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Hydronic Heat and Hardwood Floors

Ken_40 Member Posts: 1,320
so-called Brazilian cherry; full 3/4" T&G and a 3/4" subfloor of AvanTec. The cherry came in various widths, we selected the "thinnest," which is around 2-3/8" wide. We used PEX with aluminum plates with water temps in the 110F range. The beauty of the arrangement is the floor never feels hot, nor cold, just neutral.

The floor shows absolutely no sign of shrinkage, warping, cupping or stress of any sort. Brazilian cherry is the seond hardest hardwood known to man. I doubt that has any relevance, but that's what the "hardness" chart showed.

Hope that helps dispell the myth of wood floors and radiant being problematic.

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  • KRTaylor
    KRTaylor Member Posts: 1
    Hydronic Heat and Hardwood Floors

    I'm building a new house this spring and I love the idea of in-floor heating. My wife insists on a hardwood floor. My architect says don't put hardwood over hydronic heat because it will cup or shrink. Other sources, mostly manufacturers, say it can be done. They say it's about moisture and low temperature. I'm interested in hearing from someone with actual experience with this who's got no direct interest in selling a product. Even better I'd like to see a house with that type of installation. Can anyone help? Thanks
  • Paul Pollets
    Paul Pollets Member Posts: 3,656

    Hardwood floors work well with radiant heat. The best application is to use the "pre-engineered" wood floors that have a lower R value. We've specified Khars or Junkers on many of our projects. Some architects are not aware of these products. Strip oak, or conventional hardwoods do not work well over a radiant application. If they are to be used, 2 sheets of plywood must be laid over the radiant slab to provide a buffer, or the Oak will most likely cup or warp. The double plywood method is called the "Bollinger Method" and has been discussed previously on this site. The concerns with using the method are the additional costs of sheet goods and the increased elevation of the floors. You may want to check out the RPA website at www.rpa-info.org for more info on frequently asked questions about hardwood flooring and radiant heat.

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  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    I think it does

    The harder the wood, the more dense and less water content, therefore the more "dimensionally stable" it is.

    I agree with Paul, the lower the R-Value, the less likely you are to have an issue. This is because you would use a lower supply temp to heat up the wood surface

    The key to any successful hardwood installation is doing the math. You need an accurate heatloss to design a trouble free system. We see houses typically built with heating loads between 15 - 20 btu/sqft. Your talking about surface temps of 75 - 80F. I don't think this is going to cause damage to any floor.

    The comment of your architect simply shows his lack of knowledge/experience on the subject. We have designed well over 100 systems with wood floors over the past 10 years - virtually no problems except for contractors putting nails in the pex. Actually, we did have one other problem. The contractor left the wood in the plastic sealed bags it was shipped in and didn't let it acclimate properly. Basically, he cut the bag open and installed the wood flooring. It was a warmboard install, the wood moisture content of the wood was higher than the air and it cupped.

    We don't have a problem with wood floors. Then again - we don't guess at our designs - we do an accurate heatloss and use computer software to guide us. If the heat loads of a room approach 25btu/sqft where wood is involved, we will look for a source of supplemental heat and use a 2 stage thermostat, or try to use constant circulation to minimize supply temperature differences.

    There are limitations, the key is knowing what they are. Work from a position of accuracy and knowledge, not one of speculation and wive's tales (no offense to the women intended).

    Look around, I'm sure you can find a qualified radiant designer/installer. Check out the Radiant Panel Association for more information and a list of certified installers near you.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    While still not complete, here's a decent list of woods by hardness. Brazillian cherry is certainly very hard, but it's definitely not the "second hardest wood known to man".

    Lignum vitae (aka "Tree of Life") IS the hardest--by far. It's also self-lubricating. This unique combination makes it indispensible for propeller shaft supports in HUGE ships. To my knowledge there is still no substitute.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    "Harder" does not necessarily imply "more stable". The hardest wood I've done much work with, pecan/hickory (and by all measures they're essentially identical), is also one of the least stable I've used and while very soft by comparison, old-growth white pine is surprisingly stable.

    Woods also tend to expand and contract far more along growth rings than between growth rings. For most woods (including those traditionally used for solid strip flooring) quarter-sawn is FAR more stable than flat-sawn. Of course quarter-sawn is FAR more expensive...

    It took the dead men CENTURIES to find the ideal thickness/width proportion for commonly used flooring woods. Ideally the exposed face will be about 3 times the width. The wide plank flooring that's now en vogue is a romantic throwback to U.S. colonial times when only the richest could afford "proper" flooring. Old house people have a HARD time dealing with it as the gaps get HUGE. At least though such was generally old-growth wood and it [usually] didn't cup too badly, but I can assure you that they were carefully selecting boards cut in a way that wouldn't tend to cup (e.g. nearly quarter-sawn).

    I got a great lesson on hardwood flooring when working on my present home. The oak floors were installed in 1922. Plain sawn upstairs, quarter sawn down. When adding baths on the 2nd floor I had to remove a fair amount of the oak. Actually found tags. Bruce (they still exist). "#31 Plain Sawn Oak". Called the Bruce company. The closest they now provide is "Heritage Oak"--the most expensive plain sawn in their line but NOWHERE near the 12' or so average length in my house. Their quarter-sawn is now veneer over engineered strips...


    All this said, there's no reason that decent quality properly acclimated, natural strip hardwood that's about 3 times as wide as thick will have ANY problem with a properly engineered radiant floor heating system.

    In all honesty however, I'd probably choose an engineered flooring product with high preference to bamboo.

  • Howard Emerson
    Howard Emerson Member Posts: 111
    Even though it's really hard...............

    Hi Mike,
    The late guitar builder, James D'Aquisto, related to me a story about Pete Seeger. This is a quote from a copy of Acoustic Guitar Magazine: "Seeger's banjo dates from 1955, when he got the idea of making a banjo neck/fingerboard from a single piece of lignum vitae, a wood so dense it sinks in water. He bought the wood himself, sketched a neck outline on it, and with the help of the D'Angelico shop, turned it into a working neck (with three frets more than the average banjo) and mounted it on a Vega Tubaphone pot."

    Jimmy told me it warped anyway..............

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