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Thread: Installing radiant floor heating under subfloor / hardwood floor.

  1. #1

    Default Installing radiant floor heating under subfloor / hardwood floor.

    After removing all the old heating lines in our basement to make more headroom, the thought occurred to us that we may be able to remove the old cast iron radiators on the first floor, replacing them with in-floor heat. This would ideally gain both more usable floor space, and add add comfort.

    We would be using Uponor Wirsbo / Uponor "Joist Trak" Panels.

    I'd appreciate any guidance before we make the jump.

    Questions:

    1) Is there any real risk to the hardwood floor? Will underfloor heat destroy them?

    2) This is an old house in Minnesota with some of the colder nights in Jan. below -20F. We have a thick limestone foundation that touches about 14" of the sub floor between the joist cavities along the perimeter of the house. During the winter, this part of the floor is very cold.

    2-a) While the PEX tubing will not be able to reach this ~14" area around the perimeter of the floor to heat it directly from below, would heating of the floor near it keep this otherwise cold perimeter warm?

    2-b) Would this cold perimeter dramatically decrease the efficacy of the radiant floor heat?

    3) Is this type of below subfloor heating generally able to completely replace the heat generated by old cast iron radiators?

    4) We have an older, non-modulating, Weil Mclain he5 boiler. Will we loose any efficiency heating with radiant floor vs. cast iron with this type of boiler?

    5) Any less costly alternative to the "Joist Trak" panels? (We were told by a local supplier after doing a heat loss calculation that we would need the
    "Joist Trak" panels, especially in the colder months)
    Last edited by Terry; 10-18-2008 at 10:04 AM.

  2. #2
    Master Plumber nhmaster's Avatar
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    The sort answer is yes with reservations. Radiant floor heating (staple up or in floor ) does not have the ability to supply any where near the btu\hr that baseboard does. You may well be able to gain some comfort but you will still need to supply some supplimental heat besides the radiant.

  3. #3
    DIY Senior Member seaneys's Avatar
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    There area reputable and seasoned hydronic vendors who may be able to help out. You need someone who can run a heat sizing calculation for your house.

    I can point you towards the vendors I have used if you send me a private message.

    How old is the house? What type of flooring?

    Steve

  4. #4
    DIY scratch-pad engineer leejosepho's Avatar
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    1) Your floor will be fine as long as your system is correctly engineered and the wood is not overheated.

    2-a) Your cold perimeter might be a little warmer ...

    2-b) Since wood is not a good conductor of heat, your stone foundation is not likely to pull a great amount of heat directly from the floor.

    3) Like someone has already said, you will likely not get enough heat from the floor alone.

    4) I would guess your boiler would have to be throttled down a bit if it is only supplying the in-floor heat ... but I do not know how that might affect its efficiency in relation to what is lost through its chimney.

    5) Before spending the money for "Joist Trak" panels, I would talk with someone about how much (or what thickness of) aluminum you would actually *need* in order to carry away the maximum heat that can even radiate from the tubing being used. In other words, I suspect "Joist Trak" might be great with copper while being a bit of overkill with PEX.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    You can't run the water to the radiant floor as hot as you do into the radiators. As a result, you need much more area that is heated. This might require more loops and tubing runs closer together to ensure that the exit temp of that loop isn't too low. I don't see why you can't run the heat strips on the perimeter. If you are worried about it freezing, you could run antifreeze in the system, but normally, that wouldn't be a problem and is probably unnecessary. Course, you are there, and we aren't. Note, if you do run antifreeze, it will derate the boiler (it won't transfer as much heat as when running pure water). It's generally a good practice to have the first part of the loop run to the perimeter of the room. This puts the hottest water in the coldest place. I don't know if they sell the tracks with a varying pitch (i.e., varying distance between tubing runs). You may need that to get the heat transfer if the foundation isn't well insulated. I'd look into what it would take to add insulation...it will probably pay for itself quickly in energy costs and improve the comfort levels significantly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Master Plumber nhmaster's Avatar
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    I have found that staple up systems need to run pretty high water temps to be effective. Somewhere in the 160 to 180 degree range which is pretty close to baseboard temps. The problem here usually is the overall cost. It becomes a backwards slide because the cost of installation is higher than the benefit derived.

  7. #7
    DIY Member edlentz's Avatar
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    Hmmm I have a staple up radiant floor system. I used Alum plates and insulated under the plates to make a closed space. My water runs at about 145 all winter. My floors are 3/4 60 yr old oak T&G with a 1" subfloor. I live in the middle of Michigan. We until this summer had only about half of the outside walls (Bad blown in insulation job in through a brick exterior that my inlaws had done). Our first year our gas usage dropped about 40% My boiler also supplies our DHW. I have learned alot about this subject at heatinghelp.com . It is a site where the pros gather. They will answer questions and make suggestions. They even have a search for a pro in your area. Before doing anything get a survey done and maybe two before you commit to doing anything. It is real easy to change the paper and not as easy to change a heating system.

  8. #8
    Master Plumber nhmaster's Avatar
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    Yep, a good solid oak base will give you much better transfer than 3/4 plywood will. The overall density of the substrate has a big effect on the floors radiant ability. Note however that 145 degrees is still a lot hotter than the 85 degrees needed for in floor radiant. You start adding carpet and pad and the temps start climbing. Insulating the cavity makes a huge difference also. We have been experimenting with 1" foil faced styrofoam installed foil side up and foamed to the joists. Initial results are encouraging, labor however is not so much so. If you were doing the insulation in your own home I think it would be well worth the time though.

