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Community and Q&A

What Can Bring In Outside Temperature When It’s Favorable?

jbeuckm | Posted in Energy Efficiency and Durability on

We are in Minneapolis (USDA Zone 4) and I often notice that it’s uncomfortably hot inside while nice and cool outside or we have the heater running but the front sun room is baking. What kinds of systems are there for moving a lot of outside air in when the temperate delta is favorable? 

I am also starting to measure air quality and our CO2 is too high. I am considering an ERV and I wonder if that system would be capable of bypassing the heat exchanger when outside temp is what we want inside?

We have a high velocity AC in the attic and modern water radiators for heat.

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  1. BirchwoodBill | | #1

    MSP is in ASHRAE Zone 6A — I am in White Bear Lake. It sounds you are getting Solar Gain through windows. Maybe some shades can cut down on heat during the day. Some ERV have controls for free cooling/heating. Is the Sun room included in the high velocity system?

    1. Expert Member
      DCcontrarian | | #2

      Shades don't reduce solar gain. The sun light has already crossed the glass, the shades don't bounce it back through the glass. They do reduce the unpleasant feeling of the sunlight landing directly on your body but they don't reduce the heat gain of the room.

      1. Trevor_Lambert | | #7

        Why wouldn't heat go back out the glass? I mean, we know it does. Even without blinds, heat is radiating outward from the window. It's not a perfect insulator. If you increase the temperature of the air immediately adjacent the glass, as you would with blinds, it will radiate more heat. I'm guessing it will pass some IR light back through as well; it's not a perfect one way valve for IR.

        I installed blinds in an older house, and it measurably lowered the temperature in the summer throughout the whole house.

        1. Expert Member
          Michael Maines | | #10

          If the blinds have a reflective surface and if the glass does not have coatings that reflect heat back into the room, blinds would be somewhat effective. But most blinds don't have surfaces that reflect heat, and many windows have coatings that retain heat.

          1. Trevor_Lambert | | #11

            Every material reflects some heat, and no window is perfect at retaining heat. If it wasn't the case, we'd be replacing walls with windows for their magical heat retaining properties. The blinds don't even have to reflect at all in order to keep the room cooler. They just have to keep the temperature between the window and the blind higher than the rest of the room, and the window will move heat outward by conduction.

        2. Expert Member
          DCcontrarian | | #13

          Even if you had a perfectly reflective shade, windows typically have visible light transmittance in the 50% range. So only half the energy would be reflected out. But that's not the whole story. Windows will typically have solar heat gain in the 30% range. The reason that SHG is less than VLT is that windows have coatings that all the transmission of visible light but block other frequencies.

          Since the shade is not a perfect reflector, when light hits it the light is absorbed and the shade heats up. Since the shade is at a much lower temperature than the sun, the energy radiated by the shade is at a much lower frequency than the sunlight, and it's those frequencies that are blocked by the window. The shade is going to radiate in all directions, with as much radiation going toward the room as toward the window.

          Sure, some of the heat goes back through the glass. But interior shades reduce very little of the solar gain.

  2. Expert Member
    DCcontrarian | | #3

    The classic answer is a whole house fan, although they've gone out of style because they're hard to seal properly when they're not running. In commercial HVAC they have what they call an "economizer," which redirects the return to the outdoors and sucks in outdoor air instead of recirculating indoor air. In commercial settings it's a lot more common to be needing cooling when the outdoor temperature is below the set point than in residential.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Isn't the classic answer: open the windows?

      1. Expert Member
        DCcontrarian | | #5

        Maybe I shouldn't have assumed that had already been tried.

  3. BirchwoodBill | | #6

    You may want to hire an HVAC company to perform a manual-J and look at how your system is zoned. A manual j should calculate any heat gains through a front Sun room.

    Adding an overhang to window such as awning - can reduce the un-wanted heat gains.

    The front Sun room may need to be on its own zone. In terms of an ERV — some have smart ventilation controls to bring in outdoor air and cool. The Building Equinox CERV2 has that type of control system. The RenewAire with a Carel control system can also do free cooling with its ventilation system. It measures the temperature and humidity of the outdoor air and compares in the temperature and humidity of the indoor air.

  4. Trevor_Lambert | | #8

    Zehnder is the only brand in North America that has a cooling bypass mode. I have one. I have read that the amount of cooling you can get from this is not very significant, like it will be dwarfed by just opening windows. But it's a very nice ERV regardless, so it doesn't hurt.

  5. maine_tyler | | #9

    If you want to consider using open windows and fans, this is an interesting video on best fan placement (tldr: blow through the window, don't suck).

    1. jbeuckm | | #12

      I had seen this great video - love Matthias.

  6. shed_time | | #14

    In our next house (San Francisco, CZ3c), I'm planning to install several Broan Fresh IN supply fans to bring in cool air in the evenings:

    I'll build my own control system for it that takes into account outdoor temp, indoor temp, indoor CO2 and thermostat demand signals. I have an prototype I installed in our current bedroom and we *love* it. Keeping the temp below 68F and CO2 below ~1100ppm makes for much more comfortable sleep! I think I've heard of ERVs with a bypass, but I don't think it's worthwhile in our extremely mild climate.

    1. Expert Member
      DCcontrarian | | #15

      In my climate (DC) you also have to look at humidity, you don't want to be bringing in outdoor air when the dew point is higher outside than in.

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