The Evaporative Cooler (4 easy steps to using a psychrometric chart)



A few years back in mid-2015 I moved to a dry hot place and happened to rent out a house that had an extremely inefficient air conditioner. The electric bill was horrible, and I had no water bill. The most obvious solution was a swamp cooler. Being inexperienced in everything related to evaporative coolers and how they work, it took some time to come to understand the ins and outs of these machines.


First, the basics:


Put some rubbing alcohol on your hand, and let it dry. This will feel cool on your hand. After you get out of the pool, you feel cool as the water evaporates off your skin. Once all the water dries off your body, you feel hot and want to jump back in. This describes the mechanism at work here. As water evaporates it cools. Thus if you blow a bunch of hot dry air over/through something wet it cools and moistens the air. Thus the basic thing to visualize is a fan, a ‘medium’, and some water.


We could go into the science and start talking about the phase change of the water, and all the specifics. However, the basic idea is a wet towel being being blown by a fan. That’s how they did it in antiquity and it hasn’t changed much since then.


My specific situation:


My first month’s electric bill was $600! The half ton A/C unit was a rust bucket, and by my estimate this house should have a 1-ton unit. The thing ran 22-24 hours a day and used 4 kwh. 22-hours x 30-days x 4-kwh x $.30. This comes out to ~$800. To explain the discrepancy: In California they have this screwed up electric bill system that increased the price per kilowatt the more you use, with the max being around .30 cents, so some of the cost was in using up the lower cost tiers.


It took some time to figure out exactly how much energy the A/C system was using, but not until after a whopper $600 electric bill had come in and been paid. I made a spreadsheet to calculate the california electricity billing system, it was quite complicated. I looked at the meter a lot; When I turned off the ‘always-on’ A/C the drop in usage was 4 kw! Sure enough it was an honest bill. I considered moving out, forcing the landlord to fix the A/C (very doable in CA), and a lot of other options.


Then I found the swamp cooler!


Some more details on Swamp Coolers:


There are several varieties of swamp coolers. The big boxy ones that re-circulate water, and look like A/C units. The roller ones for shop floors. Also, window type units. However, the efficiency of the unit is based on the medium inside and not the shape. Sizing the unit is based on the square footage you want to cool. Well, sizing is really based on the volume so if you have vaulted ceilings it’s not the same as regular ceilings.


The medium inside the cooler governs its efficiency. The blue-green plastic mats are not very efficient. The pink stuff isn’t either. The wood-shaving stuff is somewhat efficient. However, the cardboard looking stuff is the most efficient!


A swamp cooler must move all the air in your house through in a few minutes. In short, the best strategy is to ALWAYS oversize in order to get the best results. There should be a path for the air though the house/building, and a cracked window or two on the far side. Not a wide open window.


Lastly, in a lot of places that you can use swamp coolers, the water is hard. Thus units that re-circulate water don’t fare so well.


The Find, the buy, and the usage:


I was in the local big-box store and was wandering through the swamp cooler section. All the prices were way out of range for me, but then I noticed a $650 window unit for the exact square footage of the house I was renting. This caught my eye and I went home and researched the hell out of it.


The result of the research was that the thing would pay for itself in about a month and a week. I won’t link to the thing here, but there are various brands of window mounted plastic swamp coolers at big box stores in desert areas. They all look alike and function pretty much the same, but the key thing is to make sure they use the waxy cardboard looking medium. These window type units are not for the type of person that calls a repairman. It’s more of a diy type machine with lots of interesting design choices meant to fool you into breaking it. By reading forums online I learned from others mistakes.


It took a weekend, several trips back and forth to the hardware store, and roughly $800 in stuff (cooler + hose + fittings + other materials). The thing worked wonders. It barely used 1 kwh. It reduced the cost of electricity down to 13 cents per kwh. By the numbers it actually paid for itself in 33 days, and then kept going. The electric bill went down to $100/month. It was a great success.


However, there was a weird problem. On random days it would just not work. Out of the blue on a random day the house would get hot and muggy. These days sucked.  Also, sometimes it would just half work. No amount of opening the windows more, fans, or anything would help. It was as if the swamp cooler turned off of it’s own accord.


The solution, and the point (swamp cooler advanced lessons):


Prior to starting this I had zero experience with swamp coolers. Thus, there was a missing bit of knowledge that I hadn’t considered. I was discussing the ‘random day’ issue with a friend and his response was, “It’s monsoon season what did you expect.” Obviously it took a few days for those words to sink in.


The reason the swamp cooler stopped working randomly was because of spikes in the humidity. There are three key tools that are necessary for having a swamp cooler. Those tools are namely,


  1. A temp/humidity sensor
  2. A website that can show you the predicted temp AND humidity on a given day.
  3. A psychrometric graph


First spend some time (read days) moving the temp humidity sensor around the house and observe thing. For example, I learned that the particular swamp cooler could raise the humidity 40% on a good day, and 30% on a bad day. The reason this tool is needed is to asses if things are working correctly.


Second, find a good website that can show you humidity predictions. This is key. You need to know the humidity and temperature. My go to here is wunderground.org. They have a graph prediction for the next 10-days. On the graph you can customize it to display temp and humidity.


Third and most important is you need to print out a good large psychrometric graph. This is where the magic comes in. By using this graph I figured out how to predict the days where the swamp cooler would not work, also when it would work great. It takes some getting used to, but to the uninitiated it looks like some kind of sorcery.


4 Easy steps to Using a Psychrometric Graph:


  • Put your finger on the bottom x-axis at the temperature that it is going to be (or is) outside.
  • Slide your finger up to the curve that matches the humidity (or an estimation of the curve)
  • Slide your finger diagonally past 30% to 40% (the amount of humidity that your swamp cooler will add to the air)
  • Slide your finger back down to the x-axis and this is an estimation of the temp that the swamp cooler will output (not the temp of your house).


(see chart below)


It is key to understand that this is an estimation, and if the cooler outputs 80 degree air, your house will be hotter than that. In my case the far side of the house could be 3-6 degrees hotter than the output of the swamp cooler. Also, some swamp coolers only add 20% to the air and are horribly inefficient.




It is 100 degrees out, and 20% humidity. What temperature will a swamp cooler output if it adds 40% humidity?


  1. Start at 100 on the bottom
  2. Slide up to the 20% curve
  3. Slide up-left to the 60% curve
  4. Slide down to 80 on the bottom.


Thus a swamp cooler adding 40% humidity to 100 degree air that has 20% humidity will output 80 degree air.





http://www.cpuc.ca.gov/ (they try very hard to hide the info here)


https://docs.google.com/spreadsheets/d/18UmuYY2MZNcGBXqYW7B66MbqC8hdJqXPIF9RA_oOsV0/edit?usp=sharing (my complicated spreadsheet for calculating energy costs in my location)




http://imgur.com/a/GF0gx (imgur backup of image)


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