Sunday, June 26, 2011

Praise for Swamp Coolers (in Denver)

We've got a bit of a heat wave going in Denver right now, so I felt compelled to brag a little about my $400 air conditioning system.  If you look at this photo closely, you can see that the temperature of the top floor of my house was 59F this morning, after running the AC all night.  Doesn't this seem wasteful?  After all, the EPA recommends a 76F setting for your air conditioner.

Well, no, it isn't.  My AC is actually an evaporative cooler which is really just a ventilation fan that passes the ventilation air (3500cfm in this case) through a water-soaked pad.  It costs me only $0.80/day (7 kwh/day) to get my house this cold during 90F weather.  What many folks don't understand, however, is that when it's hot (over 86F), you shouldn't run the thing during the daytime.  It should be off from about 9am to about 8pm, with the windows in the house shut tight.  Apparently the swamp cooler manufacturers don't understand this either, or they'd be selling controls with timers at Home Depot.

And I'm not feeling guilty about contributing to global warming, because I'm actually cooling down my neighborhood when the cooler runs.


  1. we use an attic fan at night, because the night air is almost always already cooler than the house, so we don't need a swamp cooler at that time; however our old house is not (until we spend several thousand dollars) well enough insulated to stay cool through the day; so we have a window-mounted swamp cooler on the east side of the house; it is used on hot afternoons (when the sun is not beating on the pads) and keeps the house in the low 80s until it cools down outside

    what is it about 86 degrees that makes it somehow wrong to run a swamp cooler? the relative humidity is lower the higher the temp, and we definitely get good cooling results from our swamp cooler

    i don't follow the logic about "cooling down your neighborhood" -- one should still be conscious of the energy and water use of swamp coolers (and attic fans); eventually i'd like one of the more efficient two-stage evap coolers, but tightening up the house comes first

  2. Garbanzito,

    You make some great points and ask some good questions. I'm throwing out some very rough and qualitative numbers here, and they only apply to a pretty well insulated house and assume that the relative humidity is at a typical low value common in the Mountain West.

    First, run your swampie at night! Why would you use the attic fan when for the same energy consumption you can have air that is 8F-15F colder? If your house is brick and you cool it down to 55F by morning, you might not need the evap cooler in the day.

    Attic insulation is by far the most important for summer comfort because that's what the sun is beating on all day. And don't forget EXTERIOR window shades for the east and west walls of the house.

    Careful about the fancier two-stage coolers or the Coolerado, instead of $1000 installed cost, they can run $4000. Attic insulation is only $1000. Once you've tightened up and insulated the house, you'll find you don't need anything fancy.

    86F is the magic number for many houses because if the house is 72F and it's 86F outside, turning on the cooler will actually heat up the house much faster than it will if you just keep everything shut up tight. You may find that it feels good to have fast moving air sweeping through the house (which it does), but if you're not home, that effect is useless, so keep the cooler off. Your ceiling fans are better for creating that effect because the windows are closed.

    So to use a swamp cooler effectively during a heat wave, you do have to "overcool" the house significantly in order to stay cool enough during the hottest part of the day.

    As far as conserving energy, the cooler is so cheap to run, that the savings are negigible to stay at 76F versus 73F average indoor temperature

  3. thanks for the followup info; your encouragement helps accelerate my attic insulation plans

    note that my house is always warmer than 72 degrees on summer afternoons, especially upstairs, where i work during the day ...

    it seems like the "magic number" is a rule of thumb based on how much evaporation can cool the air (less the efficiency of the cooler); i wonder if 86 degrees is meant for a more humid climate? i found an online wet bulb calculator (one of many) here:

    and plugged in conditions at 6 p.m. today from my weather station: 88 degrees, 35% RH, and 29.54 inches atmospheric pressure (i adjusted the dew point until i matched the right RH)

    it gave me a wet-bulb temperature of 68 or a 20 degree drop; assuming my cooler is 80% efficient, that is 16 degrees, meaning i could expect the cooler to blow out 72 degree air, even on this fairly humid post-monsoon day; meanwhile the indoor air downstairs is 78 downstairs and 85 upstairs,; i turned on the cooler and in 15 minutes i had dropped 2 degrees upstairs and 3 downstairs and it kept dropping; i would expect even better performance on a less humid day

  4. Right, 86 degrees is strictly a very rough rule of thumb for a well insulated house, and typical conditions. Your mileage may vary, and sometimes the cooler is your only resource on those hot afternoons upstairs.

    For example, if my house starts the day at 72F, it will never exceed 80F if it is shut up tight. Your house reaches 85F.

    I would seriously consider a $99 Home Depot window AC unit for your office on those hot afternoons. But of course, never run a closed cycle AC at the same time as a swamp cooler.

    And don't forget to overcool your house at night with the swamp cooler.

    80% efficiency is pretty optimistic for these cheap swamp coolers, 50-60% is more likely. Stick a thermometer in the outlet airstream to find out. And your wife will find out that you are true nerd!

  5. Kevin, do you also have a vent outlet to balance the pressurizing of your house via the swamp cooler? I would assume ideally you'd want either motorized louvres linked to the cooler (or pressure activated louvres) located to create airflow across the house and balance the pressure generated by the cooler fans?

  6. Yes, with a swamp cooler, you must open a window somewhere in the house. The air will flow to that window, cooling everything on the way.