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Pex spacing

I’m having a new home built. All slab 40x80 footprint. My contractor installing the 1/2” pex is placing it on 24” centers. I asked him about it and he said he’s done several like that and it always works fine. Should I be concerned? Will it just take longer to warm the slab up then work ok?


  • Alan (California Radiant) ForbesAlan (California Radiant) Forbes Member Posts: 2,351
    edited August 2019
    Rule of thumb doesn’t always work. What is your finished floor (bare concrete, hardwood......), city and state, how good is your insulation, size and type of windows, ceiling height, etc.?
    A responsible professional would do a heat loss analysis.
    A wider spacing is cheaper to install, but may force you to run higher water temps. Higher water temps. will reduce boiler efficiency.
    Often wrong, never in doubt.

    Click here to learn more about this contractor.
  • rickdavis81rickdavis81 Member Posts: 6
    Bare concrete. Southern Missouri. It’ll be well insulated. R21 in the walls and going to shoot for close to r60 in the ceiling. A couple of quality windows per room. So maybe 20 windows. Part 8 and 9’, part 10 peaked to 18’ and the garage will have 12’ ceilings. Going to run it off of a wood boiler with a heat exchanger in the backup furnace ductwork in case it needs help heating the partial 2nd floor living quarters.
  • nibsnibs Member Posts: 462
    Make sure you insulate under the slab, even in southern climes.
  • rickdavis81rickdavis81 Member Posts: 6
    We installed slabshield. I’ve heard maybe I should insulate around the perimeter of the slab too?
  • GroundUpGroundUp Member Posts: 990
    While the 24" centers may meet the heat load, you're not likely to enjoy the striping that occurs on the bare concrete with your bare feet. It'll be toasty, possibly even uncomfortably hot directly above the tubing while it's less than favorable a foot away, halfway between. We have very limited information on your project, but if the contractor's excuse is simply "because it worked last time", you may want to start looking for a new contractor. That layout will give you 1600 feet of tubing, and with a standard 20 degree delta T you're only looking at about 45,000 BTU of output total- not all of that will go into the building as you'll lose some to the ground both under and around the slab, so a well insulated slab is really important here. With 20ish windows and a ceiling high enough for a 2nd story, I'd be willing to bet your heat load is close to double that on a design day in MO. That would mean either much higher supply water temps and larger delta T, or more tubing to obtain the necessary BTU. More tubing will make for a more even/comfortable heat, as well as lower water temperatures which also means higher boiler efficiency. "Rule of thumb" is typically 12" OC but a proper heat loss calculation is the only way to determine that. Most would prefer 6-9" OC for optimal comfort.

    Side note, running off the wood boiler is another concern. Is this contractor also the wood boiler dealer by any chance? Might I ask what make and model of OWB you'll be using as well as what sort of underground lines will be buried between the OWB and building? These systems are kind of my bread and butter, and the vast majority are very poorly installed so I'd like to try and save you a little wood and corrosion if at all possible.
  • hot_rodhot_rod Member Posts: 13,455
    I would not do a residential slab with tube spacing that wide, even if by slim chance it could cover the load, you will experience wide temptation variation between the tubes. Start up will be slower also.

    Probably less than 800 bucks to tube at 12" oc compared to 24?

    A quick calc on the RadPad, which only goes down to 18" OC.

    1/2 tube at . 5 gpm, 300 foot loops
    At 18" OC about 9 btu/ ft output. Guesstimate 7 BTU at 24". I doubt your load will be that low.

    As others mentioned a heat load calc is critical for a good design, good performance, and high efficiency with lowest SWT.

    Experienced installers could argue for 6" OC spacing:)

    I'm in SW Missouri and even the large truck shops I have done have been 18" at most.

    Ask for his load calc and design.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
    GroundUpIronmanAlan (California Radiant) Forbes
  • IronmanIronman Member Posts: 5,673
    Proper design is ESSENTIAL to a radiant floor. Has a scientific heat loss calculation (like a Manual J) been done? It's the foundation for designing and sizing everything.

    24" OC spacing?? We never go more than 18" on a garage! In living areas, never more than 9" OC.

    You need to get rid of that hack and get a real radiant pro.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • rickdavis81rickdavis81 Member Posts: 6
    I’m planning on the Acme wood boilers. They’re made not real far from me so a few of me friends have em and are pleased with them. I was going to go with their midsized. They only make 3 sizes. 1” pex insulated and in the black tubing buried around 2’ deep.
    He’s my home builder and his building skills are great. But I’m rethinking his floor heat design. We’re not pouring concrete till next week due to rain so I’ll have time to change it up.
  • IronmanIronman Member Posts: 5,673
    Tell me you're not planning on connecting the radiant floor directly to an unpressurized wood boiler.

    Tell me that you ARE using a properly sized heat exchanger between them.

    Tell me that you are using O2 barrier heat pex in the floor.

    Tell me that you ARE using good quality manufactured manifolds like Caleffi, Rehau, Uponor or the like.

    Tell me the the SWT to the floor will be controlled with OutDoor Reset so that the floor won't overheat for hours and hours.

