We live in San Francisco, which is not known for high temperatures. But our apartment is on the third floor and we get a ton of direct sunlight through our 5'x8' windows. We open the windows enough to ventilate the apartment with fans, and otherwise keep them covered with heavy, reflective-backed curtains, but somehow the place still turns into a greenhouse. It can get above 85F inside, even when it's considerably cooler than that outside.

I've started thinking about evaporative cooling. I don't want to use an evaporative cooler in a closed room because of the risk of growing mold, but what if we were to combine evaporative cooling with ventilation? Normally when you're changing the air temperature you want to prevent air exchange, but we actually want to bring the inside temperature closer to the outside temperature. So, could we run an evaporative cooler to suck room heat into water vapor, and then vent the moistened air to the outside to keep the humidity from getting too high? Or am I failing to understand something about the physics here?

2 Answers 2


The way evaporative cooling works is that it forces air through moistened pads, evaporating the water and cooling and humidifying the air. The cooled and humidified air is then blown through the house and back outside again, lowering the interior air temperature and producing a cooling breeze as well. If I understand you correctly, you are asking if you could use evaporative cooling to use the heat already inside the house to evaporatively cool the rest of it. I don't think this will be any more effective than simply using outside air. After all, if it works, the temperature of the interior air will rapidly fall below the outside air temperature, eliminating the high temperature inside.

You have correctly identified that ventilation is necessary for evaporative cooling to work. You basically have two options: crack some windows, or use a dedicated ventilation system. Here in the southwest, there is a product called UpDux that is basically a vent that allows air exchange from the room to the attic above. They are used in warm-season houses that are evaporatively-cooled, allowing the cooled and humidified air to escape through the attic, with two benefits: 1) they allow you to keep your windows closed, and 2) the air cools the attic as well on its way out of the house, preventing the attic heat from heating up the interior so much. In your apartment, installing such products is not likely an option. I'd crack some windows in the rooms farthest from where you locate the evaporative cooler.

However, you have a potential problem: San Francisco is fairly humid for evaporative cooling. Less than the national average, but I see at http://www.city-data.com/city/San-Francisco-California.html that the afternoon relative humidity in July only falls to 30% in the afternoon, and it's higher than that most of the time.

You can figure out exactly how effective evaporative cooling will be using this calculator: http://www.srh.noaa.gov/epz/?n=wxcalc_rh

For example, plug in 90 degrees for the temperature, 30% for the relative humidity, and 30 inches of mercury for the atmospheric pressure. That will give you 67.55 degrees for the wet-bulb temperature. That's the theoretical cooling power of a 100% efficient evaporative cooler. Unfortunately, 100% efficiency is impossible unless you get a two-stage unit that is very expensive; assume more like 70-90%. Here is the formula for the real-life cooling power of a less-than-100%-efficient evaporative cooler:

Air temperature - ((air temperature - calculated wet bulb temperature) * efficiency)

So, 90 - ((90-67.5)*0.8) = 71.6. So an 80% efficient evaporative cooler, such as the Bonaire Durango window unit I own and love, would blow 71 degree air at 100% humidity into your house.

I can tell you from experience that once your evap cooler is blowing air that's in the 70s, it stops working that well, because the cooling power you're receiving is offset by the mugginess of the interior relative humidity rising to 60% or higher, which is also dangerous for a wood-framed house because that's approaching the mold-friendly level.

This may work anyway, since 71 degree air is certainly more comfortable than 85 degree air, but you'll need to keep it running constantly because 71 degree air is probably not going to cool the house down to below 77 or so, based on my knowledge of the heat gain you're receiving in a likely awfully-built, basically uninsulated San Francisco building with terrible clear-glazed single-pane windows. Additionally, you'll want to keep the windows open wide to ensure maximum air exchange to the exterior to keep the interior relative humidity down. If this is a plan worth pursuing, I would buy a hygrometer so you can measure the interior relative humidity for yourself.

  • Thanks for the thorough answer! I didn't know it was possible to calculate the cooling potential like that, and I'll definitely go ahead with humidity measurements.
    – octern
    Oct 7, 2014 at 15:26
  • 1
    Per standard SE protocol I'll wait a couple of days to accept, though I can't imagine getting a more useful answer unless someone tells me how to get a little demon who strategically opens the window to let hot air molecules out and cold ones in.
    – octern
    Oct 7, 2014 at 15:30
  • It is indeed! I can verify that my own evaporative cooler performs at almost exactly the calculated performance, so it's accurate as long as you feed it the right data. :)
    – iLikeDirt
    Oct 7, 2014 at 15:30

You already have a source of cooling: the outside air. The "little demon" you are looking for is a thermostat. A thermostat that controls a window fan. Something like this: http://www.amazon.com/Bionaire-BW2300-Window-Remote-Control/dp/B000065DKJ

Install the fan vertically with the upper fan set to blow air out, and the bottom fan set to suck air in. Depending on the size of you apt, that may suffice by itself. One optimization could be to put a divider of some sort between the fans to ensure the incoming air isn't getting sucked back out to any appreciable degree. Another possible optimization would be mounting a duct (something like a dryer duct perhaps) to the exhaust fan with the duct arranged to suck in air from near the ceiling (where the hot air will collect). The cold air should naturally sink when it comes in. All this ducting etc sounds kludgy, but probably no more so than dealing with and evaporative cooler, even if it would work.

Other random thoughts.... If the heat gain is that bad with the curtains blocking the sun, it seem to me you're likely getting most the heat gain from the lower floors. However, you may want to think about some window tint film to block some of the heat. Judging by the reviews, this film appears to be well suited to the task: http://www.homedepot.com/p/Gila-3-ft-x-15-ft-Titanium-Heat-Control-Window-Film-HRT361/100616385

Second, an evaporative cooler in SF would be useless most of the time... the humidity is too high. Also, you'd be injecting a lot of moisture into your apt, something you really don't want IMHO.

Third, you may want to look at the cost since water isn't exactly cheap in California and will only get more expensive as the drought continues. That is, you have to pay for electricity for the fan and for the water. With the proposed solution, it is electricity only...

EDIT: Adding additional evaporative cooler info....

There is a chart here that shows evaporative cooler ranges of operation: http://www.air-n-water.com/Common-Swamp-Mistakes.htm

I could include the chart here, but it appears to be copyrighted. At any rate, optimum humidity for cooling from 85 is around 35 to 40%. It is almost never that dry in SF. See here: https://weatherspark.com/averages/31587/San-Francisco-California-United-States

Indeed, it appears that a lot of time, you would achieve little to no cooling (i.e. relative humidity would be 70% or higher).

Perhaps the strongest evidence against the use of evaporative coolers in SF might be: have you ever seen or heard of someone using one there? If it were a viable option, people would do it.

  • The fact that nobody else uses them isn't automatic evidence that they wouldn't work. Evap coolers would work great in the south bay, but nobody uses them there for reasons that I have difficulty understanding. Perhaps they seem too "poor."
    – iLikeDirt
    Oct 7, 2014 at 22:28
  • Actually the demon I was referring to was this guy: en.wikipedia.org/wiki/Maxwell%27s_demon Otherwise, this sounds sensible. The film is a great idea, though I'd prefer something that's easier to remove for the rest of the year, when we want as much solar heat as we can get.
    – octern
    Oct 8, 2014 at 3:20
  • @octern, Ah, I get it now. My only question is, are the faster molecules alive or dead before the demon opens the door? Oct 8, 2014 at 21:56

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