Swamp coolers and humidity

I am trying to find out how much swamp coolers affect humidity, or to what degree a swamp cooler moves the humidity needle.

I realize the higher the RH is, the more uncomfortable heat is, so if a swamp cooler is not working in an optimal scenario where the humidity is low, is there a point where comfort is burdened even more by using swamp coolers?

For example, If the air temperature is 90 and the RH is75, then at best you can expect a 4 degree drop, but does the interior RH increase and by how much? The question I have is can if it is 90 degree temp at 75RH, will be more comfortable than say 86 degree temp at 85rh?

If you live in the southwest such as the Mohave desert rh increase is a good thing… If the cooler is functioning properly it should be decreasing the air temp relative to the ambient temperature. If you live in Florida it may not be if the ambient temperature is in the nineties… Probably why they are common out west and not in Florida.

In your particular example it is unlikely you can “feel” a change in comfort between those two points.

There is a difference of about 15 grains per pound of dry air between the two points you describe.

If you look on your own chart you’ll see that 90/75 is on the very outer edge of the parameters for evaporative coolers. Anything beyond that would obviously indicate the use of evaporative cooler is ineffective.

Relative humidity is “relative”. Higher relative humidity does not necessarily mean “uncomfortable”.

Measure the indoor and outdoor air humidity on the coldest day in Los Angeles. You’ll find a large swing in relative humidity readings but it has absolutely nothing to do with the amount of moisture in the air. The indoor air and the outdoor air moisture content will be the same (unto you add an additional source).

Comfort conditions are also relative.
Age, gender, nationality, climate zone of residence all have effects on perceived comfort (people from Phoenix do not have the same comfort as those from Anchorage). Look up ASHRAE comfort level psychrometric charts and you will see there is a huge variance in conditions that are considered comfortable.

As for how much the humidity needle swings, it depends on the house. You must calculate psychrometric mixed air formulas based upon building air leakage rates to determine just how much is too much. Just because you add humidity through evaporation to the building does not mean the ambient air within the building will remain the same as the supply air levels. Moisture has different vapor pressures and high vapor pressure will want to normalize with low.

Airflow within the house with the swamp cooler operating (without any drop in temperature or change of humidity) in of itself improves comfort levels which is also difficult to calculate in your “comfort formula”.

If you’re looking for a “rule of thumb”, there is none. There are too many variables.

Thx for the response!

I found more information that helped me get a more overall impression of the relationship between air temperature, humidity, and comfort levels.



Swamp coolers apparently blow in air at 80-90% humidity. How that affects the interior of a given space is of course harder to understand given that there are alot of variables to consider. In time I expect to be aware of some case studies or samples to illustrate to what degree swamp coolers can effect humidity levels. Im guessing there is a complicated math formula out there that can help just waiting for google to uncover.

Heat Index/ Apparent Temperature Explained

It’s called psychrometrics.

Go to Trane or Carrier.

What exactly concerns you referance to “to what degree swamp coolers can effect humidity levels.”?

This is a very broad subject. Are you concerned about the effect of moisture on the building, indoor air quality, mold growth…? Or just comfort.

If you want to determine how much humidity is being pumped into the house, you need to measure the volume of water being used by the cooler. You then normalize it with reference to the building parameters.

Trying to measure from the air side is nearly impossible with what is available to you. You can measure how many pounds of water is in the air, but you don’t know how much was pumped in to reach that level.

Give us what you are looking to achieve. I don’t think you will find the answer in Google which you will fully understand because it will be written in Engineer.

BTW: That Heat Index chart has little to do with what we are talking about. It will just confuse you more.

It only takes in account dry bulb temp and rh (which we pointed out as being “relative”). It only feels like the chart temp if certain other conditions exist (which likely do not). Heat Index is not a “measurement”, it is a feels like condition. It is quantitative , if you will.

Primarily I’m concerned about comfort and effective/efficient use of swamp coolers.

According to “Residential Energy: Cost Savings and Comfort for Existing Buildings” (6th Edition) by John Krigger, evaporative coolers exhaust air that is at 80-90% relative humidity. Im not looking for exact numbers, just a general ball park understanding of how humid a swamp cooler can make an environment.

Well I am working in a company with several warehouses. Swamp coolers are used on hot days but there is alot of ideas and theories on how to use them. One theory is to turn on swamp coolers early in the morning and let the temperature get as cool as possible to combat the heat. Some dont want it used at all and believe is just makes things muggy, some say to use them over night. Long story short there is alot of ignorance about how evaporative coolers work and how they should be used and Im simply looking to be more educated on the subject.

If a swamp cooler is turned on in the mornings when humidity is high (60-80%), then the reality is that a swamp cooler is not lowering ambient temperature at all and only serving to increase the humidity levels inside. The higher the humidity, the more uncomfortable its feel as the temperature rise. To what degree was elusive but not nearly so much now.

IMO its important to understand how quickly a swamp cooler can effect RH so as to turn it on or time it so that humidity levels can be kept to a minimum.


A swamp cooler is basically self-regulating as to the amount of moisture it injects into the condition space.

When you have 80% relative humidity in the morning the amount of moisture added by the swamp cooler is negligible. As the heat of the day arrives and the relative humidity falls, more moisture evaporates at the swamp cooler (subsequently lowing the air temperature).

Again, 70°F 80% relative humidity is comfortable. 95°F 36% relative humidity is not (and they contain the same volume of water vapor). The reason is a sensible rise in temperature, not the percentage of relative humidity. Adding humidity resulting in lowering or keeping the temperature at 70°F does not make you uncomfortable because of the increase in percentage relative humidity.

Two of the comfort factors that come to play here are temperature and humidity.
When the temperature gets up to or above your body temperature you begin to feel hot.
Higher temperature migrates towards lower temperature. The lower the rate of heat transfer the warmer you feel.

Based upon the amount of perspiration you are generating (proportional to your activity rate) relative humidity affects evaporation rates off of your skin.
If you are not perspiring, the relative humidity of the air has less effect on your comfort. If you’re highly active and perspiring, the relative humidity of the air is significant because you become the swamp cooler.

If you are sweating a lot, relative humidity is a greater factor. If you’re sitting behind a desk, dry bulb temperature is a greater factor.

Do not think of a swamp cooler as a humidifier. A humidifier uses heat to change the state of water to a vapor. A swamp cooler does not. The rate of evaporation is based on the relative humidity of the air passing through the machine. Heat of the air is used to change the state of water to vapor. Vapor still contains the same quantity of heat energy but the sensible heat you feel is lower.

From a different perspective; being a warehouse you probably have a lot of air changes going on. If you lower the temperature within the building from 95°F 36% relative humidity down to 70°F, you are creating the muggy atmosphere because you lowered the temperature without removing humidity. It’s not because the swamp cooler is adding to the percent relative humidity, it’s because a swamp cooler does not operate below the dewpoint temperature and cannot remove the latent heat which subsequently lowers the sensible heat like a refrigerated air conditioner does.

Because you have a warehouse in this situation and subsequently a large air exchange rate, there is likely to be a point where you will exceed the capabilities and therefore move outside of the comfort zone utilizing an evaporative cooler. We need the specifics to make that determination.

hello guys,
I am new here,

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