# Evaporative cooling fail

Back story: I am trying to make my attic (where I have my home office) comfortable during heat waves. I live in the Paris area, where heat waves are comparatively dry (in fact, they are the only time when it is comparatively dry).

So, I decided to get a medium-sized evaporative cooler ("swamp cooler"): https://www.amazon.fr/KLARSTEIN-Skyscraper-Rafraîchisseur-Humidificateur-Refroidisseur/dp/B085HM8N2Z?th=1

It does produce a cool breeze, so I suppose it is helpful if you stand or sit literally in front of it and quite close to it. I didn't notice enough of an effect if sitting at 1-2m from it.

So, wanting to determine the truth, I ran an experiment.

My attic consists of two compartments (not counting the bathroom): I closed the windows in both, and ran the evaporative cooler in one of them; I put a thermometer/hygrometer in each compartment. After a while,

• the half without the cooler was at 30.9 C, 50% humidity,
• the half with the cooler was at 30.9 C, 60% humidity. That's a remarkably total and precise failure! The cooler was humidifying without cooling. My question is: how is this even possible?

Wouldn't just about anything (indoor fountain? cat in cold bathtub?) do better than this? How can a tool manage to be so bad? How do you set out to make an evaporative cooler that doesn't work?

Further details

Some data (from https://www.kwangu.com/work/psychrometric.htm): 30.9 C at 50% corresponds to a wet-bulb temperature of just 22.75 C (i.e., we could be very comfortable if were just willing to do math while wrapped in wet towels). An evaporative cooler that keeps enthalpy constant (I thought that was a reasonable assumption - these are low-powered devices, so the heat they dissipate by operating is likely negligible) would keep wet-bulb temperature constant. Then, at 60% humidity, we would have expected the temperature to be lowered to 28.7C, a significant change.)

I ran the experiment again the following day. Starting conditions (at 4:50pm): outside temperature 28C, attic room with cooler: 32.8C, 42% humidity; attic room without cooler, 32.9C, 47% humidity.

End conditions (at 11:25: attic room with cooler - 30.3C, 60%, attic room without cooler - 31.2C, 51%). All right, this time, the cooler did something, but it seems to be remarkably bad at what it does. What does the effectivity of a cooler depend on? Is the performance of this one typical, or is it particularly bad?

(Further information: yes, the attic is reasonably well-insulated - I had the side walls insulated myself. Ceiling insulation was done by the previous owner, presumably to the standards of about 15 or 20 years ago; there is about 15cm worth of it. There are plenty of (double-glazed) skylights, but they are covered by outside shades in this season (the shades are partly see-through but pretty effective; you do not want to know what it was like before I got them). The roof is clay tiles.)

• they stop being effective when relative humidity is high. Adding a vent or leaving a well-positioned window open to create circulation and egress for the moisture helps. New technology evaporative coolers with sandwich sheets of water do better. Commented Jul 2 at 8:48
• Sure, but why would they stop working at 50% humidity? See the above: just going from 50% to 60%, temperature should drop by several degrees (for constant enthalpy). Commented Jul 2 at 9:22
• Can one install a temperature-sensitive vent, so that it goes on high when the outside temperature drops? I can’t always leave the skylights open during the night due to the possibility of rain. Also, when it is hotter outside, what to do to expel humidity without letting in heat? Would double-flux single-room heat exchangers also work in summer? Commented Jul 2 at 9:24
• I suggest that 50% relative humidity is not "dry." Dry is Saharan Africa. Commented Jul 2 at 16:44
• Isn't the problem just that you run the cooler in a closed room? The cooler just "moves" some heat from the air to the water vapour, but it remains in the room. Could you run the experiment with open windows in both rooms? Commented Jul 2 at 17:36

Too long for a comment.

What was the actual water consumption of the device? I suspect you have a lot of air exchange (stack effect); and probably had too much thermal mass and not enough time in the first case. What is the rated cooling performance of the machine and how does that compare the the amount of water that was consumed?

Note that cool air sinks. At constant temp, humid air rises in dry air, but if evaporation is used to cool the air, the airmass will sink (down the stairwell, I imagine). A circulation fan will be important. A temp of 87 F is marginal for evaporative coolers. They work way better when the air temp is higher than body temp.

I'm also curious about the actual outside conditions at the time. What was the air temp outside at 2 pm when the indoor temp was 31? What is the actual ceiling temperature? If there is a large heat flux already in the room, ie. large radiant and convective heat gain being exhausted by mass transfer, this heat flux may be all out of proportion to what the cooler can handle. Basically, it mostly just reduces the stack effect and the temp doesn't change much.

Just for reference, 1.8 gallons/hour of evaporation equals one ton of cooling.

