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Community and Q&A

Radiant Heating and Cooling in a Hot-Humid Climate

Xeven | Posted in Mechanicals on

My wife and I are currently in the planning process of building our new home.  We’ve decided to move forward with an ICF house, especially with the price of lumber currently.  As such, we have also been looking into radiation heating/cooling instead of the conventional forced air systems.  Some people I’ve spoken with keep saying that radiant heating is not not a good idea to do in Texas due to the thermal mass of the concrete, stating we’d basically bake ourselves out of our own house.  Maybe I’m thinking of this wrong, but with the way the ICF house is built, wouldn’t it keep the temperature of the house relatively stable regardless of how much heat the concrete is keeping?  It just wouldn’t have to “work” as hard, or am I looking at this incorrectly?

Secondly, would radiant cooling be a viable option to do as well?  Condensation is obviously the biggest issue, but wouldn’t the ICF hold this more constant versus the traditionally stick build?

Thanks in advance for any assistance on this.

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  1. user-2310254 | | #1

    If you build a "pretty good home" (ICF or conventional), the heating and cooling loads will be much lower. It often seems that people want radiant heat for that toasty toes feeling. You are not likely to get that in a tight house since the system won't run that much (especially in Texas). With radiant cooling, you will need to be careful about managing indoor humidity levels.

    FWIW. It's likely you could save a bundle -- and have equal or greater comfort -- by installing a properly sized forced air system.

  2. GBA Editor
    Kiley Jacques | | #2

    To address the second part of your question, take a look at this article: Does Radiant Floor Cooling Make Sense?. It addresses considerations related to condensation, which, as you note, is a major factor with radiant cooling in your climate.

  3. Expert Member
    NICK KEENAN | | #3

    Put all thought of "thermal mass" out of your mind, it is irrelevant. Nowhere on the Manual J does it ask you to weigh your home and input the value.

    The reason they call it "air conditioning" instead of just "cooling" is that the AC has to do two things to keep you comfortable: remove heat, and remove humidity. In fact, an overly humid house is not just uncomfortable but can also be unhealthy. Hydronic tubing can be an excellent way of removing heat, but it won't remove humidity. But if you have a device that removed humidity hydronic coils can be used for cooling.

    The only practical way to remove humidity is to run humid air over a cold surface and collect and dispose of the condensation. That's what an AC does and that's what a dehumidifier does.
    I'm not a fan of using dehumidifiers in cooling conditions because they are basically highly efficient space heaters, they introduce a lot of heat into the space.

    If you want to go down the hydronic route, you should look into hydronic fan coil units. These are units that have a blower that blows air over a radiator that can have either hot or cold water running through it. When the water is cold they also have provisions for collecting condensate. Basically you would have a mixture of floor tubing and fan units to provide the mix of humidity and cooling you need.

    This system needs to be engineered by someone who knows what they're doing.

  4. Expert Member
    BILL WICHERS | | #4

    The only issue with thermal mass is that it slows down temperature changes. If you want it to heat up quick, too bad -- the thermal mass FORCES you to wait a while. If you want it to cool down quick, same problem -- the thermal mass makes it take a while. The absolute temperature doesn't matter, thermal mass doesn't make things get any hotter or colder, it just makes things take longer to get hot or cold.

    Radiant cooling doesn't do any real dehumidification. I say "real" because the water DOES condensate out on the cold "radiant" surfaces, but it stays there and makes them wet. I wouldn't try this in a humid climate -- you need something that can REMOVE moisture from the air and send the condensate away, which pretty much means some kind of air conditioning system. You can accomplish this with either a traditional air conditioner and air handler, or with minisplits.


  5. charlie_sullivan | | #5

    Everyone is pointing out that you need a combination of dehumidification and cooling in the summer. If you build a really well insulated house, that reduces the amount of cooling you need. But it does not reduce the amount of dehumidification you need. So you are left with a high ratio of how much dehumidification you need vs. how much cooling you need. In technical terms a high latent to sensible ratio. If you had a *low* ratio, you could get some benefit from radiant cooling. But with your high ratio, the amount of useful cooling you can do with the radiant system is near zero. So it's pointless to put it in, especially given the special controls that would be needed to avoid condensation.

    You could do the heating with radiant. You'd need good controls, but there's not much reason to put that in--whatever system you use for cooling, whether it's central ducted, ducted or ductless minisplits, or hydronic with small fan coils, will provide plenty of heat, and the good envelope will give you plenty of comfort.

    Note that concrete makes sense now in terms of lumber prices, but makes less sense when you consider CO2 emissions.

