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Max BTU Sq Ft radiant in concrete

bob eck
bob eck Member Posts: 930
How many BTU can you get with radiant pex tubing per SQ FT in concrete without having the concrete to hot or cracking the concrete?
Customer has a home owner building a pool in their back yard and in the fall, winter and spring the home owner will put a removable dome over the pool area and they want radiant pex system installed in the concrete around the pool to heat the pool area.
There are no permanent walls around the pool to install backup panels to help heat the pool dome area.
Pool water will be heated to 88 F all the time the dome is on the pool.
Anyone ever do a job like this?


  • Zman
    Zman Member Posts: 7,606
    Snowmelt around pools is done all the time. Concrete will take almost as many BTU's as you can throw at it, especially if the slab is being constantly cooled with snow. Snowmelt designs range from 100-200 btu/sqft For snowmelt, we try to keep the water temp under 130 to prevent thermal stress.

    Generally, for heat, the rule is that the slab will become uncomfortable on the feet at ~30 btu/sqft. That number is likely a little higher outdoors.

    My gut is that with no walls to block out the wind, you may end up with some thermal comfort issues. Super warm feet with cold wind blowing by your body will feel like a campfire on a really cold night. You won't die of hypothermia but you won't be very comfortable either.

    I am curious how this one turns out. Maybe they could do some canvas roll up walls.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • bob eck
    bob eck Member Posts: 930
    The pool will be in a portable bubble type room but the contractor did not give me any type of heat loss yet. He is working on that. Just wondering if we can get enough heat out of the radiant in the concrete walkway around the pool inside the bubble to make the pool area comfortable to be used in the cold winter days and nights in Pennsylvania. Would be cool if they could build a temporally wall for when the bubble is up that would also have radiant panels in the wall to help put more heat intoned this area.
  • hot_rod
    hot_rod Member Posts: 22,476
    The delta T will determine the slab output. Slab surface- ambient air temperature around it.
    120F slab temperature in a home at 70°, or 120F slab outside at 30F.

    The rule of thumb, 2 btu/sq.ft. for every degree difference. So basically a home slab at 82F surface temperature, in a room with ambient 70F would be 12X2= 24 btu/ square foot output. That is a barefoot friendly slab surface temperature, lower the ambient to 68F 14X32= 28 btu/sq.ft. Mid 20's btu/sq.ft.i a reasonable expected output for a bare concrete resi application.

    Obviously outside you have a much larger delta at play and 150, 175, even 200 btu/sq.ft is possible.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Any time tounwnclose a pool, especially with 88f water (that’s hot, multiuse is Normally 83-85) you need ventilation or it will
    Rain and or mold in there. Need constant airflow across the walls.

    So I’d suggest having a boiler that uses
    a hot water air handler (can form a plenum in the pool deck for a clean install) to supplement the radiant floor.

    In a commercial radiant install a few years back I was told the upper limit was 130f water temp. So concrete would be maybe 120 which is pretty warm on bare feet.