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Feedback on pex layout in slab

Gods
Gods Member Posts: 15
I'm planning a new build and am currently mapping out the pex layout. Are there any problems that stand out?

5/8" pex, the longest run is 330'


Comments

  • Why 5/8"? You could do it all with 1/2"; much easier to work with.

    Does each room get a thermostat? Even the half bath?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    GodskcoppEdTheHeaterMan
  • Gods
    Gods Member Posts: 15
    edited May 2022
    Would 5/8" allow me to run a more efficient pump? I was under the impression that the bigger the pex, the smaller the pump needed due to the lower pressure drop.

    I actually only plan on a single thermostat for the entire house but wanted to separate the loops for better control and the possibility of future thermostats.
  • Gods
    Gods Member Posts: 15
    edited May 2022
    I should also add, I'm running it with 12" spacing.
  • Three hundred feet of 5/8" tubing at .9 gpm will have 3.3 feet of head.
    Three hundred feet of 1/2" tubing at .75 gpm will have 6' of head.
    Both can be served by a Grundfos 15-58 pump on low speed up to 3-4 gpm plus the head loss of the piping from the boiler to the manifold. And definitely both on medium speed depending on how many other manifolds there are in the house.

    Zoning should be done for different sun exposures, different floor coverings and different usage. If all those rooms are bedrooms, OK, but I'd take a look at sun exposures. East facing rooms will turn off the heating in the morning when west facing rooms are cold. The opposite is more of a concern.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Gods
  • Gods
    Gods Member Posts: 15
    That's a good point. I'll do some more number crunching before deciding.

    Do you have any suggestions on the layout?
  • Gods
    Gods Member Posts: 15
    Correction: longest run is 360' (green loop in the big room)
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,717
    edited May 2022
    I see that you're trying not to get the hallway too hot, but avoid running under walls where it could easily get nailed. Try to go through doorways when entering or exiting a room. There is also the possibility of heating two or more rooms with one loop given the zoning limitations.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    GodsEdTheHeaterManPC7060
  • Dave Carpentier
    Dave Carpentier Member Posts: 543
    A heat loss calculation could let you know if you really need 3 zones in that big room.
    I laid my walls out with tuk-tape on the aerofoil and made sure not to cross a wall when we strapped the wirsbo to the mesh. No way was I going to trust my carpenter to not nail a pipe when he was anchoring the walls.
    Re - eventual floor covering. On our tile floor, we notice slight differences in floor-feel because it conducts so well. On our laminate floors (bedrooms), the differences are much more muted. Apparently carpet is even moreso (but we dont have any carpet).
    Re- small zones. We have one very short zone, a bathroom with about 70ft of line. This short line is the principle reason why I need to use a buffer tank to prevent short-cycling a modcon boiler. If you have all zones tied together, or lumped in groups, this isnt a worry.. but someday if you chose to split off a small zone like a bathroom onto its own control/zonevalve, you might get short cycling.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • hot_rod
    hot_rod Member Posts: 19,846
    Any rooms that will have the doors closed, like bedrooms, may need to have thermostats
    Especially  bedrooms with two outside walls.
    I agree 1/2 Pex with 250- 300’ loops would be much easier to handle
    What about floor coverings? Bedrooms with carpet and lots of furniture can be another reason to zone those rooms.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    EdTheHeaterMan
  • With three full length loops at 12" oc, that room is 1,000 square feet? Is that correct? What is it? An auditorium? My house could almost fit in that room.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    EdTheHeaterMan
  • archibald tuttle
    archibald tuttle Member Posts: 1,034
    edited June 2022
    is this slab on insulation on grade?

    i'll be the first to admit that my methods are more emperically than theoretically supported but I always use at least 2, often 4 loops in each room so if one gets holed I still have the others although I totally agree about going through the doors and reducing the "no nails" marks that are often honored in the breach it seems. And I use 1/2" or 3/8 inch pipe on 6" centers which are readily provided by 6" mesh. generally speaking i'm not looking for high velocity circulation so i'm not that worried that much about the head and plan for the hottest water to go to the coldest spots with the ends of the runs coming toward the center of the home/building. i guess you would have electric savings if you went with similar low velocity circulation with the lower head of larger piping. i'd have to spend some time with a thermal imaging camera or studying the work of others who have to see how efficiently the heat spreads through the mass with the wider spacing, but i tend to prefer smaller pipe because you are getting more wall material per cubic unit of water and my impression is the limiting factor in the all this is the ability of the heat in the water to be transfered into the floor once a relatively steady state is obtained. So smaller pipes are better transfer agents in my thinking by relating are of pipe wall to volume of water.

