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Community and Q&A

Running Evaporative Cooler and AC Window Unit Simultaneously

etting | Posted in Mechanicals on

We’re getting afternoons above 105 here in central AZ, and the window AC that keeps the small living area in my temporary house tolerable most of the time isn’t quite keeping up.  I have a portable evaporative cooler big enough for my space with less extreme outdoor heat, but it isn’t enough by itself now either.  An input of dry air is recommended to prevent the evaporative cooler from eventually making the interior air too humid, but would it work to use the AC’s drying effect instead and get the cooling benefit of both at the same time?

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  1. Expert Member
    BILL WICHERS | | #1

    I don’t think this will work very well. The evaporative cooler will likely add more moisture to the air than the regular AC unit can remove. I’m going to assume you’re in a very dry area since you’re in Arizona, so a larger evaporative cooler will probably work well for you. Remember that evaporative coolers need crossflow to work best, you don’t want to operate them in a sealed room the way you do with a conventional AC unit.


  2. etting | | #2

    Thank you, Bill. I have floor fans to supplement the evap's fan if needed. A bigger evap isn't an option. I'll be interested to see what others think. In case it helps, the AC is 12,200 BTU and 10.8 EER, and the evap moves 1300 CFM, rated to cool 500 sq ft.

    1. Expert Member
      BILL WICHERS | | #3

      You need new dry air coming in, not just extra fans. You basically need some outside air circulating too.


  3. etting | | #4

    As you indicated, Bill, it depends on how much humidity the AC can remove vs. how much the evap puts out. You're probably correct that the AC can't remove enough humidity, but I hope someone who has a good sense of the numbers will weigh in.

  4. tommay | | #5

    If the outside air is 105, any mechanical means is going to have a hard time removing and dumping heat. Keep windows shaded and closed during the daytime and do your cooling at night, and hopefully this night time cooling will get you through the day.

  5. etting | | #6

    Thank you, Tom. That's certainly a good principle, but my current house isn't insulated well enough; I'm building my new one so that it may be.

  6. maine_tyler | | #7

    Does an AC units's efficiency (capacity?) depend on overall enthalpy? Evaporative coolers don't appear to change the enthalpy of the system (they swap sensible for latent load—an isenthalpic process).

    So if the AC performs based on enthalpy, it would see the same load with or without the swamp cooler.
    I'm not sure if this is the right way to think about it... I'm not at all familiar with swamp coolers.

    It intuitively seems like it would depend on the location and air-flow currents of the respective systems, but in a truly closed system, it seems like a wash. ?
    (Could the two systems theoretically operate somewhat isolated and 'split' the load given proper air-current management?)

  7. Jon_R | | #8

    Direct evaporative cooling is beneficial up to the point where the AC starts taking the moisture back out (or you get uncomfortable). Indirect (with a heat exchanger) is useful as long as the resulting air is cool enough to cover the cost of operation.

    Increasing outside ventilation air with direct depends on the outdoor conditions. For example, if direct evaporation brings the air to 75F and 60% RH, then it may be worth using (with or without AC assist). Higher temp or humidity wouldn't be.

    1. charlie_sullivan | | #9

      This is a good guideline. I would inspect the A/C coils while they are running. If they are dry, you can run the evaporative cooler a bit until the A/C coils get wet. Then stop running the evaporative cooler to keep it just getting a little wet. If this seems worthwhile you could then get instrumentation to monitor the dew point of the air to be able to predict that without looking inside the A/C. But you aren't likely to gain much that way.

    2. Jon_R | | #10

      Got me curious - with the current 99F and 28% in Phoenix, I calculate that direct evaporative cooling of outside air can't work without exceeding 60% inside. Needs more like < 90F and 28%, which may not occur, even at 5am.

      Indirect will work in a wider range and could be worth considering.

  8. etting | | #11

    Thank you, Tyler, Jon, and Charlie, for lots of interesting input. If you don't mind explaining how you're doing your calculations, Jon, or pointing me to a digestible reference, it would be good to learn.

    1. Jon_R | | #13

      I don't have an explainable system (I wrote some software), but there are some online calculators.

    2. charlie_sullivan | | #15

      I'm not sure what to recommend as an introduction. I had bookmarked some tutorial info from "coolerado" a company that made indirect evaporative coolers, but they are now owned by another company, Seeley International, and I can't find the information there anymore. As far as a calculator, Here's a minimalist one I like:

      and a super fancy one:

      A calcalation method would be: enter 99 F, 28%. Wet bulb temperature is 71.8. Now add a line (on that first calculator) and enter 71.8 wet bulb and a target temperature to cool to, say 80 F. Humidity is 70.5%. Ugh. That's without factoring in anything about offsetting heat gain through the envelope, etc.

