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.]
Step 1: Prepping the Fan.
After doing a good bit of comparison shopping on the web we chose the Q Standard 36" belt drive drum fan model #10265. The fan has a 2/3 HP 2-speed motor and delivers 11,200 CFM of air transfer which is more than adequate for our home. We paid $307 (Northern Tool via Amazon) delivered to our door. The cost per cubic foot of air movement is typically much better for drum fans than for whole house fans. Our fan did arrive with a few dents and dings due to poor packaging. But these were easily hammered out and made straight.
To mount this fan vertically and draw air up out of the house a large plenum must be built. This can be constructed of sheet metal or plywood. For our fan we used two 4x8 sheets of 7/16" USB plywood. The first step in constructing the plenum is to remove any wheels, handles or fan guards that might be attached to the drum fan. Then level the fan on a flat, level floor. Note the stack of 2x4's used to get the fan mounting bracket to a vertical position. Also note the closed cell foam under each foot of the fan. The foam is used to help reduce vibration and noise transfer and must be in place during mock up and final assembly to insure the plenum opening will be at the proper height.
Step 2: Building the Plenum
The front and the back pieces for the plenum are 42" tall by 47 7/8" wide (USB is not quite a full 48" wide). Center the fan against the front USB panel piece and temporarily clamp it in place. Make sure the panel is resting flat against the floor. Scribe a line on the plywood panel around the inside AND outside of the fan casing.
On the outside line mark each spot where the fan guard screws were attached. There will be a slight indentation for each screw position as show by the arrow in the photo. Make your mark as close to the center of the indentation as you can.
Remove the plywood panel from the fan and extend each of the marks you made and drill a hole half way between the inside and outside line. For my fan the screws required a 5/16" hole.
To cut out the opening for the fan, first drill a pilot hole just to the inside of the inner circle. Then cut out the fan opening using a saber saw or sawzall staying just inside the inner line. The plywood panel should look something like the final photo.
The screws which came with the fan for mounting the guard screen are not long enough to go through the plywood panel. Substitute proper length screws (I used 1 1/4") and use the original flat washers and lock washers if possible.
Screw the plywood panel to the fan.
Cut four corner supports from 1x3 pine. These supports will allow you to screw the four sides of USB plywood together. The four corner supports are 41 9/16" which will allow you to install a 7/16" top on the plenum which will then end up flush with the side height.
Cut the side panels from USB and screw the corner supports to the edge of each panel. The side panels for this plenum are 31 7/8" wide x 41 9/16" tall. The width of the panel is determined by the width of your floor opening and the position of your floor joists. So it may vary depending on your situation. The finished plenum should sit directly on top the floor joists.
Set the side and rear panels together and screw pilot holes through the plywood at each corner so that the screw tips will hit near the center of the 1x3 corner support. Cut and screw in place 1x3 supports for the lid of the plenum. You will note that the front 1x3 support can not extend the full width of the plenum since it will interfere a bit with air flow at the top center of the fan. So make the front support in two pieces with enough gap in the center so that the fan is not covered. Also note that the 1x3 pieces are mounted 7/16" below the top edges of the front and rear panel. This will allow the lid to slip down inside the box just far enough to make everything flush on the top.
The plenum mock up now completed. The unit is disassembled and all the inside surfaces are painted with a couple coats of flat black paint. The paint is necessary to help protect the USB from moisture and will also prevent being able to see the inside of the plenum through the grate once the unit is installed.
Step 7: The Ceiling Opening.
Cut out the plenum opening in the ceiling. The opening is cut just to the inside of each ceiling joist and the center joist is left in place. Our opening was 32" X 45". The photo shows how the vent opening is framed out on each end with 2x6s (arrows). A small platform of 7/16 USB is constructed in the area where the fan legs will rest. The USB rests on 1x3 cleats which are set 7/16" below the top edge of the floor joists. This allows the top of the USB to remain flush with the top of the floor joists.
Step 8: Putting the Unit in Place.
Most homes have very limited access to their attic areas. In our case we had to move the fan and disassembled plenum pieces all separately into the space and then assemble them in the attic. The fan unit is BIG, so make sure during your design and construction phases that you will be able to get the unit into position. The fan unit is also relatively heavy. So if you are lifting the unit up though your ceiling opening as we did, make sure you have some strong helpers.
Step 9: Grill, Cover and Cost.
The electrical wiring is not shown since there are a variety of ways you may want to hook yours up. We routed our wiring down through the ceiling into a closet and then hid our switch in there. You can turn the fan on and off manually but it is much better to put it on a time so that it can be programmed to come on during the coolest part of the morning when you will probably be fast asleep. We used a nifty timer switch from AutoChron (model #ACRT) that fits right over the top of traditional light switch and wall plate.
The hole to the plenum should be covered with a grate or grill. We used six 16x16 metal grills from Home Depot mounted on a framework of 1x4 pine. During winter months when the fan is not needed, the grill should be covered to prevent hot air from escaping from the living area. We use a simple panel made of 1" insulation and painted to match the trim of the house.
The total cost for the fan installation was:
Drum fan $307
Plenum materials 33
Grill sections 67
Digital timer 30
Total spent $449
We have been quite pleased with the effectiveness of the WHF. The Central Valley is typically 90 to 100 degrees during the day throughout the summer months. This past summer, after installing the WHF, we only had to run our A/C on 5 or 6 occasions. Our normal practice was to manually run the house fan for approximately an hour after the sun went down, primarily to blow the accumulation of hot air out of the attic. Then we programed the timer to turn the unit on from 3:00am to 7:00am while we slept to draw in the cool 50-60 degree temperatures that sweep through the valley at night. Granted, we have learned to tolerate short episodes of temperatures in the high 70's to low 80's during the heat of the day, but we save an estimated $200-$300 a month over what our neighbors are paying for operating their A/Cs. Whole house fans have become increasingly popular in this area and easily recoup their cost in energy savings and in the value of the home if it is ever sold.