    1. Expert Member
      NICK KEENAN | | #6

      If you can reduce air leakage the latent load goes way down, it becomes essentially whatever the occupants generate.

      I had a Manual J done recently for new construction in Washington, DC -- which is a hot, humid climate -- and on the design day the sensible heat ratio (SHR) was 93%.

      A hydronic fan unit with the water at 45F gives a SHR ratio of 70%. (The colder the water the lower that goes.) To get an average SHR of 93% you'd have to have 25% of your cooling from the fan units and 75% from radiant coils. So that's not insignificant at all. If you're going to go down the road of having a chiller it's not unreasonable to say a quarter of your capacity goes to the fan unit and three quarters goes into the radiant coils. And in the winter the fan units can supplement the coils.

      The problem is the control mechanism, there isn't something off the shelf you can buy that will just work. What you would need is both a humidistat and a thermostat and then control logic that senses where to send the water.

      1. Expert Member
        BILL WICHERS | | #7

        There are off the shelf systems for this that are normally sold into commercial systems. The problem for a typical residential system is that the commercial systems are complex, expensive, and require programming to make them work. (there are specialty contractors that do only control systems). BACnet is an open standard for these controllers. Honeywell makes many options, as do others.

        The neat thing with hydronic cooling systems is that you can adjust both the airflow over the coils and the temperature of the coils (by adjusting the chilled water temperature), to optimize the entire system for optimum efficiency. The newer datacetners do this on a big scale. We put temperature sensors in equipment racks that tie into baffles or sometimes fans on the vent tiles on the raised floor (the entire under floor area is pressurized with cold air). The air handlers sense the static pressure under the floor and ramp their blowers up and down to maintain a constant pressure. The chillers usually maintain a constant water temperature, but each air handler has a bypass valve allowing it to control it's own coil temperature. Heat is ultimately dumped to a big evaporative cooling tower with a fan that -- you guesed it -- ramps up and down with temperature.

        The really cool thing is that with modern servers that have processors that can ramp up and down with system load, the actual compute load on the servers affects the temperature of the rack. The temperature sensor adjusts the cooling airflow per rack to maintain a constant rack temperature. The rest of the cooling plant adjusts to keep the under floor temperature and air pressure constant.

        The end result is that the entire building cooling plant ramps up and down based on computer processing load on the servers. If you ever wondered how "cloud" computing improves overall system efficiency, this is one of the big ways that is done. Cool stuff -- literally :-)


        1. Expert Member
          NICK KEENAN | | #8

          What I like about hydronics is how much tinker-friendlier it is than other types of systems. Which can be a curse.

          1. Expert Member
            BILL WICHERS | | #9

            Hydronic also allows for the source of input heat to come from just about any energy source. I don't know of any other heating system that allows for relatively easy changes between different energy sources without a complete replacement of the system.

            In commerical projects, I just contract out the controls and everything gets automated. I also always require some logic be in the control system that does something along the lines of "I've put in a lot of makeup water over the past few hours/days/whatever, that probably means there's a leak somewhere. I should beep to tell people there is a problem somewhere". You'd be amazed how many times I've seen systems that happily add massive amounts of makeup water without alarming, and later we find a big leak somewhere (often somewhere difficult to access, like a utility tunnel). A simple logic routine to alarm with excessive makeup water would minimize this issue.


          2. Expert Member
            NICK KEENAN | | #11

            In my area it's the norm to leave the makeup water valve open on boilers. I think that's nuts. If my system is leaking, I want to know about it. If you close the valve, you'll realize you've got a problem when the upstairs radiators stop putting out heat. If you leave it open you'll realize when your boiler rusts out.

        2. Jon_R | | #12

          > makeup water valve open on boilers

          +1 , sound an alarm and/or shut down on low pressure. Don't quietly refill.

          I'd like a similar alarm on the water plumbing - at 3 am every day, turn off a main supply valve and see if pressure holds.

      2. charlie_sullivan | | #21

        When you say the latent load becomes just what the occupants generate, you are assuming the moisture recovery rate of the ERV is 100%. In fact, the ERV is much less efficient at recovering moisture than heat. So the load driven by ventilation is primarily latent. Your building example worked out differently, but in the limit of a perfect envelope and ERV, the ratio will be pretty different from your example.

        1. Expert Member
          NICK KEENAN | | #22

          I spoke with Equinox, manufacturer of the CERV2, and they said I could assume zero latent load from the ventilation it provides.

          1. user-2890856 | | #23

            They also probably assumed you have an IQ lower than dewpoint

          2. charlie_sullivan | | #24

            I would describe that differently: it's an active dehumidifier that is taking care of the latent load, and, in doing so using a lot more electricity than an ERV. It's a valid approach, called a DOAS (dedicated outdoor air system) in the commercial world. But it's not magic--you don't get that for free.

