radiant question

Discussion in 'Boiler Forum' started by phillymaster, Nov 16, 2009.

  1. phillymaster

    phillymaster New Member

    Messages:
    5
    Location:
    Philadelphia PA
    newbie here.

    i just put radiant heat on the my first floor and i love it. i put 3 zones in. two of them are less than 100 square feet, and one is about 500 square feet. my 2nd floor has old fashioned radiators, 3rd floor has nothing right now.

    anyways, the rooms warm up nice (and i haven't even insulated the joist bays yet :)) but i noticed when i go in the basement and feel the pex loops (i put in pex between the joists with transfer plates), all of them get fully up to temp except for the return on the larger loop. my house is almost 100 years old. it's a pretty darn long loop for the living/dining room. it doesn't really bother me that much, but i'm curious as to why that return isn't heating up fully on the one loop. i'm using a taco mixing valve and a 007, with 3 zone valves. i'm not using a manifold with metering gauges because i figure that the 1st floor will be self-correcting, although some zones may heat up a little quicker. but even when i only run the big zone by itself, the return only gets up to probably 100 or so, whereas it's supply is 140ish. the other zones get even return and supply temps. maybe it's losing some heat as it gets to the end of the loop?? maybe insulating the joist bays with reflector and r19 would help?

    i guess this long loop is just too far away from the boiler (in linear pipe)??? maybe it should have been two loops?

    thanks for any input!
    Last edited: Nov 16, 2009
  2. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    600 sq ft of residential with one loop is a poor design especially with plates. Who did the design? Split up the loops, lower the water temp to get an even and efficient system.
  3. phillymaster

    phillymaster New Member

    Messages:
    5
    Location:
    Philadelphia PA
    everything seems to work well. it's a pretty simple system. feels like it heats up evenly from above. i designed it but am teaching this stuff to myself right now so that's why i'm posting here to get some help. can you elaborate on any of what you wrote, or give some more recommendations? i'm guessing i should have one loop for each room? what's the maximum sq ft for one loop?
  4. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    600' with 1/2" is way way too long. I like to keep them to 200' or less.
  5. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    Loop lengths are designed based on way it is attached, flow rate, water temp, floor material, thickness of floor, tube size and a few other factors besides heat loss and room size.
  6. phillymaster

    phillymaster New Member

    Messages:
    5
    Location:
    Philadelphia PA
    cool. so what's the ideal temperature for pex with plates between joists? the floor boards are 7/8" pine. 120? what about in the back room where i have 3/4 plywood and 1/2 hardibacker, and then 1/4" tile? does that require hotter temps? thanks again!
  7. patsfan78

    patsfan78 Web Development | HVAC

    Messages:
    33
    Location:
    Maine
    The more flooring material you have between the pex and "your feet" the higher the water temperature may have to be for that loop.
  8. Dana

    Dana In the trades

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    2,864
    Location:
    01609
    ... and the water temp required to actually heat the room is a function of the heat loss in that room, and the temp it takes to get that much heat into the room, which is affected by the R-value of the material above the PEX, the type of heat distribution plates you have to get it from the PEX into the sub floor, and the total flow rate, etc. For under-floor installations heavy extrude plates work substantially better than thin sheet-metal versions and can be run at lower temps. But you can get the heat into the room at even lower temps if you use an above-the-subfloor approach with distribution plates (like Warmboard, etc.) But there is no "ideal temperature"- it's all application specific.

    With plated under-floor systems the performance improvement you get out of radiant barrier is less than you get with filling the radiant-gap you would have needed with the RB with more insulation. Radiant barrier is a waste of time in this application- the emissivity of aluminum is very low, and you get very little downward radiated heat flux- it's nearly all convective & conducted. Radiant barrier also stops the convection, but the convection is also quite low in warm-side-up situation. You're better off snugging up the batts to the plates, but using denser &/or thicker batts.

    R19 is the minimum for over cool unconditioned (but unvented) space like basements, but R10 is fine between heated spaces. Over ventilated crawlspaces, go R30+ (or better yet seal & insulate the crawlspace with R10 min rigid unfaced XPS or EPS foam board at the walls, and go R19 under the radiant.)
  9. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    "cool. so what's the ideal temperature for pex with plates between joists? the floor boards are 7/8" pine. 120? what about in the back room where i have 3/4 plywood and 1/2 hardibacker, and then 1/4" tile? does that require hotter temps? thanks again! "

    It's not about the water temp its all about comfortable and even floors with the surface of wood floors shouldnt be over 78 degrees.

