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Radiant cooling

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Big Will
Big Will Member Posts: 396
in THE MAIN WALL
Its a sun room and it gets hot. Not a surprise. However it was intended to be an entertaining area so the heat is bothersome to the client. They want me to look into radiant cooling.  The room is 40 foot circle with polished concrete with radiant in the slab. The tube is 12" oc. The whole room is window that arch into the center to make a 14' height in the middle that starts at about 9' on the perimeter. They have a set of four large ceiling fans and operable windows at the top. The humidity stays pretty low in this area for instance today with an outside temp of 73 the room was getting a bit uncomfortable the humidity at the time in the room was at 38% as the day warmed to the peak of 86 deg the room rose t0 92 deg with a humidity of 30%. my hope is with the combined low humidity and ceiling fans moving air across the floor I can get some capacity out of the floor to at least help with the heat. I would think even if the air was still warm but the floor was at 65 deg it would be more comfortable. In the end I am looking for opinions I have never designed a radiant cooling system and have yet to find any decent design guides.

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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Sure worth a try...

    Radiant cooling is like radiant heating. It doesn't directly affect the air temperature. It affect the MRT, and that does drive the bus of human comfort, regardless of wether you are heating or cooling.



    There are automatic dew point controllers now available from manufacturers like Honeywell and Dannfoss.



    Using an air to water heat pump, you could not only cool this space, but could also use the capacity of the HP to base load the heating needs of the system as well.



    Radiant cooling is no longer a pie in the sky concept, but is a proven means of delivering efficiency cooling.



    Here's a link to a Dew Point chart. You could be down around 45 degree F water temperatures, which would probably over cool the slab, resulting in a morgue-ish feeling. The ceiling fans may need to run to keep that feelling from being the predominant feeling, but shouldn't be a problem.



    http://www.buildwithbps.com/assets/downloads/resources/Dewpoint%20Chart.pdf



    Let us know how it goes if they go forward. Forgot to mention loading. Anything you can do to reject the incoming solar energy will be to their advantage. WIndow films, external shutters, internal shades, etc.



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Big Will
    Big Will Member Posts: 396
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    BTU per foot

    Can I use the same ratios based on the difference between the slab temp and air temp. For instance a 60 deg room and 85 deg slab = 50 BTU per foot so does a 50 deg slab in a 75 deg room = 50 BTu per foot?



    And thanks for the help.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited July 2012
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    You're a little off...

    The R.O.T. as it pertains to slab output for heating is based on 2 btu's/square foot per hour per degree F differential between slab and air temperature. But remember, that is just a rule of thumb. In the REAL world, the output capacity of the slab is also significantly affected by other things, like fenestration directly on to the slab, AUST, or in the case of cooling AUCST (new term, bonus points). AUST is the average unheated surface temperature. AUCST would be the average unCOOLed surface temperature. Thermal energy flows from hot to cold.



    Of course, cooling has two segments, and you don't even want to attempt to address the latent loads with radiant slabs...unless you are really in to growing mold beneath carpet and or hardwoods, or creating slick puddles in the middle of the floor. ;-)



    Your radiant cooling slab should be able to handle most of the sensible loads, but to be sure, you need to run the numbers for fenestration, lighting and human loads.



    If the latent loads (humidity) do become problematic, they will have to be addressed via (pick a means) dehumidification.



    EDITION: I consider Geoff McDonell THE go to expert in North America as it pertains to hydronic radiant cooling. Here is a link to one of many articles he has penned on the subject.



    http://www.industrialheating.com/Articles/Feature_Article/09e0e5e8ab298010VgnVCM100000f932a8c0____



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Big Will
    Big Will Member Posts: 396
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    we are fairly dry

    which will help with the latent load. However the room is literally all glass. The load in reality is huge.I don't expect to be able to actually meet the true building load. They open the upper windows and the doors around the perimeter every day and even with that the space gets hot in comparison to outside. My goal is to improve the level of comfort by absorbing  the heat radiation off the occupants and making it feel cooler than the air temp actually is.



    In regards to a load caulc. I don't even know how to figure for that much solar gain. I use the standard manual J sheets and they ask which way the window faces so I guess I could come close by treating it like a glass square of the same cubic area and same square feet so that I could set up the windows facing directions.
  • Big Will
    Big Will Member Posts: 396
    edited July 2012
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    Dan foss

    Makes a wireless control that acts as a thermostat and a dew point control using a infrared sensor that picks up the floor temp. It would be perfect as there is no reasonable way to run a wire into the space for a thermostat of a floor sensor. One problem they wont sell it in the US due to its radio frequency. The infrared incorporated in the stat is really neat though. I love innovative controls like that. I wonder if they would sell it in Canada. I have friend there.



    Here is the link to the product although you may have seen it already.

    http://heating.danfoss.com/xxTypex/180157_MNU17463662_SIT54.html
  • Paul48
    Paul48 Member Posts: 4,469
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    It

    seems a little unrealistic to think you could provide enough cooling to a slab in the direct sun.I know it's a sunroom, but wouldn't it make more sense to shade it based on slab temperature.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Don't worry about the floor temperature...

    Worry about the fluid temperature, because if you don't, you will have pipes dripping in places that can't be seen and will support bacterial growth. Control the supply water temperature, not the emitting (absorbing) surface temperature.



    The other hurdle you have to over come, is the possibility of a fast moving thunderstorm that can raise the dew point significantly in the blink of an eye. With a concrete slab, as Geoff pointed out in his articles, they don't change temperatures real quick, hence the recommendation that the slab not be run as cold as possible, because it may become necessary to actually raise its temperature in order to avoid condensation production. Give yourself some buffer room...



    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Big Will
    Big Will Member Posts: 396
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    That makes sense

    Fortunately the mains and all of the tubing are in the slab until they daylight near the mech. room so I can add pipe insulation were necessary.

    Regarding the thunder storm I won't say never because occasionally we do get aggressive weather but I cant remember the last time we had a sudden storm thunder or otherwise. We don't get weather extremes  we get earth quakes.
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