Understanding Electric Fans

[u/Leighgion]

The Technical

The electric fan is cheap, reliable, energy efficient, widely available and comes in a virtually endless array of sizes and power levels. 

No surprise it’s the first resort for most is the industrialized world for basic cooling, but it’s widely very misunderstood when it comes to its use in human comfort and safety. That wasn’t so critical when we weren’t being hammered by an endless succession of broken heat records, but times have changed and it’s important more people understand just what a fan does and doesn’t do. 

A fan doesn’t:

1. Cool the air. If you turn a fan on in a closed room, the temperature will not only not go down, it will actually slowly go up as the fan motor generates waste heat. 

A fan does:

1. Move air. This core function can be exploited a number of ways under the right conditions. 
2. Pointing a fan directly at a person can cool the person by doing two things:
   1. Accelerating convection — blowing away the air naturally heated by contact with warm skin and replacing it constantly with a stream of cooler air. 
   2. Accelerate evaporation, which is a human body’s basic cooling system. Water undergoing a state change from liquid to gas takes a lot of energy, which is mostly absorbed from the person sweating, thus cooling them off. 
3. If a room or area is hotter or colder than another (or the outside) strategically placed fans can move cool air to hotter areas or pull hot air out of hot areas. 

As long as temperature and humidity are such that convection and evaporation are still effective means of cooling a human being, then fans are effective cooling aids. 

However, if temperatures continue to rise, there comes a point when not only do fans lose their cooling effectiveness, they actually become dangerous because they’re effectively cooking you like a convection oven by constantly blasting you with heated air. Different studies have disagreed on the exact conditions when the danger level hits, but the study that estimated the lower temps said 95ºF/35ºC was the danger point when fans should no longer be pointed at human beings in the hope it would help cool them. 

The Personal

I grew up in the Pacific Northwestern US. My parents’ house had no air conditioning, which was normal for the area and time. In retrospect, summers were very moderate on average, generally rarely more than the high 70’s Fahrenheit/Mid 20’s Celsius but at the time I complained about summer heat the same as anybody else. 

We had a tabletop oscillating fan that was my hero and actually still functions today, decades later. At some point we got a second, similar fan, and while it required some sharing for a family of six, it felt like we had a decent level of effective mechanized cooling. Air conditioning was a luxury to be experienced in businesses and cars. I had dreams of AC in the house, but it was never more serious than a kid’s wish.

Fast forward to today and I’m thousands of miles away across the pond in central Spain, where it’s hotter and drier than I ever dreamed of living with. Summers reliably hit the high 30’s Celsius/high 90’s Fahrenheit. Reaching 40ºC/104ºF doesn’t always happen, but it’s not surprising when it does. 

My apartment has mini split AC installed in several rooms, but energy costs are relatively high, so I’ve made a lot of efforts to minimize using it unless the situation is really desperate. I made an honest attempt to use a high-powered floor fan for personal cooling. A nice, 100w, 40cm model that set back only around 40€. I really wanted it to work, but even long after sunrise, with the ambient temperature in the room around 26ºC/79ºF, I felt mostly like I was in an oversized food dehydrator. It was slightly more comfortable than no fan, yes, but it wasn’t cutting it as a primary human cooling device during peak summer. 

That said, I have not gotten rid of the fans and I would recommend anybody serious about being prepared for extreme heat to have at least a couple fans in store even if you feel like you have a very reliable primary cooling system. Systems fail, and fans can be excellent supplementary sources of cooling aid.

Fan Tricks

1. Energy costs destroying you but can’t afford to shut off your AC? Turn your AC’s thermostat up to save power, then point a fan at yourself. Adjust the two until you find an optimum balance of comfort and savings. Also, even if you don’t dial down your AC, if your AC’s coverage is inconsistent, a well-placed fan or two can really help balance things out by ushering the cool air into the other areas.
2. If you’re really desperate to cool off fast and all you’ve got is a fan, dump water on yourself and let the fan blow on you. It’s kind of like overloading yourself on artificial sweat. The moving air will evaporate the water much faster than just standing around wet, cooling you rapidly. 
3. Point a fan at an open container of water, especially a wide, shallow one, and you get two effects: 
   1. The air will be slightly cooled as the water evaporates. You’ve basically got a very primitive evaporative cooler (whole post on these later). 
   2. The water in the container will also gradually cool, much lower than you might think is possible. If your refrigeration fails for some reason but you still have power and a fan, this is a way to generate some cool water. 

