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Reducing humidity without AC

mculik5 | Posted in Mechanicals on

I live in NJ. Typical summer weather here is highs in the mid-80s/low-90s, lows in the 70s, and very humid.

Our house has a “natural” overnight temperature of about 73. What I mean by this is that if I set the AC to 74 before we go to bed, it cycles on/off until about 3AM, at which point everything has cooled off enough outside that no more AC is required to hold 74.

While this is nice from an electricity use perspective, no AC means the interior air gets stuffy and the humidity starts to creep up.

What is the most energy efficient way to address this?

I could set the thermostat to 72, but this seems to me to be a waste given that we are comfortable at 74, provided humidity stays low. Would a dedicated dehumidifier be the way to go? Something else? Or do I have no choice but to set the temperature to keep the AC cycling?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matt,
    You have correctly identified your choices: you can use a stand-alone dehumidifier, or you can lower your thermostat setting.

    For more information, see All About Dehumidifiers.

  2. Expert Member
    Dana Dorsett | | #2

    By opening up the windows you reduce the sensible cooling load (temperature), but when outdoor dew points are in the 60s and 70s it adds a significant latent load (humidity).

    A dehumidifier converts latent load into sensible load- it takes the heat of vaporization out of the water by condensing it, but puts that heat into the air, raising the temperature, along with a bit more heat from the compressor motor (typically about 500 watts, or ~1700 BTU/hour.) An air conditioner takes that heat of vaporization from the room moisture and puts it outdoors, where you want it.

    The most energy efficient way to manage this is to run the AC, but pay attention to the outdoor dew points. If the dew point is under 60F opening the windows won't add a significant latent load, and when dew points are under 50F and it will even take away latent load. On nights with low dew points letting the house convection cool (or use a whole house fan) works pretty efficiently. On nights when the dew points are north of 65F you may still get the sensible cooling, but you'll be adding more latent cooling load than it's worth.

    Running the air conditioner under dehumidistat rather than thermostat control can work too. Some higher-end programmable thermostats can be operated in either dehumidistat or thermostat control and you can switch between modes as-needed.

  3. mculik5 | | #3

    Thanks, Martin and Dana.

    Dana - Your response makes a lot of sense. To sum up what you're saying, it sounds like any situation where I'd be running the humidifier, I may as well run the AC, and any situation where neither the AC or humidifier are necessary, I should just open the windows (and use whole house fan, if I install one).

    Is that correct?

  4. Expert Member
    Dana Dorsett | | #4

    That's pretty much it. The oversizing factor of the central air can be an issue though.

    If the AC is significantly oversized it might overcool the house overnight in dehumidistat mode- sometimes a half-ton window-shaker running all night handles the latent load better than oversized central air, and you won't be waking up at 5AM shivering.

    With programmable thermostats you can also play some games with pre-cooling the house in the early to mid afternoon, which dries out the house, then stepping it back up for the late afternoon/early-evening hours, which in the northeast corresponds to with the absolute daily grid load peaks in summer. In the ISO-New England region summertime peaks typically fall between 3:30 PM and 5:30 PM. They chart the regional load in quasi-real time against the prediction, updated on five minute intervals:

    http://www.iso-ne.com/isoexpress/web/charts/guest-hub?p_p_id=systemloadgraph_WAR_isonesysmonitorportlet&p_p_lifecycle=0&p_p_state=pop_up&p_p_mode=view&p_p_col_id=column-5&p_p_col_pos=1&p_p_col_count=3

    So if you set the temp down a degree or three at noon, then up a degree or three at 3PM the house will still be dry & cool through the dinner hour, then and step it back down at 7-8PM (or later) you can sleep in cool-dry comfort even if it's sticky-fog outdoors. The optimal set back timing will change from day to day, but if you pre-cool and avoid adding to the grid peak it's good for power quality and grid reliability (even if you don't get properly paid for the benefits your strategy is delivering to the grid and other ratepayers.)

  5. GBA Editor
    Martin Holladay | | #5

    Matt,
    Q. "Is that correct?"

    A. Yes, except for the fact that you called a dehumidifier a "humidifier."

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