• I will have to run the experiment again in the next heat wave, but - outside temperature on that day peaked at 31C. . I have a tiny stairwell at the end of the attic (it wouldn't be up to code in the US), with a door at the bottom - and the part of the attic with the cooler in it was not connected to it. There's no chimney, and the only windows are new, airtight skylights. So, I suspect the stack effect is not responsible in this case. (But really! 87F at 50% humidity is uncomfortable enough!) Commented Jul 2 at 12:31
• Next week it will be fairly hot again. I’ll run the experiment again, this time with open skylights. Perhaps the stack effect will be enough to keep cold air down - but will humidity will go up? If it is water vapor, yes, but if my evaporative cooler is a fake evaporative cooler that just produces droplets that fall lazily… Commented Jul 2 at 18:31
• The manufacturer claims a 180m³/h debit with a 0.8l/h consumption. Commented Jul 2 at 20:50
• You've only got 2000 btu/hour of cooling. That's nothing. You can buy a tiny window unit ac for under 150 euro that would do 3 times as much. Commented Jul 3 at 0:49
• The issue is not unit cost (it was also under 150eur) but power consumption. But you are right - for the price one would expect more cooling. Commented Jul 3 at 3:29

Evaporative coolers require a constant flow of outside air pushed through their cooling pads and then exhausted out through opened windows in the interior space as far as possible from the cooler discharge location. If you run them with all the windows closed, then the room is humidified and not cooled.

Best results are had when the outside relative humidity is 10% or less. At 50%, the cooling effect will be significantly impaired. This is why "swamp coolers" are used in the desert regions of the American west, and not in the midwest and southeast.

• I agree that these devices are best run with windows open. However, a true evaporative cooler deserving of its name will in fact lower the temperature of the room while humidifying it. The total heat in the room will go up very slightly, yes (in the sense that the enthalpy will go up slightly). Commented Jul 3 at 3:31

As observed by @NeilsNielsen, Swamp Coolers depend on continuous flow and require quite low humidity to be effective; they won't work in a swamp.

I moved into my house outside Phoenix, Arizona, I turned on the swamp cooler with the windows closed and the house was plenty hot. I thought, "This thing doesn't work for beans!" After a little thinking, it occurred to me that if there is not air flow, it can't keep adding water so it can't keep cooling. I was getting higher humidity without the cooling, not what I signed up for. I left the windows open a few inches and it made a world of difference. When it was 111F, about 44C, we would have about 10% humidity outside. With adiabatic cooling (following the lines of constant enthalpy on the Psychrometric Chart), if we went from 10% humidity to 50%, the air temp would be about 86F or 30C, which after a few years of living in 100F plus temperatures from May to September, we found comfortable. The bad news is in August, the humidity would go up and the swamp cooler did not cut it so we had to switch to the A/C with the compressor. (People say, "But it's a dry heat." Don't let anybody kid you. 118F is brutal no matter what the humidity.)

Some checking on your numbers, if it is 30C and 50%, if you go to 70% humidity, you will get about 25C, not much of a difference. You won't get to 100% humidity (as uncomfortable as it is) for the wet bulb temperature with a swamp cooler.

My suggestion is put a fan in the window to keep getting fresh air. It does not help much in the afternoon but in the evening it allows getting cool air in, displacing the hot air from the afternoon.

• After typing up my answer, I clicked on the amazon.fr link for the humidifier. That device won't do much of anything, anywhere unless it is actively blowing on you. My A/C in Arizona had a "piggy back" swamp cooler. There was a box on the roof, next to the A/C condenser that sucked air through water soaked grass pads into the house. There was a pump that would cycle water from the (auto-filled) reservoir at the bottom to dribble over the pads, The water was hard so I had to replace the pads every year or two when I flushed out the calcium from the reservoir. Commented Jul 3 at 1:35
• Yes, I think now the device is not a true evaporative cooler and won't do much of anything. How do I tell whether something advertised as one is really one (other than by buying it and testing it)? Commented Jul 3 at 3:34
• (NB. 25C would be great. Yes, opening the windows in the evening obviously helps - the problem is that, in this season and at this latitude, "the evening" means basically "bedtime", and there's the small risk of rain coming in while I sleep and damaging the floors.) Commented Jul 3 at 3:36
• @HAHelfgott, How to tell? The device listed on Amazon, has pictures of smiling people with the device blowing directly on them. This really will work if you have lots of fresh air and have it blowing on you. Heat is not destroyed, it can only be transferred, either pumped out with an A/C or carried out with replacement air with a swamp cooler. There is no shortage of things for sale that just don't work. In the US, the FDA keeps Snake Oil off the market but everything is anybody's guess. Commented Jul 3 at 4:30
• Again, not to repeat myself, but, if the device worked (which it doesn't) then it would lower the temperature with closed windows (though the amount of the heat in the room would obviously not go down; it would go up slightly). Actually losing the heat would involve opening the windows, yes. One would think one should be able to get a genuine evaporative cooler somewhere in Europe... Commented Jul 3 at 4:40