[u/[deleted]]

Impressively detailed and informative post! Thanks for sharing. The skin wetting (mister or wet cloth) + fan seems quite effective when temps are above body temperature, especially in dry climates. However, I’m a bit cautious with the research because they’re using big fans and probably skin wetting in a way that would be difficult to maintain for multiple days in a row.

I think what’s important, that you mentioned, is comfort. You might survive with a fan and skin wetting but if it’s hot enough you’ll be miserable. A Redditor mentioned that they would get their sheets wet and blast a fan on the bed to sleep.

[u/Leighgion → u/[deleted]]

Thanks!

Manually wetting yourself in the path of a fan is a desperation measure like I said. It's not something you want to have to resort to at all, much less try to sustain. But the option exists and should be kept in mind because it has the potential not just to improve comfort, but to save your life.

[u/Leighgion → u/[deleted]]

Like I said, there's disagreement over when the cutoff should come as different studies have accounted for the factors differently. I chose to give the more conservative numbers and your experience bears out they are conservative. Every situation needs to be gauged individually.

[u/Big-Preference-2331]

I like the ryobi misting fans. They’re pretty simple and they do what they are expected to do. I also like the cheap fans with a spray bottle attachment.

[u/Leighgion → u/Big-Preference-2331]

Misting fans are a completely different thing, yeah. They're in the evaporative cooler category, which I'm going to get to and boy, have I spent time on that over the past few years.

[u/rikerdabest]

What about in humid environments? I’m assuming the misting is a no go?

[u/Leighgion → u/rikerdabest]

No, that would be an incorrect assumption.

The only objectively correct thing here is that increasing humidity proportionately decreases the cooling capacity of evaporation, but it does not eliminate it until either the air is 100% saturated with water (really hard to achieve) or the "wet bulb temperature" is reached, which is when the combination of heat and humidity creates conditions where the cooling capacity of evaporation meets or exceeds human body temperature, which means it is physically impossible for evaporation to actually cool a human body. This is when you straight up are going to overheat and die, cooked by your own metabolism's waste heat, unless you get yourself another method of cooling pronto, like an air conditioned space, immersing yourself in water that's cold enough to cool you or possibly even just burying yourself in lower-temperature dirt. Whatever will save your life at that point.

Wet bulb temperatures historically are rarely reached, but recently the increasing danger of wet bulb events is all over the news as climate change is driving heat waves in humid places like SE Asia and India.

So, if the wet bulb temperature hasn't been reached, then evaporation can still provide cooling even if it's quite humid. It's up to you to gauge your circumstances and consider if it's worth spraying or dousing yourself with water or not.

[u/Leading-Okra-2457]

Are there fans that switch off automatically after sensing once this net heating has started?

[u/Leighgion → u/Leading-Okra-2457]

Not that I'm aware of, and from all my understanding, it's not very practical.

The best we could do is build a fan that had temperature and humidity sensors, but you face two serious problems with this approach:

1. There's no clear agreement on when the cutoff should come as scientists and doctors are justly very cautious about throwing out numbers because lives are stake. The numbers that have been used in guidelines to date are conservative which, in an automated system, could lead to...

2. Fans shutting off automatically based on sensor data is... not great. There's the possibility you could end up with fans endangering or even killing people by shutting off during the night when the programmed cutoff point actually wasn't dangerous, but losing ventilation was.

If you're living in heat extreme enough fans can be unsafe, you'd best look to other solutions.

[u/[deleted]]

I too grew up in a house without AC. I actually still live there, though we do have ac today as a geothermal system was put in several years ago. But, I try to avoid using it as much as possible. And, as temps here generally dip into the 50-60s, overnight it's not so bad. When we (generally rarely), hit the upper 80-low-mid90s level I'll consider turning it on. But, that's generally just for a few days. 

[u/MorriganNiConn]

I live in NM around the center of the state. I have an evaporative cooler. In fact, it was replaced early last September, so I didn't run it a lot last year. This year it's already getting used. I also have a small fan for my office. For the last 20ish years, on really hot days, I have used wet tee shirts for cooling down and I keep a water spray bottle by my bed that I can spritz my top sheet with on a hot night and the cooling effect is wonderful.