    i'm totally in agreement with multiple posters regarding 'zoning' relative to wall and sun/compass. again, my zoning tends to be sioux chief valved manifolds with multiple loops per room so I can throttle for comfort. you could use automated valving that responds to photo electric sensing or time of day or other indicia. I don't see using room thermostats given the response time you have to anticipate zone needs. maybe an algorithm using rate of temp change.

    my 2¢. worth what you paid for it . . .

    best success with project which will surely be successful if you even get it half right. at least that's been my experience also.

    brian

    Alan (California Radiant) Forbes
  • Zman
    Zman Member Posts: 7,466
    The hallway is going to cook. It has very little heat loss and will have more tubing than any other space, not to mention a very tight mech room with all that tubing.
    I would suggest remote manifolds with insulated supply and return lines, you can run a control wire with it to allow for future zones. Using tighter tubing spacing on exterior walls with high heat losses is also a good idea.
    A 360' loop should be split in 1/2 with either tubing choice.
    Let's talk insulation, particularly at the edge. What did you have in mind?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,231
    Why do all the manifolds have to be in one location?
    steve
  • Gods
    Gods Member Posts: 15
    A remote manifold sounds like a good idea, but I'm unsure where to place it. Possibly the furthest right closet in the same horizontal line of the current setup. Should the remote line be run in the slab as well? What size supply/return to the remote manifold? I also have the option of running up and through the attic.

    We won't be laying any flooring over the slab. Only stained and sealed concrete.

    As for the thermostat in each bedroom, is it feasible to control each room's temperature with the manifold flow valves? I also considered running the pump/boiler on a timer instead of thermostat controlled.
  • GGross
    GGross Member Posts: 694
    You would benefit greatly from having a complete radiant design done, you seem to want to do this the correct way and are on the right path. I think that LoopCad still offers a 1 month trial. You can do a complete load calculation for each room, position manifolds where you want them, draw your circuits and then connect to the correct manifold. One of the benefits of doing this is that all of the relevant information will be shown, flow rates, pressure drops, loop lengths, heat loss, supply and return piping info for remote manifolds. And it will tell you if you have gone outside of recommended parameters (keep your pumps small). Once you have all of that data, setting up your boiler to run on outdoor reset is a snap and will increase the efficiency of your system to its potential.
  • I also considered running the pump/boiler on a timer instead of thermostat controlled.
    They do that in apartment buildings. Makes people unhappy. Stick to a thermostat.

    How thick is your slab?

    And is that large room truly 1,000 square feet?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Gods
    Gods Member Posts: 15
    4" slab. Would running the pex directly on the insulation (below the mesh) lower the efficiency? That would protect me from nails under walls.

    That room is roughly 32x31, mostly open. Living/gathering space, dining, and kitchen.
  • Gods
    Gods Member Posts: 15
    I'm also constructing the building, so running under walls won't be a major issue since I'll be nailing the inside walls and will be cognizant of the pex layout.
  • Gods
    Gods Member Posts: 15
    edited June 2022
    I may also liquid nail the interior walls instead of nailing them. This is a pole barn style building with clear span trusses, so the interior walls provide no structural support.
  • Gods
    Gods Member Posts: 15
    edited June 2022
    Newer layout according to suggestions here. I also separated the left perimeter wall into its own loops as that wall is mostly windows as well as doubling up on entry doors. Any suggestions?

    I appreciate all of the feedback!



  • hot_rod
    hot_rod Member Posts: 19,846
    Tube at the bottom of the pour will require a bit warmer water7-8 degrees. Ramp up time will be slower, and output a bit less. Try to get it mid slab as best as possible. Map all the tube for future reference.  Liquid nail is a common method for interior walls. 

    Do you have a detail for edge insulation? That can be tricky on pole barn construction. The slab edge is a huge loss area on slab on grade construct 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gods
    Gods Member Posts: 15
    The slab will be filled in after the structure is built, allowing me to line the entire perimeter with 2” EPS. The outer most layer will be the splash plank, followed by insulation, and finally slab edge.
  • archibald tuttle
    archibald tuttle Member Posts: 1,034
    edited June 2022
    as to your new diagram, i like the multiple loops in the big space. I tend to run the 4 loops comingled beside with the feed side of all 4 loops hitting and running along the outside perimeter two legs as directly as possible (obviously one loop is very outside and the others step in according to your spacing) and then u-turning on itself and stepping into the room like multiple lanes in a 4 lane plastic slot car track so the coolest return temps are ultimately the pipes closer to the center of the building. i don't know if this program lays out like that but that's what I have always done.

    brian
  • hot_rod
    hot_rod Member Posts: 19,846
    I see the Euro installers on IG use the counter flow method, mostly. It puts a hot line next to a cooler line and allegedly spreads heat more evenly?

    Tighten spacing on exterior walls of high load rooms is another option.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream