Evaporative Humidifier to Supplement A/C

Factual Questions
1

Here’s one that’s been puzzling me for a while. I live in a 100 year old building with uninsulated brick walls, so needless to say my two window air conditioners struggle to keep the place cool on hot sunny days. That said, I have the windows well weather-stripped and everything is quite thoroughly sealed up. So most of the heat load is sensible rather than latent, from heat just bleeding through the brick or solar gain through the windows.

Because of this, and since the a/c units are a bit undersized, they actually dry out the air significantly. If left alone the indoor RH will drop to 32% easily. I have one of those big Kenmore console evaporative humidifiers that just sucks air through a moist pad. It can blow 60 degree air, so I was wondering if allowing it to run in the summer would help cool things off, or if it does more harm than good, or if the benefits just wash out. Note I don’t let the RH go above 45% because at about that point the air coming out of the humidifier isn’t that much cooler, so it’s a case of diminishing returns.

Anyway, I know in a desert situation with a swamp cooler, it’s dry outdoor air being blown through the wet pad and into the room, then simply exhausted outside. In my case, the humidifier is merely converting some sensible load to latent load, but since the RH is so low to being with that’s not really a bad thing. What does that do to the air conditioners though? My guess is that they’re removing the same total amount of BTUs, just more of it is latent heat rather than sensible heat. So does that mean the a/c units are losing some sensible heat removal capacity, offsetting the cooling done by the humidifier?

The one possible advantage I could see is that more cold condensate water is available to sling onto the condenser coils, but I don’t know how much that impacts everything (I think it mostly reduces the power draw of the a/c unit rather than lowering the discharge air temperature).

Even if there is a benefit to running the humidifier, I realize it’s small. Maybe 1,500 BTUs or so. That’s what makes it hard to gauge. I tend to think it doesn’t yield any net benefits, but I just can’t seem to wrap my head around all of the different factors and how they interact with one another. What’s the SD?

2

It gives you Adrian Monk’s “balanced” setup of a humidifier and a dehumidifier in the same room. I say that only half in jest, but it’s true.

3

Increased humidity in the room increased conduction through the walls also. So you’re sending some condensed water outside, but first it has condensed on the coils affecting AC efficiency in some way, while the room heats a little faster at the same time. I’m just guessing that there’s a net benefit too small to measure.

4

The only thing is that a dehumidifier dumps the latent heat back into the room as well as the heat generated by the compressor, whereas a window or central a/c dumps it outside.

5

Wait what?

6

[quote=“jjakucyk, post:1, topic:761201”]

n my case, the humidifier is merely converting some sensible load to latent load,/QUOTE]

But the latent load takes more energy to cool. The sensible heat is hot air, the latent load is water vapor. The water vapor takes more BTUs to cool, and really takes more if the window units condense it.

I guess you could give it a try, but I don’t think I would. 32% RH is by no means too low for hot conditions.

If money is tight, hit up Craigslist for a bigger window A/C, or at least add another cheap. new one. Walmart has 5000 BTU units for $112.

Dennis

7

I’m just saying that the more humid air is a better conductor, transferring heat to and from whatever it touches, whether it’s the AC coils or the walls, roof, floor, etc.

8

Is the effect anything more than negligible though? I’ve never heard this property before. Yes water is a much better conductor than air, but water vapor is a whole other animal. One link I found says that conductivity decreases as air gets more humid, though at indoor temperatures the differences look to be entirely irrelevant. The Thermal Conductivity of Moist Air | Electronics Cooling