Closely related to Does a humidifier cool the air? but with a different kind of humidifier, and I'd like to know exact numbers.

First question: for the same production volume of vapor, does an ultrasonic mist maker cause the same amount of cooling as an evaporative humidifier?

Second, more practical question: My apartment already struggles sometimes with keeping the unit warm, but it's also far too dry, so I need to humidify. I bought a mist maker with stated vaporizing capacity of 1600ml/h. Mist maker

It seems since I've had it running, the heater struggles even more than before. How do I convert from ml/h to BTUs I am losing to humidification? Do I need to provide other parameters (like a starting relative humidity and temperature)? I looked at Philip's chart at: http://www.truetex.com/psychrometric_chart.gif but I'm not really sure how to read it.

2 Answers 2


What you're asking about is the latent heat of vaporization.

It is 970 real BTUs per pound of water, or 2260 joules per gram. A joule is a watt-second. A BTU raises one pound of water one degree F.

That means if you have an ugly bag of liquid water, 1 pound, and it is 112F temperature, and you inject 100 BTU, it is now 212F. If you inject another 970 BTU, it is still 212F but is now vapor. If you inject 30 more BTU to make it an even thousand, it is now 242F. The 970 BTU was eaten by the phase change. Yes, you can use this to air-condition, but it works better with freon, propane, etc.

There's no free lunch. That energy has to come from somewhere.

A gallon of water is 3785 grams (also 3785 ml, by wild coincidence, not). It will take 8.5 million joules or 2400 watt-hours, or 8200 real BTU, to evaporate it. If the humidifier avoids paying it (e.g. By misting), it is stolen from the air, cooling the air.

By "real BTU" I mean BTU is a timeless unit. Remember when a furnace talks about BTU, that is slang. It actually means BTU/hr.

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    Thanks for the hard numbers! The mister uses 350W of electricity. If I run it for an hour, vaporizing 1600ml water, that requires 1004 watt-hours. Do I subtracted 350 watt-hours from that to get the amount of heat that was taken from the air to make up the difference? Commented Feb 25, 2019 at 20:19
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    Yes, you got it. Commented Feb 25, 2019 at 20:26
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    @JoeFala no, but thanks. Commented Feb 25, 2019 at 21:04
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    "at some point the humidified air will return to equilibrium" - are you referring to air exchange with the outside? It is a pretty leaky apartment: I estimate I need about 15GPD to humidify the whole thing. So the cooling effect of the humidifier is actually added to the cooling effect of air exchange (because the air outside is much colder). Commented Feb 26, 2019 at 15:37
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    But due to air exchange in a leaky building, the warm humidified air is constantly being exchanged with cold, dry (in absolute terms) air outside the building. Commented Feb 27, 2019 at 16:04

Technically a humidifier heats the air but it's not in the form of a temperature change. Well it sort of is. It's a little complicated but it adds latent heat and also reduces evaporation from your body making you feel warmer. It's different than misting water on your body. In that case water deposited on your body will evaporat due to the difference in moisture levels of your body and the air. The evaporating water must take heat energy with it. It's a very fascinating but unfortunately overlooked subject. The rabbit hole goes very deep for this one but I could talk about it all day. The enthalpy chart in your link will only confuse the average laymen and even most professionals.

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    The evaporating water must take heat energy with it. That's what I'm curious about. How much heat energy is it taking out of the air (putting into latent heat) to put moisture into it? Our skin can't feel (and a the thermometer can't measure) that latent heat, right? Commented Feb 25, 2019 at 19:31
  • Yes it does and that is coming from the mister, the remaining energy of evaporation from the air does not leave or disappear. So the net effect is an increase of btu
    – Joe Fala
    Commented Feb 25, 2019 at 19:33
  • In this case it's not a big increase but it's not a negative effect
    – Joe Fala
    Commented Feb 25, 2019 at 19:34
  • The body can feel latent heat due to the evaporation effect but a thermometer cannot unless it is a sling psychrometer or a digital hygrometer
    – Joe Fala
    Commented Feb 25, 2019 at 19:36
  • I'm actually at in a customer's driveway I'll be back, but Google things like sling psychrometer, wet-bulb and dry bulb, and relative humidity.
    – Joe Fala
    Commented Feb 25, 2019 at 19:38

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