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radiant cooling via slab

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hot_rod
hot_rod Member Posts: 27,790
in THE MAIN WALL

I turning part of the shop into a ADU. About 750 sq ft, well insulated, just an insulated OH ad man door right now. 4" slab 1/2" tube 6" on center, 250' lengths

90° outside yesterday, 12% humidity in Salt Lake

The system dropped from 80 inside to 75 over the course of about 8 hr. SWT 51°

I turned off the HP but left the pump run through the night.

72° this morning in the space. So the flywheel effect causes some overshoot, as expected.

The loft, 15' ceiling area, was just a few degrees warmer, a shot of the hanging thermometer at 74.8°, which was good to see,. No air movement at this point.

I was thinking a 2 ton hydronic air handler for cooling . Maybe not, just a HV unit to move some air around? It would be nice not to add ductiong with these exposed ceilings.

Screenshot 2026-06-06 at 9.52.03 AM.png Screenshot 2026-06-06 at 9.52.39 AM.png Screenshot 2026-06-06 at 9.53.28 AM.png Screenshot 2026-06-06 at 10.11.04 AM.png Screenshot 2026-06-06 at 10.11.38 AM.png Screenshot 2026-06-06 at 10.12.14 AM.png Screenshot 2026-06-06 at 10.12.55 AM.png Screenshot 2026-06-06 at 10.15.22 AM.png Screenshot 2026-06-06 at 10.17.52 AM.png
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream

Comments

  • Kaos
    Kaos Member Posts: 917

    Hydronic air handlers generally want reasonably cold water. Even in your dry climate, that could mean condensation on lines/pumps/valves.

    I think a bit of radiant ceiling would be simpler and cheaper.

    hot_rod
  • DCContrarian
    DCContrarian Member Posts: 1,494

    My usual advice is that slab cooling won't work because it provides no dehumidification, and most of the time people don't realize how humid it can get where they are.

    But in your case I'm willing to make an exception. You can look up humidity data here: https://ashrae-meteo.info/v3.0/index.php

    The also have a psychrometric calculator that allows you to express the data given in different ways. I'm seeing that your 99.6% humidity conditions are dew point of 60.2F with a mean coincident temperature of 72.4F. That's a relative humidity of 65%. Those are conditions where you don't need any cooling, and if you need to remove humidity you can do it with ventilation. So you can just turn off the cooling when the dewpoint is above the water temperature — and 99.6% of the time it's less humid than that, or about all but 35 hours a year.

    I'm in DC where right now it's 72F, but the humidity is 87%!

    PC7060

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