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I'm considering getting an evaporative humidifier for my super-dry Northeast house, but I'm wondering if the evaporation cools off the room and is going to add to my heating bill.

It seems like evaporative humidifier work exactly the same was as evaporative air conditioners — i.e. blowing dry air across water to cause evaporation, thus transferring the heat of the air into water vapor's latent heat.

Am I wrong about the thermodynamics? Or is the effect small enough that it doesn't make much difference?

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It's a closed system, so unless you're sucking in external air to run the humidifier, there shouldn't be any net gain/loss of temperature (at least nothing that you'd notice.) –  DA01 Feb 2 '13 at 5:32

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up vote 4 down vote accepted

You are quite correct about the thermodynamics. The heat involved in a phase change of water is so significant that professional HVAC guys specify the performance requirements for cooling as two different parameters: sensible and latent heat to be removed. The latter is the heat involved in phase change of water and is often more demanding than removing sensible heat.

"For example, in 1 kg of waste air at 20 °C with a relative humidity (RH) of 60%, the total energy is 42.5 kJ, more than half of which (22.5 kJ) is latent heat."

You can analyze your situation with a psychrometric chart like this: http://www.truetex.com/psychrometric_chart.gif

Just pick your desired temperature and relative humidity and you can find out how much water exists as vapor under those conditions.

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You are correct. That's why some humidifiers have additive heating.

If you put your hand into the stream of created vapor, you can feel the coldness of it.

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the vapor isn't any colder than the water, which is likely room temperature. It just FEELS cold because it's evaporating from your skin. –  DA01 Feb 2 '13 at 5:33
Evaporation of water lowers temperature. Do you think the water waits to hit your skin before it will start to evaporate? –  Philip Ngai Feb 2 '13 at 18:23
@DA01: Have you read Philip Ngai's answer with the explanation? I agree with him, and I said there is even a counter-measure built into humidifiers. –  Peter Ivan Feb 2 '13 at 20:06
It lowers both the temperature of your skin and the temperature of the room. Look up swamp cooler on wiki. –  Philip Ngai Feb 3 '13 at 1:14
@PhilipNgai I stand corrected. And I learned something. Thanks! So, here's the link. en.wikipedia.org/wiki/Evaporative_cooler Looks like a swamp cooler can cool the air 5-7 degrees fahrenheit. Granted swamp coolers tend to be a magnitude larger in volume that a humidifier, but yes, it does look like they cool the air. –  DA01 Feb 3 '13 at 16:38

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