Thursday, June 24, 2010

Cheap Air Conditioning - Part 2

This is a follow up on my last post about cheap air conditioning.  I started looking for a whole house fan but the ones I found looked fairly expensive.  About $200 to $900 depending on what I was looking for.  I didn't want to cut a hole in the ceiling and most of them wouldn't fit in my back window.  I started looking at regular fans and finally box fans.  A typical 20 inch box fan has about 2,000 cubic feet per minute (cfm) of air flow.  A typical 16 inch fan has about 1000 cfm.  So, I decided to pick up a couple of $24 box fans from Canadian Tire.  The same fan in the states at Target has been seen for $12 or so.

I cut some plywood to fit the back window and cut holes in it to accommodate the fans.  I took the front faces off the fans and used the grill as a template to mark the size and screw holes.

I cut the hole about 1 inch smaller all around than the grill.  I used zip ties to hold the fan to the plywood by using the front face screw holes.

Here they are in the back window. The top one is upside down so it is easier to turn off and on. You have to have both running at the same time or it does no good.  If just one is on, then the air just goes in a small circle. Both these fans blow out and that causes air to come into the house through the windows.

It is really nice.  I turned them on and every window I opened had a really good breeze blowing.  It completely refreshed our house air in about 2 minutes at a whopping 4,000 cfm.  As soon as the sun set we actually started to get cold and had to shut it down.

The above picture is from a website for whole house fans.  You can see what kind of savings can be accomplished based on where you live.  I checked the wattage with my trusty kill-a-watt meter.  It showed 142 watts for low, 153 watts on medium, and 164 watts on high.  And that is both fans together.  A window mount AC (that would only cool one or two rooms) uses about 1000 to 1500 watts.  A full house AC can use 10 times that.



  1. Great tutorial Richard!
    We have done the same here, although having central air conditioning our electric utility offers a time of use meter and a remote controlled energy management device for the compressor condenser unit. Our "daytime rate" is almost $0.25 per Kilowatt hour [7:00 A.M. to 7:00 P.M.] But only $0.05 per KwH weekends and nighttime
    There is additional cost savings with the load control receiver where the utility may, during peak loads turn off the A.C. for a 3 hour period. We never run the AC during that likely period regardless.
    Back to the "whole house" fan concept, often you see these fans tossed into the trash. If the blades are not damaged chances are good the motor shaft bushings ran tight and may be freed up with a penetrating oil or W D 40, after that a light oil, even engine oil will keep it running for a long time, all it takes are just a few drops front and back.
    Another tip, when the season winds down for the use of these fans, they often look a bit dirty. I have found a spray down with a foaming bathroom cleaner and a gentle rinse either with a garden hose or a plant prayer bottle does the trick, any water that may have entered the motor will have long dried by next season. Most fans of this type are considered to be disposable, access to lubricate them is at best a shot in the dark, but a once a year end of season attention to it will offer that lube to "wick" along the motor shaft chasing away dampness, and at best help to keep the motor spinning freely.
    Of course any maintenance is to be performed with the power removed ;>)

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  2. Craig, excellent comment and great advice.

  3. Hi Richard,

    I bought a HUGE whole house "window" fan (7,700 CFM) early last year, but here in Mississippi it is so darn humid that my wooden floor started to absorb the humid outside air, causing the floor to rise and buckle. Also, the nights here don't get very cool, so even when the inside/outside temp is stabilized, it still doesn't cool the house enough to make a big difference (when I lived in California the concept worked a lot better!).

  4. I am in Northern Nevada and I did almost exactly what you did, only had a 12" and 16" fan with about 2400 cfm combined, on hand to test with. By mounting them in a piece of plywood and then to a window in a spare room, like you show, we got excellent results.

    We open windows and turn on the fans once the outside temp gets down to the inside temp, cools off early at night here, and we were able to reduce our power consumption for the summer by 85-90%, using old parts and materials.

    Our July/Aug/Sept power bills were $21, $32, and $25 respectively for a 3,100 SF home. The Neighborhood average, from the power company, was $180, $220, and $200 for those months, so we saved big time with no direct cost. This was keeping the inside of the house at 69 degrees or below for the entire summer with record summer heat this year.

    This next year we are going to try using a couple Lasko 20" Wind Machine fans (about $35 each) that have a 3-speed output from 2500-4900 CFM each and we can get two of them in the same type of configuration you show. This should allow for much faster cooling at warmer temps, so we can open up the house earlier in the evening.

    Because the days get 90-105 during most of the summer, we also installed an attic gable fan a couple years ago. We found that reduced our A/C cost about 35% at that time. The combination of the two are what has been very effective with our climate. The gable fan reduces the heat radiation from the attic and the whole house fans bring in the cooling at night that is held throughout the day by not having the heat radiation.

    Just thought I would share another success story for you, hope it encourages others to use simple techniques to make a big difference.

  5. Thanks, erought, for sharing that.

    CW, yeah, hot and humid is tough to fight. I grew up in Mississippi myself. The solar air conditioning article I wrote would work better for you.

  6. Richard-

    I'm just starting to go thru your blog but I am enjoying it. I'm down in Seattle and your summer cooling tips will work awesome for me. I also liked your windmill. Sadly, I'm in a little dip and have almost no wind to speak of. However, it might work for my cabin on Anderson Island (south Puget Sound area). It looks like a good idea and might work there.

    I'll cruise thru here some more. I have a pool for the kids (in the summer) and read the solar water heater blog post fairly quickly. It looks promising!

    Are you not keeping up with this blog. I noticed you haven't posted anything since December 2010. I did come over from the windmill ebook portal so maybe you are focusing on that more?

    Thanks again for a great helpful blog! Drop me an email anytime!

    Kevin Hellriegel

  7. Do you think that this fan idea would work in place of an attic door?

    We have a large attic with a door that comes right down into the middle of our house. I would think that if I put the fans in that space with the air blowing through the attic and out the attic windows that the attic as well as the house would stay much cooler.

    By the way, I live in Central NY so I have cooler nights and the summer is only really warm in late July - August.



  8. Christopher,

    It would work, but there are some things to think about:

    1) This type of fan doesn't push air as well as a squirrel cage fan will. So, you need plenty of openings in your attic to allow easy air flow.

    2) You would want some rigid foam on top of the fan to keep the cold air (and who knows what else) out of the house in the winter. It could be hinged or it could rise on 4 metal guides like a floating pier does. It would open or rise only when the fan is turned on.

  9. Thanks for the post Richard. I am wondering though, how did you get the plywood to stay in the window?

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  11. I don't understand. I would have to put it in and pull it out every night?

    1. nope, it just stays in the window all summer. I put a timer on mine so that it comes on at sundown and shuts off at about 5am.

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  14. Thanks for the great advice. I will look into this when I am getting air conditioning in Fort Mcmurray. Keep up the great advice.

  15. I dont understand how this works when it's a dew point of 70F outside and you want the house 75F when it starts out at 90F inside with an inside dewpoint of 55F (say, from previous ac'ing?).

    Do you jsut train yourself to live at 75F and 80% rh? gross.

    (ie your system exchanges AIR not heat, so you get all the moisture in that outside air too.)

    I have the same system setup but will not turn it on unless the outdoor air is 10C (50F) dewpoint. Otherwise, any higher, say 15C (58F) dew point in a 23C (73F) house becomes 60%+ RH, too sticky.

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