My new house has a 20+ year old evaporative cooler on top of the roof. The cooler is ancient and rusted, difficult to service due to the location, and the forced air heating ductwork it feeds into is undersized for the volume of air it pushes. I would like to replace the unit with a (surprisingly inexpensive) new, highly efficient evaporative cooler mounted at ground level that blows air straight through a wall. The cooler would sit in the shade and have a short, straight duct going right through an exterior wall and into the main living/dining/kitchen area.

My hope is due to the direct installation, I could get a very low level of static pressure and choose a small, energy-efficient 1/3 horsepower motor. However, I worry about this installation being able to serviceably cool the entire house (≈ 1100 sf) with all of the air coming through only a single supply vent.

I think I would be able to get adequate airflow into the bedrooms by opening bedroom windows but NOT opening windows in the living/dining/kitchen area, thereby forcing the flow through the main living area, down the hall, and out through the bedrooms.

Does this sound like it would work? Is there some kind of chart or formula for determining the required CFM of the blower given a static pressure value, a square footage, and/or (pipe dream) a house layout? Should I just get a bigger cooler or motor for some wiggle room and be done with it? I would prefer the more efficient, better designed approach. ;-)

3 Answers 3


To answer my own question, I went ahead and installed a Bonaire Durango 4500 CFM window unit permanently through a wall in my main living area. So far, it has been doing a magnificent job of cooling the entire house, including the bedrooms, and it's even acceptable right now during the monsoon season. When the outside temperature is in the 90s and the outside relative humidity is between 5 and 15%, it gets the house down to an average temperature of 71-74. When the outside humidity goes up to the 20 to 30% range, it keeps the house between 74 and 79. As predicted, the bedrooms are a few degrees warmer than the living area, where the cooler is located. But I only expect this temperature differential to decrease over time as I further improve the insulation and air sealing of the building envelope, so that the cooled air it yields won't be doing as much heat exchange with the building materials as it flows through the house. The actual temperature of the air it blows is always in the low to high 60s. And I can calculate its performance with this calculator, which has proven to be fairly accurate so far.

  • the old one didn't cool enough?
    – rogerdpack
    Commented Jul 7, 2015 at 17:48
  • 1
    Nope. The new one does!
    – iLikeDirt
    Commented Jul 7, 2015 at 17:58
  • When a "soggy" day is only in the 30% humidity range, a single unit is reasonably more than enough! When a "dry" day is 50-60% humidity, don't count on it! Glad this worked out for you.
    – FreeMan
    Commented Nov 1, 2023 at 12:48

The optimal CFM is related more to the cooling capacity than house size, since you have low pressure (usually the other major factor). The cooling capacity is related to house size, climate, and many other factors, so there is some indirect relationship. I couldn't tell you what that is. Consider this a verbose comment and not an "answer".

For the bedrooms to be adequately cooled with that setup, the living area would need to be colder than normal, assuming the unit is adequately sized for the whole house. On the average, your house will be kept properly cool, but you will find the temperatures will vary noticeably from room to room. Neither extreme may be all that comfortable.


The majority of swamp coolers I've seen come in through a single tunnel, FWIW...their own tunnel, not the heating ducts, though I have heard of it both ways. Each swamp cooler has a "tunnel size" coming out of it, it's larger for larger models, so as long as you use that tunnel size the cooler's fine. And open your windows appropriately? :)

Only problem I could imagine might be getting too cold right near the duct entrance?

Most models list some kind of square footage. I'd say opt for something that says it can cool a bit larger than your house.

See also Is it OK to use an oversized swamp/evaporative cooler?

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