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PEX Size (SA/V ratio)

The application: Under tile kitchen floor constructed of T&G pine subfloor, OSB (ca. 1/2" ?), Hardybacker board, 12" ceramic tile in Wilmington, DE, USDA Hardiness Zone 7.

Presently there's a baseboard heater that will be removed. I need to cut off all the nails that poke through the subfloor and then install PEX for under-floor heat.

This will be my first install of this type. I see that 1/2" seems to be the most popular, but I'm wondering what the effect of running 2 loops in every bay of 3/8" PEX. Because of the greater surface area/volume ratio, it seems like more smaller dia. tube would be better, but I haven't found any examples of this type of application.

Input appreciated.

Comments

  • hot_rodhot_rod Member Posts: 14,425
    I like 3/8 for that installation, easier to install. 250' per loop max with 3/8. You want two tubes per bay regardless of the tube diameter.

    I've found a die grinder with a cutoff wheel to be the fastest by far for cutting nails, and easy to get tight against the plywood. Air or electric versions, very inexpensive Harbor Freight versions are fine for a one job use. Buy a stack of extra discs :) Cover up will and use a face shield, ear plugs dust mask, etc.
    Watch for sparks around flammable materials!
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ewangewang Member Posts: 56
    Oscillating tools with bi-metal blades also work and throw no sparks.
    mattmia2
  • iconoclastheroiconoclasthero Member Posts: 4
    hot_rod said:

    I like 3/8 for that installation, easier to install. 250' per loop max with 3/8. You want two tubes per bay regardless of the tube diameter.

    I've found a die grinder with a cutoff wheel to be the fastest by far for cutting nails, and easy to get tight against the plywood. Air or electric versions, very inexpensive Harbor Freight versions are fine for a one job use. Buy a stack of extra discs :) Cover up will and use a face shield, ear plugs dust mask, etc.
    Watch for sparks around flammable materials!

    Ok, so my question was both theoretical and practical...but some more details first:

    The floor area to be heated is 8x10-12' depending on where I stop under the fridge and stove...I wasn't planning on heating under the cabinets/sink/pantry. I have what I assume to be an old gravity system with really large trunk pipes in the basement feeding radiators. I have 3/4" Cu going to the soon-to-be-removed baseboard with ball valves on both sides... Now, it looks like 3/8 inch gets one vote. Any particular reason over 1/2 inch?

    Now, onto the theoretical question. Decreasing the volume of the tube, e.g., by going from 1/2" to 3/8" increases the surface area/volume ratio and thus, theoretically, the greater amount of surface area/mL H2O and ultimately better heat transfer. What I was really asking is could I run double loops of 3/8" instead of say single loops of 1/2" in each bay to get heat radiated faster or more heat? I.e., in the middle of the run of each bay, there'd be four 3/8" tubes...
  • iconoclastheroiconoclasthero Member Posts: 4
    And yeah, I was planning on using an angle grinder. I think it would take forever with my oscillating tool. Not much flammable to worry about under the kitchen.
  • hot_rodhot_rod Member Posts: 14,425
    I I think your tube question is overly complicating things? I’m not sure you will see a measurable difference. One loop down and back with plates, either tube size, will be fine. The plates are doing the heavy lifting with the energy transfer.
    A cutoff wheel works best regardless of the tool. Die grinders spin 20- 24,000 rpm, twice what a 4” angle grinder spins, so about 1/2 the time per nail🔪

    Agreed that some nails and grabber screws are hardened and can destroy saw blades quickly, abrasive wheels do much better and cost less 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Dave H_2Dave H_2 Member Posts: 418
    When it comes to what tubing size you want to use in a radiant floor heating system is irrelevant to the heat transfer. The pex is not directly heating the space, it is your entire floor assembly that heats the space. Sure we can perform a Finite Element Analysis and will be able to see the difference but no one would ever "notice" the difference in the space.
    There is no one here that could walk into a radiantly heated space, stand there and say "Yup, this space is heated with 1/2" pex, not 3/8"
    The surface to area ratio is not the important part, its the ease of installation, size of the space, heat loss......in order to get what you want.

    So I say, go with the 3/8", it is so much easier to install.


    Dave H.
    Dave H
  • You will also have to use a mixing valve and pump for this reduced temperature application, no? And, if you want to zone the kitchen with a thermostat, there are some control issues.
    Often wrong, never in doubt.

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