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Summer Setting, Radient Floor Heat, and TriangleTube Prestige

Thanks for laying out all that thinking for us! I couldn't have done it so well.


  • Ann Willis
    Ann Willis Member Posts: 3
    Summer Setting, Radiant Floor and TriangleTube Prestige

    Last year we purchased and remodeled of a conventional 2 story walkout; the remodel included radiant floor (ceramic) and thermostats on all three levels. TriangleTube Prestige provides domestic water and floor heat. The system was primed and started up in October. This winter we enjoyed even, efficient heat. SE Minnesota Spring and Summer heat/humidity are just around the corner. What is the most efficient radiant floor thermostat setting to prevent condensation/mold in the lower walkout and not compete w/central AC, when in use. We prefer to leave windows open whenever heat and humidity are not outrageous.
  • carol_3
    carol_3 Member Posts: 397

    This is the first time I've thought about the idea of using in-floor heat to cut down humidity in the summer. (I live in a very dry climate.) What comes to mind is that any heat you create in the floors is going to add to the heat load that your AC needs to remove. It seems you might get to the same result by keeping the AC setting a little higher and leaving the in-floor heat off.
  • Brad White
    Brad White Member Posts: 2,393

    the goal is humidity control (and as Carol said, who ever thought of this before?), let me think a bit about what the applicable principles are.

    Firstly, are you controlling the AC system to humidity versus just temperature alone? Some systems have an over-ride to automatically control the greater load of humidty or temperature. If temperature is satisfied but humidity is still high, the unit goes into dehumidification mode, running at low speed and with possible secondary heating to wring out the air. Check that first.

    The second part is, may I assume the goal is to elevate the slab temperature to avoid condensation and to reduce the RH near the slab? There is a narrow band in which to work but it can be done.

    Here are some of my thoughts and the approach I would take:

    1) The radiant floor will only add to the cooling load if the floor temperature is above the ambient (cooling setpoint) temperature. Any lesser temperature will not be an issue in this regard, but you must have the ability to control the slab temperature by itself, specifically as opposed to space temperature.

    2) The dewpoint of the air is critical; anything over 60 degrees is considered "muggy" (but this is subjective and what you might or might not be used to). You want to keep your floor above this level with a margin.

    3) Know the AC conditions you wish to maintain. If say 75 degrees and 50% RH, the space dewpoint will be about 55 degrees. This would be your absolute minimum for slab temperature. I would add about 3 degrees for margin of controllability.

    4) From the above, you could maintain the slab temperature at between 58 and 72 degrees without inducing condensation nor adding heat to the cooling load. Higher would be better, say the 68-72 degree range.

    Keep in mind that, as I see it, the cold slab on grade effect would occur in spring when the earth is still cold and the first bursts of spring/summer moisture come streaming in.

    The downside is, increasing the slab temperature can increase evaporation but if that is the case, it is revelatory of other problems such as inadequate vapor barriers.

    Nothing I am suggesting is irreversable. Try it and see.

    Never did this before nor ever thought of it, but those are the principles as I see them.

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Mad Dog_2
    Mad Dog_2 Member Posts: 4,825
    It would be very helpful if the contractor had installed a slab

    sensor in the mud job. A quick phone call could determine this. If that is the case, slab temp control would be easier to achieve. Mad Dog
  • Ann Willis
    Ann Willis Member Posts: 3
    Condenstion prevention at grade level

    I appreciate your thoughtful response.

    1st- I am familiar with central humidistat AC which we installed in our previous home; however, the current home in question has relatively new standard central AC.

    2nd- Radiant floor heat is part of our major remodel of current home built in '81. Blue prints filed with city show hydrocide mastic to grade w/4 mL poly barrier applied over mastic. There does not appear to be any barrier under slab. The Roth floor w/PEX tubes is enclosed in 3/4 inch, 90 psi dense foam. Roth floor is covered w/ 1/4 inch Den shield to which the ceramic tile was applied. So the floor application may provide some insulation and hopefully not condensation between Roth floor and slab.

    3rd- No heat sensors were applied under the Roth floor. There are manifolds and thermostats on each of the 3 levels. We are impressed the temperature is even and holds steady at thermostat setting; interestingly, a thermometer placed on floor usually reads identical to thermostat display.

    Given the wide variability of SE Minnesota temperature/humidity and in light of your comments, it appears 68-70 F is cheap insurance against condensation and mold when RH is 60% plus. We want to maintain the house as 'green' as possible, responsible w/non-renewable energy use so the line between comfort, condensation, and respondible energy use is at times fuzzy. Where is reference for the formula to calculate dew point which may be relevant for setting grade level thermostat? In past Minnesota summers in other homes we have run grade level dehumidifiers concurrent w/central AC to minimize condensation and subsequent mold at grade level. You may have guessed asthma is primary basis for mold prevention here.

    Thank you again,
  • Brad White
    Brad White Member Posts: 2,393
    You are welcome, Ann

    You are exactly right about the magic number of 60% RH and mold. (For other readers, that is near the threshold at which spores of certain fungi -mostly of the mildew variety- can take the moisture they need from the ambient air without the presence of sensible moisture.

    I have good things about the Roth system and appreciate your observations.

    A slab thermostat may still be applied by drilling and inserting the sensor in a discrete corner of the tile floor, perhaps near a cabinet toe-kick or where concealable by trim.

    Might you consider a solar solution for tepidly hot water for this summertime function? As long as you are going green, that is one asset I would aquire.

    To your question about calculating dewpoint, the answer is a psychrometric chart. There are some available on-line. I have pre-printed ones and programs that map them (Linric.com is one resource). Absent buying the program, see what you can find on-line. If you come up empty, I can send you one by snail mail with instructions on the back, if you contact me off-line.

    Essentially, the psychrometric chart maps out the temperature of the air along the L-R or X axis, and associated moisture along the vertical or Y axis. Any intersection of two variables will give you a third or more conditions. I can go over this with you if you like.

    For example, at a given temperature and RH, the dewpoint and specific moisture content can be read.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Ann Willis
    Ann Willis Member Posts: 3
    Neat stuff


    I googled quite a few psychometric charts. This is neat stuff! I also purchased a mid-range temperature/humidity weather station which can handle several remote sensors, so in addition to outside and base, I can track high and low for grade level and possibly attic. There is a little low carpeted 'closet/shelf' under grade level stairs which we did not tile that may provide easy access to floor temperature with a probe (as yet to be identified).

    As I studied the psychometric charts I wondered: where does floor temperature (under tile) factor into (chart) calculations? In other words, which psychometric chart variable correlates with floor temperature under the tile?

    Thank you in advance,
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Glad you find pychrometrics neat, Ann.

    I sure do... :)

    To your questions, the floor temperature does not come into play except that it is the controlled medium. You want your floor temperature to be warmer than the dewpoint of your air.

    See the curved line on the psychrometric chart going up the left side? That is the 100% RH or saturation curve. Draw a line vertically from the temperature scale (along the bottom) until it intersects with your measured RH line, then to the left until it intersects that saturation curve. That will tell you your dewpoint. Air at the condition of temperature and RH (or specific humidity) you pinpoint will condense out it's moisture at that point. Hope that makes sense!

  • JJ_4
    JJ_4 Member Posts: 146

    A/C will de-humidify the air as it runs. This is why you generally have to have a condensate pan and drain at the coil. Commerical rooftop units have large pans with a p-trap that drains onto the roof.

    I've had several units where the p-trap gets clogged and the water gets sucked into the ductwork...which of course reverses the de-humidification and makes a mess of the insulation.
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