass (including the nature of any material other than copper) is actually why a hydronic system is slow even if its water temperature is higher than that of an electric system, correct?

    Oh yes, that is a fact! It was a bit of a disappointment to discover there would be no need for me to tweak the actual voltage available at the wire, but it sounds like my thermostat is going to do a far better job anyway!
  16. yes, and yes.

    So it is not an issue how warm the floor could become;; the upper limit is only reached when the thermostat is set to that (high) temperature. And, the thermostat being better than expected, means you don't need to simulate the thickness of a concrete installation. Combining these two pieces of information, we can come to the conclusion that an insulating underlayment is far more preferable than a CBU. It also saves you money in operating costs, but that is only half-pennies less than the pennies a day you would have spent.

    david
  17. Bob NH

    Bob NH In the Trades

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    If you are going to heat a concrete floor, you MUST have very good insulation between the concrete and the earth. Otherwise, you will be spending a lot of money heating the earth, which is a VERY LARGE heat sink.

    If you try to store heat in the thermal mass of concrete, the heat is going to be leaking out the bottom to the earth faster than it is leaking out the top to the air because conductive heat transfer rate is greater than convective heat transfer rate.

    Any hard surface floor such as ceramic tile will alway feel cold to the touch unless it is as warm as your foot. That is because it is a good conductor of heat. A steel file cabinet and a carpet in a room are the same temperature. Put a hand on each and see which one feels colder. A mat that you toss on the floor to step out on will do more to keep your feet warm than heating a tiled floor.

    You don't get enough "radiant" heat from a floor to give you much sensation of warming. It will barely keep your feet warm because it is in contact with them. If you want true radiant heat, you need something that is much hotter than the surroundings, such as heat lamps or those "wire wrapped around ceramic" heaters.

    Leave the exhaust fan off when showering in the winter to keep you warmer when you step out of the shower and keep that good humidity in the house. Evaporation from your wet skin makes you cold. One ounce of water evaporating from your skin in one minute is taking more than one kilowatt of heat away from your body. If you want to add some heat, try a heated fog-free mirror. When I Googled "heated mirrors", most links seemed to be from the UK.
  18. leejosepho

    leejosepho DIY scratch-pad engineer

    Some structures are heated entirely from nothing but "radiant heat" in the floor, yet I do hear what you are saying there.

    If it is still cool enough for a legitimate test in a few weeks, we will see what difference our heating wire in our bathroom floor actually makes ... and at the very least, we will be standing on tile that does not suck heat from our feet as quickly as it would with nothing but mortar inside!
  19. Cork, Wedi, Cerazorb, Acousti-tech, or other pad type underlayment

    Bob is right about many things.

    I agree with his conclusions 99.9% of the time.

    We may find after a long discussion that I also might agree on the one thing that I cannot lend my support to here. Or he may have been attempting to drive home a point, and using a strongly worded conclusion to do so.

    Skin temperature is always lower than body temperature.

    When discussing heat transfer between two objects, we could use the strictest definition of heat which means we ignore skin temperature and always refer back to body temperature. However, people only feel heat on their skin. (More later about this. To be continued). When someone's skin temperature gets raised by a new heat source, that immediately creates a new point of reference.

    So skin temperature is a moving target and that fact alone makes any discussion of "feeling heat" extremely complex. Phew! :eek:

    Repeat: once your cold winter feet get warmed up by the tiles, they won't feel so warm anymore. The tiles I mean won't feel so warm any more.

    Furthermore, the net heat transfer will still be in the opposite direction. I mean away from your body. You will be losing heat to the tiles. (More later. See below, about ocean water). Bob mentioned the rate of heat transfer; tiles are a good heat conductor. This is a crucial part of the analysis.

    To complicate things even more, I'll say this: that the psychological feeling of warmth (heat) going towards/into or away/out of the body does not depend on object/skin temperatures, and also not on the rate/speed of heat loss, but on the comparable heat loss rate -- with other surfaces nearby! Comparable in one's immediate physical memory. Phew! :eek: Phew! :eek:

    It is true that tile (porcelain or ceramic) will pull heat away from your skin.

    It is true that tile (porcelain or ceramic) will pull heat away from your body (and thus skin), even when the tile is heated -- except initially when your feet are cold. After your feet get warmed up, the tile floor is still cooler than your body temperature. The RATE of heat flowing away from your body is much lower when the tiles are heated, and this makes the tile surface feel warm, to some, and not warm to others.

    A heated tile floor, when built on an insulating underlayment like cork or synthetic membrane, will produce enough heat to feel really warm to anyone whose house is surrounded by cold radiating in from the outside. At first, your cold feet will be warmed up. Later, when your feet are warmer, the heat loss (from warmed feet to tile) will be less than the heat loss from feet to wood floor -- assuming you have a wood floor in the rest of the house. This is what I meant when I mentioned "comparable surfaces nearby".

    I think a heated floor will also feel warm to anyone in a warm climate too. When direct sun is shining on it you can still notice the difference but it's no longer as significant; the sun is the strongest heat radiator we have.

    A comparable phenomenon I think is this
    - A.) hypothermia in warm ocean water, versus hypothermia in water that "feels cold" to start with. Staying for hours in warm water will suck heat energy out of your body.
    - B.) Feeling, and enjoying feeling, warm ocean water, versus feeling water that "feels cold" to start with -- and not enjoying the sensation.

    Part A.) is reality, regardless of feelings, and Part B.) is all about feelings, sensations. Both waters, warm and cold, are below body temperatre, and yes they both remove heat energy from you. One feels warm; the other feels cold. For how long does the warm ocean feel warm?? Everyone can have a different answer, since it depends on one's point of comparison.

    Conclusion: three things to do: use an insulating membrane, use an insulating membrane, and use an insulating membrane. Then you will have heat so high that even the most sceptical person will appreciate the warmth. Not only will it feel warm at the first second, and the second second, and so on for many seconds, but also AFTER it warms your feet up, it will still feel warm to your senses.

    Many people feel frustrated when the heated tile floor only seems to work for a short time while before it feels cold again. Their feet have gotten warmed up to a level that now causes the point-of-reference to change, and the sensation of warmth disappears. That is frustrating. The shifting point of reference.

    I believe the most sensitive people are engineers who understand heat transfer. They can feel the differences in heat flow rates (heat loss or transfer), to a finer degree than anyone.

    David
    Last edited: Feb 7, 2007
  20. Bob NH

    Bob NH In the Trades

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    leejosepho and geniescience:

    I try to use precise language. That is why I said, "You don't get enough "radiant" heat from a floor to give you much sensation of warming."

    I was talking about real radiant heat. The kind that comes from thermal radiation. The kind that varies as the 4th power of the absolute temperature as represented by the Stefan-Boltzman law. The kind that gives you the sensation of warming that you get from a heat lamp, or from standing in the sun, or from a fireplace.

    You don't get any real sensation of warming from a floor at 75 degrees F. You get a sensation of warming from a radiant source if the source is at a higher temperature than the target. You get a sensation of warming from convection of warm air that is hotter than your skin temperature, or from conduction as when you put your hand on a hot object.

    Most "radiant" floor and ceiling systems heat the air and surfaces in the room to the control (thermostat setting) temperature. But that doesn't help much when the dry air that exists in the winter heating season evaporates a lot of water from your wet skin. You can be cold in 75 degree air if you are wet and the dew point in the room is 35 degrees. At that point, you want high intensity heaters such as radiant or convective heaters operating at higher temperatures to replace that heat that is being lost by evaporation.
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