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Thread: Consequence of Running Humidifier AND Air Conditioner Simultaneously

  1. #1
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    Consequence of Running Humidifier AND Air Conditioner Simultaneously

    Good day all,

    My mother in law's house is maintained at 72*F (usually around 50%RH) by a central air conditioner overnight, but the dry air bothers her throat so she's been running a small humidifier in her bedroom (same conditioned envelope, humidified air will be returned to the air conditioner). The humidifier is simply blowing air across a saturated media so energy input is minimal (lets say 5 Watts for the fan vs. a steam humidifier with a 1 kW heating element).

    Question: what is the energy consequence of running a humidifier and central air conditioner simultaneously? Obviously the moisture evaporated by the humidifier will eventually be removed by the indoor coil as condensate, but is there any serious net loss in efficiency? Other sources online suggest there is a penalty, but I'm thinking no!

    My understanding of a simple humidifier is that it does not change the dew point of air, it just trades humidity for temperature aka reduces sensible heat and increases latent heat (again, for a humidifier with minimal added energy, steam humidifiers would add energy and therefore increase dew point). When water evaporates in the humidifier it would absorb sensible energy from the dry air (evaporative cooling) and the cooler moist air carries the latent heat through the house instead of sensible heat. I don't see why 1 BTU latent heat would be any different to extract from the air compared to 1 BTU sensible heat if evaporation is reversible. Is this understanding correct?

    Thank you for your thoughts!

  2. #2
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    You could ask a technician to alter the blower speed so the air conditioner does more sensible cooling and less latent cooling. That's how they run them in dry climates. If the blower speed is not changed, the cooling system will expend more energy removing the latent heat, that's the humidity you're working to increase.
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    Thanks for the reply. I should have mentioned we're in New England so I definitely want to keep dehumidifying the air for the health of the house. Online it looks like anything higher than 60% RH can encourage mold? Plus the insulation on the house isn't great (built 1957) and the AC would have to cool the house to 67*F to keep the same comfortable dew point!

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    Sometimes people's mind tricks their body based on numbers. Does she feel better when she sees the right 'number'? What RH does she see in her house in the winter, and is her throat still dry?
    @kdean1 gave you the best advice.
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  6. #5
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    The recommended RH range is 40-60%. But the inside design conditions for summer comfort is 75 at 50% RH. Most people are comfortable with RH lower than 50%. I am very cautious of anything higher than 50% because of concerns about condensation within the wall, especially when the thermostat is set lower than 75. That is how people tend to run an oversized system.
    Last edited by kdean1; 08-08-2022 at 03:02 PM.
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    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  7. #6
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    @STEVEusaPA - funny enough I did a test messing with the temp setpoint and quizzing her on how she felt, we landed on 74*F relaxing during the day and 72*F at night (A/C maintains house at 50% RH so I didn't have enough control to manipulate and quiz). She uses the humidifier in the winter too but I haven't taken any measurements, I plan to this winter and I might suggest a whole house humidifier.

    @kdean1 - regarding the 50% RH that's exactly how I feel so I don't want to mess with the system balance, so if she uses a humidifier as a "patch" for comfort at night, will that impact energy efficiency? Thinking about it a little more, it seems like the only energy penalty would be transporting the water from her bedroom to the AC drain pan (liquid -> vapor -> liquid should be net zero energy). This suggests the only energy penalty is the extra blower motor runtime needed to bring the moisture from her bedroom to the air handler, I'm sure I could quantify this if needed, but it would probably be low since we're talking about small quantities of water.

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    Quote Originally Posted by kent_a View Post
    Good day all,

    My mother in law's house is maintained at 72*F (usually around 50%RH) by a central air conditioner overnight, but the dry air bothers her throat so she's been running a small humidifier in her bedroom (same conditioned envelope, humidified air will be returned to the air conditioner). The humidifier is simply blowing air across a saturated media so energy input is minimal (lets say 5 Watts for the fan vs. a steam humidifier with a 1 kW heating element).

    Question: what is the energy consequence of running a humidifier and central air conditioner simultaneously? Obviously the moisture evaporated by the humidifier will eventually be removed by the indoor coil as condensate, but is there any serious net loss in efficiency? Other sources online suggest there is a penalty, but I'm thinking no!

    My understanding of a simple humidifier is that it does not change the dew point of air, it just trades humidity for temperature aka reduces sensible heat and increases latent heat (again, for a humidifier with minimal added energy, steam humidifiers would add energy and therefore increase dew point). When water evaporates in the humidifier it would absorb sensible energy from the dry air (evaporative cooling) and the cooler moist air carries the latent heat through the house instead of sensible heat. I don't see why 1 BTU latent heat would be any different to extract from the air compared to 1 BTU sensible heat if evaporation is reversible. Is this understanding correct?

    Thank you for your thoughts!
    How much water does she add to the portable humidifier per day? Adding moisture from a humidifier and an occupant breathing raises the dew point of the air. I doubt that the dew point is raised very much. Also the a/c does not run much overnight. Monitor the %RH. Winter %RH should be <40%RH to avoid window condensation. Most small humidifiers add small amounts of moisture. Occupants that are mouth breathers are more sensitive to this problem. Some get relief from wearing a cloth mask to retain moisture for the breath. An occupant breathing/evaporation adds +-.5 lb. per hour.

    A humidity monitor that records the %RH would provide the info you need to monitor what happens 24/7.

    https://www.amazon.com/Govee-Thermom...%2C109&sr=8-38

    Although I have never used this monitor, this is an impressive looking monitor for the money. Collects long term data to view on your phone. Most homes are more humid evenings and rainy days. Is there a dehumidifier in the basement?

    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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