    Design is required for the correct answer with windows, insulation, exposures, infiltration and a few other factos known
  10. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    I have seen water temps for staple up run as high as 180. Very very often, staple up alone will not be enough to heat the envelope and supplemental heat will need to be added. There needs to be an understanding that though radiant allows you to run slightly lower room temperatures because of the "radiant effect" if the envelope requires say 30,000 btu's it makes no difference how it gets there. Radiant is not necessarily more efficient than say copper fin tube is. In fact, when you take cost\payback ratio into account is is markedly less economical. What you are paying for is comfort. Radiant installation costs can and do often run 3 to 4 times what a conventionally installed system would cost. I love selling radiant jobs. The money is very good but in all honesty, I would talk my friends and relatives out of it. Oh and as an aside. Radiant is getting a whole lot of press in the trade publications and such, but the truth is that it has been around for over 80 years. It is not new technology by any means and the basic engineering principals have been around a very long time indeed. It's like the saying "Everything that is old is suddenly new again" BTW the trend is swinging back to high mass, cast iron boilers. Imagine that
    Last edited by a moderator: Nov 22, 2009
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,053
    Location:
    New England
    If you ever have a chance to visit some old Roman bathhouses (there's a neat one in Bath England), you'd see that radiant floor heat has been around a lot longer than 80 years or so. Course, burning a fire under the floor has other problems, and modern plumbing and boilers didn't exist back then, but radiant heat has been used for a couple of millenia.
  12. Dana

    Dana In the trades

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    Location:
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    There's NO payback on radiant, beyond sheer indulgent luxury. It's the cushy factor, nothing else. If you're doing it for cost savings, fuggedaboudit!

    Fin tube baseboard will heat up the room, and yes it's pretty comfortable, and with enough linear feet you can run it at condensing temps to squeak the last little bit of efficiency out of a mod-con... But cruisin' in yer socks in a 68F room heated by fin tube on design day just isn't the same as with a radiant floor, eh?

    Whether a staple up will deliver design-day heat at any temperature is a matter of the design day heat loss & the radiant layout. In a leaky uninsulated wreck of a house in MN it probably won't get there without cooking the floor, but in TN it probably can, even in a wreck. Radiant in a super-insulated Net Zero Energy type house certainly will, but it's also not especially more comfortable than anything else, since the floor temp that deliver the heat are so low that it they're not perceptibly warmer (no cushy feeling on the bare feet there!) Moderately well insulated houses with reasonable design-day heat losses per square foot (say 10-25BTU/ft^2 ) yes, it can usually deliver the heat at modest water temps, and yes, it can be noticeably more comfortable under foot.

    But there's no such thing as payback on it- only luxury-factor. The payback is purely subjective. Whether it's worth the price paid is in the eye (or toes? :) ) of the beholder.
  13. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Yep, but I never turn a radiant job down. Very good for the wallet too. :eek: Besides, you get to play with all that cool stuff. :D
  14. Dana

    Dana In the trades

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    Location:
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    As long as the customer is satisfied, eh? ;)

    I like living with radiant too (don't mind payin' for it either), but I sure wouldn't be installing it a rental or some place I was gonna flip in a coupla years.
  15. phillymaster

    phillymaster New Member

    Messages:
    5
    Location:
    Philadelphia PA
    we charge a lot to install it but it really doesn't cost us much at all to install. i feel like i'm saving a ton on my gas bill too. my boiler is almost always off on limit while the radiant heat is on. i yanked the rads on my first floor and put 3 zones in. i couldn't be happier. it works VERY well.

    thanks for the tips guys. i did split the large zone into two loops and now it works beautifully. yes, there are some things i would have done differently but this house was a rush job and i'm just glad it's all done.

    only problem now is i never want to turn my heat on upstairs, lol. got the radiators up there. all i wanna do is hang out downstairs now.

    here's some pics of the boiler. it came with the house but i did a little work to it as you can see. i'm not done yet. still wanna clean up the wiring.

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