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    DIY Member edlentz's Avatar
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    nhmaster is right about the cost of installing radiant in floor heat. One of the main objections to doing this type of installation is the labor costs. We did it ourselves and took all summer doing it. Got it properly inspected and love the system. We used plain old 3 1/2" fiberglass and made a cavity. There are alot of different theories on how do make the envelope under the floor. We used the cheapest and easiest for us.

  10. #10

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    Thank you for all the input.

    I would be doing the installation myself.

    The house was built in 1910. It has 3/4" pine plank sub-floor with 3/4" maple hardwood. Walls have had cellulose blown in at some point in the past (how well it was done, I don't know). Windows are new, replacement.

    It's a 2 story house. Basement will be finished and heated. Only the first floor would have the radiant floor heat. Second floor will still use cast iron radiators.

    More questions:

    1) Other than ease of installation, is there any real differences in the ability to heat the floor between these two products?

    1a) Simple heat transfer plate (aluminum sheet metal with bump in it for 1/2 PEX)

    1b) Joist Track

    2) When referring to "staple up", does this imply use of an aluminum transfer plate, or that the PEX is just simply stapled to the underside of the sub floor?

  11. #11
    Computer Programmer Bill Arden's Avatar
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    Quote Originally Posted by brainbone View Post
    1) Other than ease of installation, is there any real differences in the ability to heat the floor between these two products?

    1a) Simple heat transfer plate (aluminum sheet metal with bump in it for 1/2 PEX)

    1b) Joist Track
    The image of the first one is hard to see. It looks like it holds the pex up and would be harder to install.

    As for heat transfer, that would depend on the material thickness and the first one does not say what thickness of AL it uses.

    I also can't tell if the channel is rounded on the first one.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  12. #12
    DIY Member edlentz's Avatar
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    Did the same builders do your place as they did mine? Your house sounds like mine. The cellulose blown in into the walls in my case was about 45% full. This summer we have spent the money and literally tore out the old button board plaster walls, insulated with fiberglass on almost all of the outside walls.

    The first track you show is what i used. It is lightweight for sure. In my case it has worked. The problem with it is securing it to the subfloor. We used staples at first and they didn't work so well. Then we used 3/4 sheet metal screws and that seems better. The problem with the thin plates is that you only get a GOOD transfer of heat where it is actually touching the floor. The joist trak IMHO would work much better if you can afford it. At the time I couldn't and I didn't know about it then. Today if I had to do it over I would use the Joist trak. The term staple up, I think means that you have tubing attached to the bottom of your floor. I guess in the beginning guys actually just literally stapled the Pex to the subfloor. IMHO anyone doing that is not doing their homework these days.

  13. #13
    DIY Senior Member seaneys's Avatar
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    I can not be sure of the generic panel you are referring too.

    I would keep an eye on two items that can have a significant impact:
    1) Check the connection properties between the PEX and the plate. Joist track has a long uniform channel that fits snugly around about 2/3 of the PEX. The larger the surface area and the more uniform the fit, the better off you'll be.
    2) Joist track is fairly solid (nice and thick). Some of the panels are simply then AL that has been stamped. One caveat to the thin plates is that it is very hard to get decent surface contact.

  14. #14
    Homeowner geniescience's Avatar
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    Quote Originally Posted by edlentz View Post
    .... The first track you show is what i used. It is lightweight for sure. In my case it has worked. .....
    Quote Originally Posted by brainbone View Post
    .... replacing them with in-floor heat. ... more usable floor space, and add add comfort.
    We would be using ,,, productX,,, .....
    2) ... thick limestone foundation ....
    2-a) .... would heating of the floor near it keep this otherwise cold perimeter warm?
    2-b) Would this cold perimeter dramatically....
    3) ... able to completely replace the heat generated by old cast iron radiators?
    4) ... Will we loose any efficiency heating with radiant floor vs. cast iron with this type of boiler?
    5) Any less costly alternative to the ...
    Heat loss to a massive stone foundation is still a subject worth answering. A heat bridge is what you have. It sucks heat energy out of your house. In places farther north, they insulate outside the foundation. Foam around the stone. Foam around the concrete.

    Previous posts have covered good subjects and given good information in response to your questions.

    In Brainbone's climate, insulating around the exterior of an uninsulated foundation will improve comfort, and lower heating bills. You have to slide rigid foam down around it. This reduces the heat bridge, a permanent path for steady heat transfer down into the ground. SEcondly, shimming a 1/16th inch gap between your wood floor and the foundation and introducing neoprene or solid fiberglas pad under the floor will be worth it to prevent heat loss. Wood IS halfway between being a good conductor and a good insulator; heat loss from warm wood to cold concrete or stone is not negligible, it is substantial when "ON" 24 hours a day. See Wikipedia "heat transfer".

    2-a) yes, warmer than before with radiators
    2-b) no, not as you have phrased that question
    3) yes, i have seen this work completely AND you will have to pay more attention to insulating the structure, to reduce heat bridging, which has the benefit of more comfort everywhere, and lower energy consumption too, so it is both money-saving, a pain to DIY, and not worth hiring someone for because it is so detail oriented. Quality of work is 90% of success. That'll take you into 2009 or 2010.
    4) no, not as you have phrased that question
    5) yes, the question to answer that question is how much you are willing to have heat that you can feel under your bare feet in stripes going up and down the floor. Plates are heat spreaders. Personally it is not a big concern to me for heat to be applied slightly unevenly. Several posts have described the usefulness of sealing under the joists so that heat remains enclosed (instead of radiating downward and convecting downward) and that helps spread heat too inside that joist cavity.

    David

  15. #15

    Default Informative site

    You might want to check out www.radiantec.com. They have been very helpful.

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