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style and substance, underfloor radiant install

I've been doing a fair amount of overfloor lately, but have done a few underfloor jobs in the past.  Got a complete gut job where it may be appropriate as the only thing left are the old floors and owner wants to save those. so gotta go under instead of over.

One thing that has always bothered me about the underfloor is working out the turns at the end of the joist cavities.  one thing i have been thinking of is dropping out of the joist space to the bottom and coming under the joist and back up and setting that up along the outside wall on two sides and then firring the ceiling down enough to cover and working out some nailing plates to cover that spot.  i often shear my own plates to size and put little holes in them with a punch so i can work out what would easily cover two or 3 pipes running under the joist.

but my old horror stories from the days before pumping away was the way, remind me that a lot of up and down, even if relatively short legs, can lead to more trouble with air binding.  And I can remember my fascination in actually watching my first airbind when the pex was coming through clear maybe 20 years ago and I watch the air bubble slowly ghost along and then slide back frustruating circulation.

that was a quick emergency 'winter is coming wife wants heat' kind of  hook up so it was kind of 'loopy' and i just repurged harder and longer and that took care of it.  But i'm wonder if i should be concerned about a lot of systematic loops down and up of about 8" or whether good purging, better air eliminator and barrier tubing combine to limit concern about that?


  • SWEI
    SWEI Member Posts: 7,356
    edited October 2013
    Of ups and downs

    A complete non-issue in my experience, assuming we install sufficient valves to isolate for high pressure purging.  Webstone ball drains by the case (literally) on most jobs.  Isolate the boiler (also installed with ball drains and/or flanges) then fill & purge one section at a time using street pressure, which can be 90+ PSI in some places here.  If it's under 60, or on a well, or too hard, we bring out the purge cart (well, not really a cart yet, but we're working on that.)

    As to the return bends on underfloor retrofits, an angle drill and Selfeed bit takes care of it.  PEX-A bends much more easily, and with 3/8" tubing makes it a snap.  Proper insulation is part of the system, not optional.  Wet-spray cellulose encases the whole shebang nicely if we keep the tubing where it belongs.
  • archibald tuttle
    archibald tuttle Member Posts: 1,033
    edited October 2013
    it isn't the drilling that bothers me

    read your post on my phone earlier today but couldn't get to computer to type a novel till tonite.

      it isn't a problem making holes in the joist so much as pulling the lengths of pipe through the holes that i find tedious and maybe minor chance for scuffing barrier and the like.

    i agree 3/8" would be better but i find the best deals on heat transfer plates for 1/2" tubing.

    i haven't been as festidious about insulation under the floor when i am heating the space below.  i have just let the relative density of the subfloor and floor compared to the air, along with gravity take care of things.  maybe that is a little unscientific. but i've never installed underfloor over an unheated space so i haven't worried about it.

    i'm open to hearing the cons of this approach.  might as well know what i'm doing wrong.

    i have done a number of over floor systems over slabs and i use closed cell foam -- as much as i can get in without scraping my head on the ceiling -- before my pipe system.

    and i always leave my legs able to be bled individually at high pressure with the boiler isolated.  i have to laugh when i was looking at a system yesterday with the quick fill hooked right up the boiler.  forgetting that the quickfills never really seemed to give me throughput to get the job done, no ability to isolate lines for purging well.  as long as you kept the drain open while you were you bleeding you didn't build up too much pressure in the boiler to cause the pressure relief to trip, but, of course, that means you never built up more than 30 lbs. to bleed with.

    thanks for your reply.  ,

  • SWEI
    SWEI Member Posts: 7,356
    Heat transfer plates for 3/8" tubing

    should cost the same or less as those for 1/2" tubing do (all other variables being equal.)

    Have you tried using PEX-A tubing (regardless of the fitting system)?  It really is more flexible.

