Instructables

Whole house fan blows away our A/C bill.

Picture of Whole house fan blows away our A/C bill.

In certain climates a whole house fan (WHF) is an effective way to offset the high cost of air conditioning. The theory works particularly well in areas with very hot days and nice cool nights like the Central Valley of California where we happen to live. The WHF is installed in the ceiling and blows upward into the attic to force the hot air accumulated during the day out your roof vents while sucking cool nigh-time air into the house through open doors and windows. In the morning, the fan is turned off and windows and doors are closed up to prevent the cooled interior from warming up.

For many homes, installation of a pre-manufactured WHF is quite simple. Calculate the volume of your home (square feet of floor space times average ceiling height) and use readily available formulas to determine the size of the fan you need. Then cut a properly reinforced hole in your ceiling based upon the size of the fan you are using and mount the unit. You also need to calculate and allow for the proper amount of roof venting. There are many pre-manufactured WHF units which can be found at big box stores and on the internet.

In our case, however, we ran into two snags during the planning phases of the project. While our home has a relatively modest 2,383 square feet of floor space, our ceilings are 10 feet tall rather than the normal eight. As a result, the volume of air we needed to move was larger than most typical homes and beyond the capacity of most lower cost WHFs.

Our second, and larger, problem was that whole house fans are designed to operate in a horizontal position. Unfortunately, due to truss design and a maze of duct work, there was virtually no area in our attic where a “store bought” WHF could be mounted horizontally.

As a result, we ended up designing and building our own vertical WHF. We used a “drum fan” which is made to operate an a vertical position and has the additional benefit of moving large amounts or air at a lower cost than typical whole house fans.

[Note to readers: After publication of this instructable, member g1981c offered some additional information about fan design and effectiveness which can be found in the comments section. Based on those comments, I just (6/12/14) swapped out the original drum fan (which is NOT designed as an exhaust fan) and installed a fan specifically designated as an exhaust fan. The improvement in air movement and cooling is clearly evident. We can run this new fan for half the time we ran the drum fan and draw in more cool night air than we did before. I was also able to find just enough unobstructed flat surface in the attic to get this new fan installed in the horizontal position. This note is just to recommend that readers not only insure that the fan they select is installed in the correctly designed position (vertical or horizontal) but that the fan be designed specifically as an exhaust fan.

Fortunately, our drum fan is being put to good use keeping out breezeway and my workshop cool and usable during the heat of the day.]

 
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g1981c4 months ago

what you may have not realized is that blower fans are not designed to work against a pressure gradient - they are designed to work in zero static pressure environment. using a blower fan as an exhaust fan is like using a sports car to tow a boat - a sports car may have more horsepower than a diesel pickup - that doesn't mean it will tow better, because it has lower torque and different transmission gearing. as such, your CFM / dollar calculations are somewhat meaningless because once the blower is placed in an exhaust fan application its CFM will plummet. it would be wiser IMO to use tools for their intended application and not to try and be smarter than engineers who designed them. i like your project overall - more fresh air, less electrical bills - but the choice of fan for the job i disagree with.

dewey302 (author)  g1981c1 month ago

To Follow Up: Based on the observations of g1981c I ordered a fan specifically designated as an exhaust fan and just this week swapped it for the drum fan shown in this instructable. I don't understand the physics of intake vs. exhaust fans nearly as well as g1981c but I can report that the dedicated exhaust fan does create a notable increase in the amount of air it moves through the house and the overall cooling effectiveness. I would therefore agree with g1981c's comments and recommend that readers not only insure the fan you buy is intended for either the vertical or horizontal position but that it be designed specifically as an exhaust fan. We are even happier now with our WHF thanks to g1981c.

How did you know that the fan couldn't be upright? Some drum fans have that position as an option?
Great idea and work and instructions!!!

dewey302 (author)  thatotherguy11 month ago

Sorry if my introduction wasn't clear. The drum fan IS built to operate in the upright (vertical) position. And most whole house fans are manufactured to operated in the horizontal position. This is why I chose the drum fan. To determine if any particular fan can be operated in the horizontal or vertical position you can usually find that information in the product description on line, by bringing up the installation manual on line (if available) or by emailing the manufacturer or supplier.

One note of caution. As another individual posted here a few weeks ago, fans are ALSO rated as intake fans and exhaust fans. The drum fan I used is, unfortunately, not rated as an exhaust fan. Therefore it is less effective and not as efficient when used in this manner. I'm no engineer so I can't tell you the design differences. But just this week I did swap out the drum fan for a fan specifically designed as an exhaust fan and the resulting improvement in air movement is quite noticeable. So folks should not only insure fan choice is designed for the position you want to install it (vertical or horizontal) but also for its use (intake or exhaust).

dewey302 (author) 4 months ago

Not sure my response to g1981c posted up correctly...so I'll try it again. Hopefully it doesn't appear twice now.

Looks like I need to go fan shopping. Fortunately, I was thinking about buying the identical fan I used in this project to cool my shop, which has no A/C. Based on g1981c's comments, I think I'll swap out the drum fan and move that out to the shop and then purchase a new dedicated exhaust fan that will work in the vertical position and mount it to the existing plenum.

Thanks for the info and education.

Dewey

KCSCAMP4 months ago

I grew up in OH using an "attic fan"....and I just loved it...plus it was in my bedroom ceiling...
I went to sleep listening to the sounds.
I had a super little room with 2 doorways..I would shut one and get twice the air....YUM!
After I moved out my Dad added a fan at the end of the house too...so they had 2 attic fans...
I miss them....

Living in TX these past 31 years sure would have had a use for them!

damianzuch5 months ago

Hi Dewey, I bought a big drum fan years ago to help cool our shop down during the hotter summer days but we had a heat wave last summer and ended up taking it to the house and placing it at the foot of the bed at night. It almost takes the sheets off. I really like this system you have!

Hi, good one. have you considered increasing the thermal capacity? How's about epoxying ally soda cans end to nd
I'm not quite sure what you mean by thermal capacity? Are you referring to more mass inside the house to store heat/cold?
rimar20008 months ago
This is a very good idea.

I am using an old turbo fan, one of these cheap chinese, put in a hole at the attic wall. I turn on it in hot nights when the outer temperature lows, at late afternoon. It is very effective.