Source: American Society of Heating, Refrigerating and Air-Conditioning Engineers. This is from the 2016 ASHRAE Handbook—HVAC Systems and Equipment (SI), Chapter 22: Humidifiers.
I’m a big believer in running a humidifier or two during the coldest part of the winter. I harped on that point just recently, in Post #1640. I do it as much for the health benefits (illustrated above) as for the comfort.
That said, I realize that I pay a considerable energy penalty for doing that.
Interestingly, a lot of people do not seem to understand just how large that energy cost is. Here’s the trick: You can’t measure it by the amount of electricity the humidifier itself uses. If you have anything other than a boiling-water humidifier, by far, the majority of energy used to run your humidifier comes from your home furnace.
Which I shall now demonstrate, and briefly calculate.
Humidifier as a house-cooling device.
First, this ain’t rocket science. Everybody knows that evaporating water cools things off. For this next part, you just have to get your mind around what, exactly, is being cooled off by the evaporation from your humidifier. And then, what you have to do about that, in the wintertime.
In the case of an evaporative humidifier, what is being cooled is the air inside your house. The humidifier literally absorbs heat from room air. You can easily prove that to yourself, as I did above. My Vornado humidifier cools down the room air by about 5 degrees when used on its medium setting.
That’s just physics, and there’s no getting around it. No matter how you do it, converting liquid water into water vapor takes a lot of energy input. Boil it, evaporate it from a humidifier pad, mist it into the air and let those tiny drops evaporate. Or just hang your damp laundry inside. If you start with liquid water, and end with water vapor, somewhere along the way, that water absorbed a lot of heat energy. From somewhere.
At room temperature, it takes just about 700 watt-hours of energy to evaporate a kilogram of water (reference). Which means that evaporating a U.S. gallon of water, at room temperature, requires somewhere around 2.5 kilowatt-hours of energy (or about 8500 BTUs).
And so, per the illustration above, if I want keep the room at 68F, I’m going to have to run my furnace to make up for the 5-degree difference between room temperature and the cool air coming out of the humidifier. How much energy will my furnace have to supply? Just about exactly 8500 BTUs for every gallon of water I evaporate. Or, if I do a typical 2-gallon day, roughly 17000 BTUs or 5 KWH of energy, per day, will have to be added into the room air, that would otherwise not have to be supplied.
That works out to a rate of power consumption of (5000 W-H/24 H =) about 200 watts, averaged over the course of a 24-hour, 2-gallon day. By contrast, the humidifier itself uses just 32 watts, run on medium speed. The upshot is that the furnace supplies roughly 85% of the energy required to run that humidifier, in a room with constant temperature.
The actual electricity use isn’t quite that bad, because my “furnace” is a heat pump with a coefficient-of-performance (COP) of roughly 3. That is, it releases about 3 watts of heat energy inside my home, for every watt of electricity consumed. So it only uses electricity at a rate of about 70 watts, on average, to offset the cooling produced by the evaporative humidifier.
What’s the difference between a humidifier and a clothes dryer?
Answer: Not much.
To drive this home, let me now compare the humidifier to a known household energy hog, the clothes dryer. A typical home dryer uses about 3.5 KWH per load. Here, if I ignore the COP advantage of the heat pump, my humidifier requires about 5.7 KWH of energy input per two gallons, including both the device itself (32 watts on medium), and the heat required to re-heat the air after it’s been cooled by evaporating water.
At which point, I’m hoping that a little light bulb goes off. Because those energy use figures are pretty close. Let me adjust them for the amount of water being evaporated.
Some time back, I figured that a typical load of laundry retained about 10 pounds of water (Post #910). So that’s about 3.5 KWH of electricity, to evaporate 10 pounds of water, in a dryer. But two gallons of water per day, out of an humidifier, is about 16.5 pounds of water. So, at the rate my dryer uses energy, that ought to take about (16.5/10 x 3.5 KWH =) 5.8 KWH of energy.
In other words, per pound of water, your home humidifier uses just about exactly as much energy as your home clothes dryer.
Because, of course it does. It has to. Plus or minus a bit of wasted heat, your home clothes dryer does exactly the same thing as your humidifier. It’s taking water and converting it to water vapor. It just does it at a different temperature.
The only energy advantage my humidifier has over my clothes dryer is that the humidifier uses a more efficient heat source. The COP 3 heat pump uses less electricity, per unit of heat, than the resistance heating elements in the dryer. So the actual electricity use is lower, due to the magic of heat pumps. (Plausibly, if you had one of the new heat-pump clothes dryers, there wouldn’t be much difference at all.)
Finally, if you have achieved enlightenment in this area, you now realize that hanging your laundry to dry, inside, in the winter, does not save anywhere nearly as much energy as you probably thought it did. Sure, you don’t run the dryer. But you run your furnace instead. That’s to make up for the cooling effect all that wet laundry has on your room temperature. Which is exactly the same cooling effect that the humidifier has.
There ain’t no such thing as a free lunch.
Sensible heat, latent heat, and conservation of energy.
Hang on, Mr. Conservation-of-Energy. You’re saying that the humidifier is, in effect, withdrawing heat out of the room air? Where does that heat go?
These devices:
- Humidifiers (both evaporative and ultrasonic),
- Personal air conditioners
- Swamp coolers
- Mist fans
- Patio misting systems
- Street-fair mist-cooling stations
all work by converting “sensible” heat — that is, air temperature –– into “latent” heat — that is, the energy embodied in water vapor as opposed to liquid water.
The energy is still there. It was neither created nor destroyed. It’s simply in a different form. In this case, it’s in the form of the energy that’s in the water vapor, as opposed to liquid water. If you could condense that water vapor back into water, it would release exactly the amount of energy it absorbed in making the transition from liquid water to water vapor.
And, as night follows day, any time you convert liquid water into water vapor, that’s going to absorb heat energy. In all of the above, the heat comes out of the air, and the air cools down. For most of these devices, that’s the entire point. For humidifiers, by contrast, that’s a regrettable downside.
My point being, physics doesn’t care about your opinion. If you like street-fair cooling stations, or patio misters, because they cool you off — up to a claimed 30 F in ideal conditions (reference) — then, logically, you have to realize that your home humidifier is also cooling you off. In the dead of winter, when that’s the last thing you need.
And that’s why running your humidifier, in the winter, takes just about as much energy as running your clothes dryer. Per pound of water, that is. From a physics standpoint, there’s not much difference between the two appliances. One of them heats up air, and converts water to water vapor. The other one converts water to water vapor, which then requires you to heat up the air. The only difference is the timing, and the efficiency of your home heating system compared to the simple resistance heaters (hot wires) used in a typical clothes dryer.