    And please tell me that the design was done by a radiant professional, not a carpenter that thinks all you have to is drop tubing in the floor.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • GroundUpGroundUp Member Posts: 990
    Just beware that if this is the wrapped type of underground piping in a black drain tile casing, you're going to have issues with groundwater infiltration somewhere down the road if not immediately. The groundwater will pull the heat out of the pex and simply heat the surrounding dirt instead of your house. If you haven't already bought and buried it, you might wish to reconsider something like Thermopex, Insulpex, Rhinoflex, or Logstor with a closed cell foam encased lineset inside a heavy jacket. Any of them will last forever and have no chance of groundwater infiltration. How far from the house is the OWB? This is another big thing- assuming your heat load is somewhere above 70,000 BTU (which it most certainly will be if you decide to heat your DHW with the OWB also), 1" pex will be undersized unless you're within about 50 feet of the house. Those ACME boilers need to run hot and have a lot of fluid movement to avoid thermal shock and spot boiling, so for good measure I would recommend at the very least considering some larger diameter underground. Being mild steel and atmospheric, you'll also want to separate the floor loops from the OWB via a water to water heat exchanger and will need a mixing valve to bring water temps down to a reasonable number. They're a lot more complicated than most people think
  • rickdavis81rickdavis81 Member Posts: 6
    I haven’t chose most of the components as I have plenty of time till I’m ready to install it. But yes I’ll be using a heat exchanger. We are using oxygen barrier pex. Planning on uponor. Please explain the outdoor reset.
  • rickdavis81rickdavis81 Member Posts: 6
    It’ll be under 50’ from the owb to the house. I’ll look into the other piping systems
  • GroundUpGroundUp Member Posts: 990
    I'll probably get some flack for this, but I generally find ODR unfavorable in a system like this. A simple 3 way thermostatic mixing valve does the job very well in most cases. If you're able to get your spacing down to 12", setting the mixer to an even 100 degrees has never done me wrong in our -40F MN winters and tighter spacing would allow an even lower SWT. Again, this will be dependent on a heat loss calculation.
  • IronmanIronman Member Posts: 5,673
    edited August 2019
    An outdoor reset control varies the SWT based on outdoor temperature. The colder it is outside, the warmer the water to the floor and vise versa. A properly designed floor shouldn't need more than 105* SWT on the coldest night of the year (design temp).

    The slab is simply a huge mass of rock. It will take along time to get it up to temp and it will give off heat for hours long after the thermostat is satisfied and the pump has stopped. It's called the flywheel effect. If the SWT is not regulated to match the actual heat loss, the slab will be over-heated and will over heat the house for hours.

    Another note on tube spacing: different areas of the house may require different tube spacing depending upon their actual heat loss per square foot. A sun room with a lot of glass will require more tubing (closer spacing OC) than a dining room with one window. This is why a room by room heat loss calculation is needed.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rodhot_rod Member Posts: 13,455
    Another helpful graph comparing tube spacing. ideally we would have as little variations possible across a residential slab, which of course comes down to tube spacing.

    What happens often with tube that is spaced too wide is you end up cranking the SWT. You may require as high a 130F SWT to get mid 20 btu/ ft output. That would be a very uncomfortable slab to walk on, especially barefoot.

    The common range is 80- 82°F floor surface temperature. Somewhere around the average persons skin temperature will be comfortable, if you cover the load.

    Hopefully these will convince you and your contractor to rethink 24" spacing. Do some number crunching of your own to determine the path forward.

    Start with a room by room load calc

    You only get one chance to put tube is a concrete slab.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • nibsnibs Member Posts: 462
    Rick, did my pex last summer, and thank heavens we paid attention to these experts. We insulated well under the slab and at the edges where possible, we spaced our 1/2 pex at 9 inches with a few variations.
    The system works beautifully, tested extensively with bare feet, there are a few cool areas where the kitchen cabs are going to be. Our slab is thin compared to most but the house has very high thermal mass. The knowledge here, made freely available will see you through as long as you study and analyze.
  • KathieKathie Member Posts: 2
    Help. I am trying to find a contractor for a job. Uponor hot water radiant heat. Add on room (on slab) needs to be remodeled into 2 rooms, living and sleeping. There is an adjoining space between this room and the main house (will make into powder room) that could hold the boiler. 2 family members could install but I need a plumber to do the match and install the boiler and the manifolds, etc. Is there anyone out there I could talk to about this? House is in Niles,Michigan. Thank you.
  • Erin Holohan HaskellErin Holohan Haskell Member, Moderator, Administrator Posts: 1,338
    Hi @Kathie, It would be best if you start a new post. Here are some tips on how to do that: Thanks!
  • TAGTAG Member Posts: 216
    I agree in the spacing .. wow ..24" no way. I did 6" on my last one to match the wood floor loops temp.

    9" would be my max and 12" for a garage. Warmboard does 12" -- but with AL.
  • John RuhnkeJohn Ruhnke Member Posts: 880
    edited August 2019
    I am against using slab shield. The reflective properties of foil do not work under a slab like that. It has been proven so in a few studies some people have brought up here on the wall in the past. So you wont get the claimed extra r value they make. The slab shield isn't thick enough to provide the needed insulative properties. I would recommend Dow Blue Board at 2" thick.
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
    ZmanCanuckerPaul Pollets
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