    Plates work by increasing conduction.  Leaving out insulation below the floor is literally throwing away heat.
  • archibald tuttle
    archibald tuttle Member Posts: 1,033
    seems funny but 3/8 plates way more than 1/2

    i think it is because 1/2" has been most used by default.  maybe for loop length or commonality of the pipe and very little discount on 3/8 pipe as well,. tthere are no bulk marketeers I can find who have good deals on 3/8" plates.  you can't even find 3/8" plates on ebay.  pexuniverse has one of the best standing deals on 1/2" plates at $136 per 100 2' plates, free shipping. they don't even list 3/8" plates for any price.

    the cheapest 3/8" ones i found listed were $80 more than that and 5" shorter each length. so you are getting 20% less so the equivalent length would actually be double the price for 1/2"

    other than that you mostly find 3/8" in the even more expensive extruded style.

    if i'm missing the bargain basement on these, by all means point me in the right direction.  i prefer 3/8 nom. for installation and management convenience even thought there is very little discount for using the smaller pipe despite significantly noticeably less plastic used per foot. but at some point i just realize i'm swimming against the stream.

    on the conduction question.  i always understood the physics to suggest that because the aluminum plates work via conduction, and because they are in contact with the flooring over the assembly along much of their length, that conduction to the floor is much more effective than conduction to the air below the assembly.

    on top of that, if the space below is heated as well and the joist cavities are not wind tunnels for outdoor air or some such, i don't see how you loose any heat.  some might be conducted through the air down to the ceiling assembly for the floor below but that isn't lost, it would radiate from there into the home. i just haven't experienced it as that important.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    edited October 2013
    Directional heat flow...

    Brian, if (and that is a BIG if) the heat load is equal on both planes of the RFH panel, then no insulation would be necessary.

    With that said, I get about 3 calls a year that I can diagnose their problem without a site visit. Conversation always starts out with"we ca'tn keep the main floor level comfortable when it gets cold outside , and oh by the way, can you install air conditioning in our basement?"

    I know immediately that someone took off on the " but heat rises, doesn't it?" road and mother nature is teaching them a valuable lesson in radiant energy transfer. It was usually caused by the unknowing GC who doesn't usually place insulation in floor joist bays because he is use to dealing with forced error distribution system, where his thoughts (hot air rising) is true and he is looking for ways to cut the budget to get the job to fit the estimate he is responsible for developing. That's a whole different story...

    If you want precise control, you will insulate. Part of the problem is the lack of conversation with the lady of the house who has had intentions all along of placing really thick wool woven Indian rugs on the floor, and didn't tell the RFH contractor, nor the architect of her intentions.

    We have provisions in our proposed USEHC Code that will require the placement of insulation behind all active radiant floors, walls ans ceilings to control this very situation. If you don't follow the Uniform codes, then it may not apply to you, but we are at least making an effort to address this ugly problem. I've been in some very beautiful homes that had basements that were TERRIBLY uncomfortable (hot) because someone yanked the insulation package, or it wasn't specified in the first place, and the level above is under heating ...

    I am sure others have experienced this same condition.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • archibald tuttle
    archibald tuttle Member Posts: 1,033
    thanks for helping me rethink

    so it isn't a heat loss issue per say, but a control/comfort/zone issue.  that i can relate to. and we could discuss the relevance and flexibility for floor coverings although the reason for the underfloor in the first place is desire for refinishing existing wood floor and this guy has spare tastes in decorating, although wife might want an oriental here or there.

    let me ask, since he is getting spray in foam from inside of outside walls, he could install and test legs before spray foam job and then ask for modest (how thick you think) sprayed coat under to provide insulation between floors.  probably not going to add so much because modest volume and doesn't need to be placed to precise level.

    any problems with spray foam as choice, i noticed early recommendation for sprayed cellulose, but since we're already bringing in foam and to keep the best ability at zoning and the like available while the joist bays are open i could make an argument esp. if we can use the same crew same day and get good price.

  • SWEI
    SWEI Member Posts: 7,356
    Spray foam and PEX

    Check with the tubing manufacturer for materials compatibility before you make a decision.
  • Gordy
    Gordy Member Posts: 9,537

    Spray foam can create problems with out some kind of break between the tubing and plates. The foam can creep between tubing, and plates breaking contact to the floor.
  • archibald tuttle
    archibald tuttle Member Posts: 1,033

    i had a friend who just had the walls done and they foam wires right in place.  think those jackets are PVC.  i think there may have been a couple pex runs as well.  get that there are solvents involved. have to look into this a little further.


  • archibald tuttle
    archibald tuttle Member Posts: 1,033
    very good point . . .

    althoug, i would think careful plate install would over come this phenomenon.  could be a case for using 4 foot rather than 2 foot plates to limit foam in from the ends, or maybe silicone exposed omega before foaming? 

    that said, i can see why cellouse might be desirable in this application. more to chew on.

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