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Are there efficient in-slab radiant systems that also cool?

Timothy Smith | Posted in Mechanicals on

A client is getting bids from HVAC contractors to design/install heating & cooling systems for a 3100 SF single-level home located in the high desert climate of southern Utah (think Alburqurque, NM). The bids are quite high, perhaps due to the fact everyone says the client will need TWO complete systems; one to heat, one to cool. The bids all specify in-slab boiler-fed hydronic radiant heating coupled with a standard forced air furnace with a conventional AC unit for cooling (and I guess back-up heat). A friend keeps saying: “Screw that! Just use the radiant system tubing to cool by running cooled liquids thru tubing in summer!”

My Question: is it as simple as that!?!

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Replies

  1. Aj Builder, Upstate NY Zone 6a | | #1

    The described system is standard high end for standard days past insulation.

    Cooling a slab is tricky and much less effective verses an air system.

    If the entire project was redesigned from scratch to superinsulated specs, split airs might be an option.

    Ultimately you are paying for a very nice rather large home.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Timothy,
    The usual problem is condensation on the cold floor. In most U.S. climates, if you are living on a slab floor, and you run cold liquid through embedded pipes during the summer, you get a cold slab that acts like an aluminum beer can. You get lots of condensation on the floor, and people slip in the puddles and sue their HVAC contractor.

    If the climate is very dry, and you install a big dehumidifier to address indoor humidity on the occasional humid day, these systems can work. But they are tricky.

  3. Chris Koehn | | #3

    Considered this a number of years ago for a build in 4b (cold winters, humid summers). Horizontal ground loop geo was a given, and primarily for heat. In the summer, we set up the system to run in reverse, but sent the cooled water to a water-to-air heat exchanger and distributed via an air handler. Worked well. Operating costs were low, front end costs relatively high.
    Cool floors are not comfortable to live on and are subject to condensation (but maybe not in Albuquerque).

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Chris,
    More information on hydronically cooled floors:
    http://www.radiantprofessionalsalliance.org/i4a/pages/index.cfm?pageid=140

  5. Aj Builder, Upstate NY Zone 6a | | #5

    Fantastic link Martin. Sounds like just having cooler surfaces but not condensingly cold works well to feel cool via radiational cooling. Like how cold winter windows cool us.

    Still, expensive verses a split air or a ceiling fan(which is what I enjoy).

  6. Chris Koehn | | #6

    Good link Martin, thanks. Sounds like a "cadillac" system would include both. Ahh, to be independently wealthy..

  7. User avatar
    Michael Chandler | | #7

    I did this ONCE in NC
    it worked okay until the day the client left the cooling on and the windows open on a hot sunny morning and was in town when the weather changed and a storm came in. When she got home the floors were wet, looked like the pipes had sprung leaks as it was a finished polished concrete slab so the water condensed IN the concrete (slightly below the surface) and bled to the surface as the condensation developed.

    It wasn't that big a deal, she closed the windows and turned on the ceiling fans and it all settled out pretty quickly but the main issue was that the cooled floors didn't de-humidify at all so she ended up installing a couple of small through the wall AC units to keep indoor humidity down.

    The whole thing was VERY low end, a 24 volt relay triggered a 110 volt zone valve that allowed water to flow from the well into the hot water injection port on the radiant floor system and out to a hose bibb that fed submatic drip irrigation in her garden through a rubber disk with a 3/16" hole drilled through it for flow control. the heat was provided by a gas water heater. The house cost $100 per sf.

  8. Kevin O'meara | | #8

    Have done radiant cooling using a water cistern that is below grade, so typically about 50F, similiarly dry envirornment, Park City, Utah. We only have about 2 weeks a year when we really need it. Ran it during a busy opne house near labor day. Outside temps in the low 90's, windows open. No condensation, even on the ground floor which is slab on grade, bare concrete floor. I also know of aleast one similiar install in SLC, taht just finished and is awaiting it's first Summer, which is much hotter, closer to what you are desgining for. In both of these projects we used a 5000 gallon cistern fed from the roof. The heat exchanger loops, an extra valve and a controller were the only additional expenses as the radiant heat system was already in the works

  9. Timothy Smith | | #9

    Thanks for all the comments. My conclusion is that while there ARE such systems available, there is certaintly not a cost savings involved over the type of dual system my clients' HVAC guys are quoting. Plus, it appears to me that if I was to incorporate radiant cooling, the best way would be to use radiant ceiling panels....Thanks Again Everyone for your insight & sharing!

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