          3. Expert Member
            NICK KEENAN | | #25

            Just to clarify --

            Yes, the CERV uses power, but it uses that power to do work that needs to be done. Ventilation brings air into the house. That air needs to be conditioned, that conditioning takes energy. The CERV can use less energy for that conditioning than an exterior air-source heat pump, because it uses the ventilation exhaust stream to either heat or cool the refrigerant, and the exhaust stream is closer in temperature to the interior temperature than the exterior air, and the efficiency of heat pumps improves with smaller temperature deltas.

            The Equinox unit introduces ventilation air that is equally conditioned to the air it replaces -- warmed in winter, cooled and dehumidified in summer. It uses energy to accomplish that. I don't know if it provides enough dehumidification to serve all of the needs of a house, but it definitely contributes some. Since it does something that has to be done anyway -- conditioning ventilation air -- it turns necessity into a virtue.

            More broadly, if you're going to be doing radiant cooling, you need some other device to provide dehumidification.

  6. walta100 | | #10

    I would avoid radiant cooling in a humid climate like the plague.

    If you are set on it understand it will need to be a highly engendered system and will be very expensive to install. This will not be a system Joe Blow puts tougher based on rules of thumb and what happened to be on sale this month at the distributor’s warehouse.

    Consider watching the HVAC YouTube videos of Matt Risinger he is in your climate and has worked out how to build high performance home there.

  7. matlockjeffries | | #13

    Consider a whole-home dehumidifier, particularly in a PGH. We installed one in our PGH remodel in Oklahoma (similar climate), through a dedicated ducting system that also allows fresh air intake and continuous filtration. Our cooling loads are inadequate for the mini splits throughout the house to dehumidify. The manufacturers don't quantify the latent heat added, but it must be less than the total energy consumption of the device...for example, the Ultra-Aire 120V we have consumes max 5.8 amp = 696 watt = 2370 BTU/hr. They also make a split whole home dehumidifier that introduces latent cooling instead of heating (the SD12), but it's spendy.

    1. Expert Member
      NICK KEENAN | | #14

      Figure on 1000 btu per pint for latent heat. Dehumidifiers are rated in pints per day, divide by 24 to get pints per hour and thence but/hr. Add that to the electric consumption to get total but/hr, and divide the total by electric consumption to get coefficient of performance. Dehumidifiers are typically efficient space heaters with a COP of three or more.

      I'm trying to figure out the use case for a split dehumidifier, how is that different from an air conditioner? What would be interesting would be a variable split system that could match the level of sensible cooling -- or even heating -- to conditions.

      1. matlockjeffries | | #19

        Probably not much different honestly.

        I agree with you, I think if there was an easy way to automatically set mini splits to enter dehumidification mode and run off of a humidistat then lots of problems would be solved...sort of like Daikin Quaternity.

        1. Expert Member
          NICK KEENAN | | #20

          I've read mixed reports on the whether the Daikin actually works as advertized.

          I think the next frontier in HVAC is having both temperature and humidity set points.

  8. user-2890856 | | #15

    These folks have it all figured out . Call them and see if it's for you .

    1. Expert Member
      NICK KEENAN | | #16

      I hate, hate, hate websites like that. I looked through just about every page and couldn't find a single word about how it works. Just a whole lot of mumbo-jumbo trying to make radiant heating and cooling more complicated than it really is. This thread has about 100 times the information content of that website.

      1. user-2890856 | | #17

        I also hate , hate , hate websites like that also . Fortunately neither of us is the asker of the original question . If he / she would like they could contact the fine folks at Messana and inquire how much it is to design and approximate pricing for a job like this . Kinda keeps the confusion level to a minimum with the round about comments on a thread like this . Hey , maybe you could design it for them , are you qualified ?

  9. BirchwoodBill | | #18

    It takes some digging into the Messana system to understand the components. The ATU series is a combination dehumidifier and HRV similar to the Minotair or CERV2. They have a fan coils that would work well in a great room, and some radiant panels. The controls are based on the Emerson IPro series so it integrates well with other system. The thermostat or HMI are Power over Ethernet panels. Sensors (temperature, humidity and CO2) are built into wallboard. Looks like a solid well designed system. It looks like HiDew makes the dehumidifier.

    I would also spend some time on Robert Beans site. HealthyHeating and go through your MRT and hydronic calculations for the house. In a nutshell you need a really good dehumidification system that is kept around 66F. The a good PID controller on the buffer tank.

    Learning Italian may help in keeping this system running.

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