      Suppose we want 60% humidity and 75 F air coming out of the evaporative cooler. Still not very useful but maybe slightly useful. That's a wet bulb of 65. Which roughly matches the 90 F, 28% humidity Jon mentioned.

      With indirect, you can approach cooling the air to the wet bulb temperature, without changing the moisture content of the air you are cooling (constant humidity ratio or dew point). So you can approach cooling to 71.8 F, and probably maintain the temperature inside at 78 pretty effectively. That might be all you need--no need to combine with the conventional A/C.

  9. Expert Member
    BILL WICHERS | | #12

    >”Does an AC units's efficiency (capacity?) depend on overall enthalpy? Evaporative coolers don't appear to change the enthalpy of the system (they swap sensible for latent load—an isenthalpic process).”

    Yes, it does on the cooling side. Sensible BTUs are total heat removed without any dehumidification. Latent BTUs allow for dehumidification and are given and specific humidity levels. Higher humidity = more dehumidification = less heat removed from the system. Basically you spend your air conditioning energy condensing water out of the humid air instead of cooling the air to a lower temperature.


  10. Expert Member
    Peter Engle | | #14

    FWIW, putting the swamp cooler outside to condition air upstream of the condenser does work. The swamp cooler chills the air entering the condenser, improving its performance and efficiency.

    You could also look at 2 stage swamp cooler (direct-indirect). In the first stage, hot outside flows through tubes that are kept wet on the outside. Air flowing across the tubes chills them through evaporation, cooling the air inside without humidifying it. That's the indirect part.
    The cooled air then flows across the standard wet filter pad, which cools it further and humidifies it. Depending on conditions, this might buy you another 10-15 degrees of temperature drop with the same final RH. It does require 2 fans, so uses a bit more electricity but still less than most refrigerant systems.

  11. etting | | #16

    Thank you all for a ton of useful information and tools and experiments to try.

  12. burninate | | #17

    I had similar thoughts to Peter, but my recommendation would be to forget the swamp cooler, and explore just pointing a hose with a low-flow mister function at the outdoor coils and position it there in close proximity; You don't want anything dripping down, ideally it nearly all evaporates. Right now the outdoor unit is trying to heat up 105F air to 120F, moving the heat from indoors to out. If you spraying water at the intake which rapidly evaporates, cools the intake air down to a damp 80F, you've increased the overall capability of the unit.

    1. Expert Member
      Peter Engle | | #18

      Yep, that works too. The swamp cooler is more self-contained, though. Spraying a mist of water right on the coils works as well, but the constant moisture does accelerate corrosion, and the mineral salts left behind can get pretty bad. All of these approaches cool either the coils or the incoming air and reduce the workload on the condenser at the expense of maintenance, water and some power.

  13. Bradleysbash | | #19

    I know I am about a year late in responding to your question, but I've run portable swamp coolers and my central AC this summer and they work fine together.

    I usually don't run them at the same time, but my 18-year old AC unit seems to be doing just fine. (I know it needs to be replaced with something more efficient.) I think that the reduction in the temperature of the air that the swamp coolers bring - I actually have 3 portable Hessaire units that I run in different rooms - outweighs the added humidity they bring. One of my portable units is a Hessaire MC26A which allows you to set a target humidity level as low as 50% and as high as 90%. What I've done on some days when the temperature is over 95 degrees is run the evap units to get the house as cool as possible, then shut them off and turn on the AC.

    My central AC unit is a 5-ton energy guzzler that uses over 7,000 watts when it is running at full power. My estimate is that it uses 4,500 - 5,000 watts on average on hot days. I don't use it that often now because I cool the house with portable evap units. In July 2021 my electricity consumption dropped by 42% because of the switch to evaporative cooling - and it we still used AC 5-10% of the time.

    I do want to stress that if you are using a portable evaporative cooler indoors you need to make sure that the air intake is against an open window or is vented to the outside. 2 of the 3 portable units I have come with side vents and I have set up mini-ducts so they pull air from outside, not inside.

    I am working on a next-generation portable indoor evaporative cooler and I've built 2 prototypes this summer. I'm hoping to have one ready for production by next May or June at the latest.

    1. etting | | #20

      Thank you, Bradley. It will be interesting to see what you come up with.

  14. walta100 | | #21

    Jeff help me understand you have the swamp cooler filling the house with outdoor air it has cooled down an equal amount of air must be getting out of the house somewhere somehow. It seems like most swamp coolers move 10 times the air your AC is likely to move. Given how fast the air in the house is being replaced by the swamp cooler it seems to me the AC is doing nothing unless the duct is blowing in your face.

    My guess is evaporative cooling is great when the outdoor humidity is under 20% but as more people move to the west with golf courses, pools and grass lawns there will be more day when evaporative cooling will not be acquit.


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