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Newbie Radiant Floor Decisions....

L_S Member Posts: 1
I recently decided to put radiant floors in my old 1927 Seattle home.  I've been scouring the internet for everything I can find, and I think I've identified most of the major decisions I need to make.  Now I just need help making them...

Over or Under:

OVER: I'm tempted to pull the old flooring up and get the pex in the subfloor.  There's some cost associated with that, but it seems to me it may be worth it to run lower water temps compared to staple up.  I would be inclined to route out tube slots in 3/4 ply sub floor as opposed to buying some of the overpriced subfloor and sandwich aluminum products.  Will I regret not having the foil in there?

UNDER:  This really appeals to me.  I'd prefer not to rip my floors up, but I'm worried about the stack up.  I've got 3/4" fir subfloor, 1/2" bead board (no joke), with 1/4" oak strip on top of that.  If I put naked tubes under there, I"m going to need a lot of insulation below the air gap to keep the heat out of the basement.  No one seems to like the thin AL plates, so now I'm buying extruded aluminum and still trying to push that heat through a lot of wood.  Seems like at that point I may as well rip the floors up?  I'm assuming that I'll need a much higher water temp with all the under options.

Water Heater vs Mod Con:

I see almost no mention of DHW heaters being used for radiant floors here, but there's a lot of talk on other sites.  Seems like this would work, but you guys are the pros, right?  If I can sandwich the pex in the sub-floor, can I consider running a gas water heater in an open system, or is the consensus around here that mod/con boilers are the only way to go?

Finally, Open or not? 

Open seems simpler, but I don't see a lot of you talking about it.  What's the drawback?

Any help you can give me on the pros and cons would be appreciated.  I'll take the time to design this system appropriately, but I'd like to get an idea of the direction I should be heading.  For reference, it's 1200sq feet over a daylight basement, with 900 square feet on the second floor.  No insulation, old double hung sash windows.


  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    good start.

    heavy plates in the joists will perform similarly to light plates or "weak panels" above the subfloor. more expensive than sandwich, less expensive than most panel products. might be a hard install though depending on your joist bays.

    high performance and more expensive panels above the subfloor can, of course, increase your performance.

    Normally I would not recommend plateless installs for anything, unless your heat load is very low. If so, I would prefer plateless only in joists to avoid heat striping issues.

    Never do an open system. Proper heat source depends also on your heat load. Sometimes a water heater is appropriate, with separation, but rarely. You almost always want something with more efficiency. There are a couple of high output high efficiency water heaters that qualify but they are in the same price range as mod/con boilers with indirects.

    If you search for open system on this forum you should find several conversations about why it's a bad idea.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013

    you can search your local news sources for massive numbers of open system failures in your neck of the woods as well. I believe they were in the seattle area.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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