What level of humidity should I expect from an evaporative humidifier?

Factual Questions
1

I live in Las Vegas and my bedroom is typically somewhere around 22-24% relative humidity at around 25c. The house has central AC of course and AC-dried air is blown into my room.

I get some issues with how dry it is (my post-lasik vision improves when I’m in a humid climate because my eyes are too dry for example) and decided to try a humidifier. Because the water here is extremely hard and I don’t want to buy 10 gallons of distilled water per week, I got an evaporative humidifier rather than an ultrasonic. They’re basically just a water resorvoir, a wick, and a fan that runs over the wick to accelerate the evaporation of the water.

The humidifier seems to be doing its job. On high fan speeds, it evaporates a liter of water every 2 hours or so. I also feel the difference, the air in the room feels more humid. But I have two hygrometers and each of them agree that it has hardly raised the humidity in my room – from about 22-24% to 28-30%. While that’s a difference, I was expecting a much bigger difference up to perhaps 40-60%. When I was reading a buying guide to humidifiers, I was reading that one of the advantages of an evaporative humidifier is that it’s naturally self regulating as it gets harder to evaporate as the humidity rises, whereas if you do not configure an ultrasonic humidifier correctly it can make your room into a swamp. This gave me the impression that normal room humidifiers are capable of making a room very humid.

I estimate my bedroom is about 50-60 cubic meters. At 25c, air could hold about 23 grams of water per cubic meter at 100% humidity. So if the humidifier is causing the relative humidity in my room is going up 6%, that’s only 1.38 grams per cubic meter extra. If the humidifier is putting 500g of water into the air every hour, that’s about 10 grams per cubic meter per hour, and yet only 1.38 grams above ambient humidity is maintained.

The room door is closed. The house has central AC, so I recognize that dry air is getting blown into my room and therefore some of the existing, more moist air must be being blown out of the cracks (I’m guessing the doorframe? I’m not sure where else it would go). Is so much air being blown in that it’s displacing 86%ish percent of the water being added to my room by the humidifier?

I thought that perhaps at night, when the AC blows less because it’s not as hot, the relative humidity would increase, but that wasn’t the case. I even closed the AC vent for a couple of hours and that didn’t seem to have much of an effect. I can’t seem to exceed about 30% no matter what I do.

The unit is working – I mean, if the water tank is being emptied, it has to be going into the air – and 1 liter every 2 hours seems like a reasonable rate to be putting water into the air. The space that’s being humidified isn’t very big. I’m not sure why it has had such a small impact on the relative humidity level. I’m going to assume the hygrometers work because two of them agree with each other. Is this a typical level of humidification from a humidifier? Do I have an unusually large amount of air leaking out of my room for some reason? Any other explanations?

Edit: In case it’s relevant I’ve only been using it for about 48 hours.

2

Yes (probably). The AC fan is probably sized in the ballpark of 1 CFM per ft^2 (whole house fans are more at 2-3 CFM). That is, a 2500 ft^2 house might have a 2500 CFM fan. Which means that with 8 ft ceilings, it only takes 8 minutes to replace all the air in the house. It’ll easily overpower the humidifier at full blast.

I’m not sure why closing the vent didn’t help at least a bit, but it’s likely that there’s still enough air leaking through to overpower the humidifier. You might try an experiment where you wrap it in foil or something for better blockage.

3

The AC system itself is drying the air. In fact, drying the air was the reason that AC systems were invented. The fact that it cooled the air was a side benefit. When air passes over the coils, the water in the air condenses on the coils. This is similar to the condensation you see on a on the sides of a cold drink glass in a humid climate. The water then drips into a pan and drains outside.

4

I agree w the folks above; HVAC systems move far more air than we often appreciate. Such that your humidifier is trying to humidify your whole house, and part of the outdoors too. And it’s way too small to make much progress against that volume.

As to this bit …

I’ll also point out all the furnishings in your bedroom matter.

All the fabric, carpeting, and exposed wood have been drying in the parched desert air for years or decades before two days ago. Now they will all be absorbing extra water from your more-humid air until they too reach an equilibrium with the new normal you’re trying to create.

That effect is not huge, but I would expect it to be significant at first. More so if you have carpet, heavy cloth drapes, etc.

5

Your numbers seem very similar to mine. I too have an evaporative humidifier, and it sits near my thermostat, which has a hygrometer in it. I typically run the humidifier in the winter, and it only raised the humidity a few percent. Maybe from 24 to 26%. That is with the humidifier nearly blowing on the thermostat. Being winter, the house is sealed, so just the normal amount of leakage.

Quoted for truth. Our tap water is not particularly hard (I never have to descale my electric kettle), but out of curiosity I put some filtered tap water in an ultrasonic humidifier, and the PM10 and PM2.5 count in the house shot up from the usual 0 into the hundreds. Makes sense if I assume all of the crud that collects on the evaporative filter is being aerosolized in the ultrasonic humidifier.

6

SenorBeef has simply created a Rube Goldberg machine to dump water outside, at the cost of using electrical power for the AC. I suggest that he stops doing this.

7

Not really. You can drive a space to any combo of temp and humidity percentage you want.

It’s commonplace in the hot humid areas to grossly overcool to achieve a humidity target when using less cooling and a separate dehumidifier can result in better comfort for less dollars.

Conversely in hot dry areas, you’re using the AC for cooling, not for humidity reduction. Any humidity reduction that does occur is an adverse side effect. So having a separate powered system to manage humidity is completely reasonable.

In cold climates it’s common to have a humidifier built into the output side of central HVAC systems. Which trickles water across an evaporator bed exactly like a swamp cooler. But unlike a swamp cooler the variable being managed is humidity upwards, not temperature downwards.

8

I came too late to the party, and they took the thermodynamics out of my course, but as I understand it, the AC will continue to set the humidity at the set point, which is set by the air temperature and the coil temperature, regardless of if it needs to be on a lot of the time to do it, or only short periods to do it.

I think you want the AC fan to blow hard, so that the air out of the AC is not as cold. Some AC systems have fan control which effectively does allow you to set humidity, although for home systems that is not the common purpose.