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Community and Q&A

Night cooling

srivenkat | Posted in Energy Efficiency and Durability on


A few ERVs offer a “free cooling” feature for cool nights by bypassing the ERV core. I am wondering as to how far this feature is useful. For example, where I live in East Central Illinois, it was 63F with 75%RH early this morning.

1. I understand the RH will drop initially since the indoor temperature is higher, but with continued free cooling, wouldn’t the indoor temp and humidity come close to 65F and 75RH?

2. If this is the case, is it not going to feel cold and clammy, requiring a stand-alone dehu to run and dehumidify while raising the indoor temp and lowering RH?

3. Wouldn’t this result in a significant Energy-penalty?

4. At what point would it be most energy efficient to stop “free cooling”?

Thanks in advance,


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  1. Expert Member
    BILL WICHERS | | #1

    These systems will ultimately bring the indoor humidity into equilibrium with the outdoor humidity. When it’s really humid out this can be a problem. At night, if it goes down to 50 but inside is 70, that “equilibrium” has to adjust for temperature, so you’ll still have lower humidity indoors. How effective the “free cooling” is always depends on conditions outdoors, so it’s difficult to say how often it can make you comfy throughout the year. Chances are, some years you’ll get more usefulness from the system than others.

    I’ve used whole house fans for decades. These work great, but not if it’s super humid outside. I just use the whole house fan when appropriate and air conditioning other times. You come out ahead in terms of efficiency, but it does take some effort on your part to know which system to use and when — it’s not fully automatic in the way that a thermostat and air conditioner is.


  2. srivenkat | | #2

    Thanks for your thoughts, Bill.

    Since posting the question, I have looked thru some suggested links at the right, and I find the same concerns with humidity mentioned. I specifically found the below comment by Dana Dorsett to be very informative:

  3. Jon_R | | #3

    To keep things in perspective, note that say 100 CFM with a 10F delta-T only moves 1080 btu/hr. Similar concept applies to humidity.

    Yes, ideally "free cooling" devices would be fully automated and only operate when it makes sense.

  4. onslow | | #4


    I lived in Illinois closer to the lake and for 30 plus years I fought with the spouse about not opening up the house windows every night it got a little cooler. The damp air that filled the house made the next days heat load worse by far. We had nothing but window shakers for AC, noisy and expensive to run. I never did convince her that the following day we would need to run the AC longer to re-dry the air and it really would be better to stay closed. I got my way less than half the time. For Illinois I would say keep the house closed from June to Sept now that the weather alterations are messing life up. Save the night cooling for the freak hot days in October.

    A friend in the neighborhood went all in on the whole house fan and while he may have felt he was saving energy, I always felt the house was muggy upstairs and clammy cool down stairs. He did have to battle kids that left the sliding door open half the time, so in his case maybe it was worth it. I have to suspect Zephyr 7 doesn't live in the middle part of the country.

    Dana summed up the "why?" pretty well by noting the dew point temps and regional humidity patterns. We tired of Illinois after 30 plus years and moved to a western mountain location to be free of AC. I designed the new house to be opened up every night in the summer to bank free cool in the house mass, since the local climate is quite a bit drier than Illinois. Finally my wife gets to have all the fresh air she could imagine and I don't have to grump about the AC since there isn't any.

    1. Expert Member
      BILL WICHERS | | #5

      I’m in southeast Michigan. The whole house fan is nice in the spring and the fall, but during the summer I know exactly what you mean about the mugginess. We run AC in the summer except on the occasional optimal night.


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