How to Install a DIY Multi-Room Whole House Fan

Living in California where the temperature gets over 100 degrees in the summer, it gets very expensive to cool down the whole house. The air conditioner needs to run most of the day to keep the house comfortable. We all know that due to climate change the temperature most likely will continue to rise. That means our energy bills will also continue to rise. I was getting large electrical bills, month after month, so I started to look for different ways to reduce our air conditioning cost. After installing a Multi-Room Whole House Fan in my attic, I noticed a significant reduction in my bills.

Working at an air conditioning company for many years, I’ve installed hundreds of units in the attics. With knowledge of how air conditioning works and how the air travels through the house, I designed and installed a multi-room whole house fan using a basic inline exhaust fan. 

There are a lot of theories out there about the whole house fans. Some people wonder if the fan is worth the money, others say it doesn’t work. And that could be the case if you have an old whole house fan with huge gaps between the louvers in the ceiling. These old fans are loud and they leak a lot of hot air back into the house. But if you install the whole house fan correctly it could save you a lot of money on your energy bill.

You might be thinking, how does multi-room fan work? I created a diagram that shows the difference between the standard and the multi-room whole house fan here. When the temperature drops in the evening, I open a window and turn on the whole house fan and let it run for a few hours. The fan sucks the cool air from the outside into the house and up the attic. So the fan cools down both the house and the attic. Also, pushes out the hot air from the attic through the gable and eave vents.

When should I turn the whole house fan on?

The best way to know when to turn on the fan is by comparing the outdoor temperature to the inside. I have a digital wireless 4 channel thermometer that reads both indoor and outdoor temperatures. If the outdoor temperature drops below indoor temperature then I open the window and turn on the whole house fan. 

For example, I set the central AC to 77 degrees in the house. During the day when the outdoor temperature is high, AC will be running to keep that 77 degrees. But in the evening when the outdoor temperature begins to drop, and as soon as it gets below 77 degrees, I open a window and turn on the whole house fan.

In the future, I want to design an automated whole house fan system. Instead of manually comparing the outdoor/indoor temperature, I want the whole house fan to do all the work. It would also be nice to have the window open by itself when the whole house fan turns on…

Inline Exhaust Fan

Several years ago when I started on this project I bought a very simple 8″ inline exhaust fan that pushes about 500 CFM. This fan had to be wired with a timer switch and a high/low-speed switch. Now if I was to purchase a fan, I would get a 12″ inline fan with about 1500 CFM. The new exhaust fan comes with a digital control that does not need manual wiring. It has fully programmable temperature and humidity controls with timers, alarms, and adjustable fan speeds.

The instructions below are for the older 8″ exhaust fan. But if you choose to use a 12″ fan, the concept is the same; you’ll just need to increase the duct size and the wye fitting.

Disclosure: Some of the links on this page as well as links in “tools for this project” and “material list” sections are affiliate links.

Step 1 – Cut an Opening in the Ceiling 

First, determine which rooms need exhaust ventilation. Then find a good location for the register in each room. You want to place the register as far away from the door as possible. This will make the air travel through the door and the entire room before being pulled up into the attic. If you place the register next to the door, the air will be cooling just the entrance of the room but you want it to travel through the room and up the attic. 

Once you figure out an approximate location for the register, next you want to find a truss above the ceiling to attach the c-can register plenum. The c-can plenum is basically a transition from rectangular register to a round duct attachment. C-can has a 2″ collar on top of the plenum where you attach the duct to. Sometimes the plenum could have a side collar for duct attachment from the side, which is called s-can. If you’re limited on space s-can could be a better option.

Whether you’re using a c-can or s-can plenum, it needs to be attached to a wooden truss. Using a stud finder, locate the truss in the area where you want to place the register. Measure and draw a rectangular box on the ceiling with a pencil. The rectangle should be up against a truss. Also, make sure that the rectangle is perfectly squared with the wall. You want the register to be parallel to the wall. Then take a sheetrock knife and carefully cut out the opening in the ceiling. Check if the c-can fits through the opening, if not, then trim the opening slightly larger.   

For this project, I used a 12×6 c-cans in three rooms. The average size of each room is about 10’x10′

Step 2 – Secure the Inline Fan to the Trusses

Next, attach the fan to the trusses in the attic space. I used an 8” inline fan for this project. The fan has a base with screw holes that could be used for mounting. Cut two 2×4 pieces that would span two trusses (should be about 24″ long) and attach the fan base to 2x4s. Then take the fan with attached 2x4s up in the attic and attach it to the trusses. You want to have the fan located centered between the rooms to prevent one duct branch from being too short and the other branch being too long. Try to keep the duct runs from the fan to registers about the same length if possible.

Step 3 – Attach a Wye Fitting to the Inline Fan

After attaching the fan to the trusses, attach a sheet metal wye fitting to the fan. For this project, I used a wye fitting with a 10″ main trunk and three 8″ branches. If you don’t have a wye fitting, you could make a sheet metal box plenum that has three 8″ inlet collars and one 10″ outlet collar. I prefer to use a wye fitting because it has better airflow than a plenum box. Since the main trunk is 10″ and the fan is 8″, install a reducer between the wye fitting and the fan. You could buy a sheet metal reducer or just make it yourself with a flat metal and aluminum foil tape. Use 1″ wide metal strap to hold the wye fitting in place. 

Step 4 – Connect Flex Duct to the C-can Box

Now take a 12×6 register c-can plenum and attach flex duct to the collar. These c-can plenums could be purchased online, or you could easily make them yourself from a flat sheet of metal. The c-can boxes are usually 4″ tall with an oval 2″ collar on top. When making the box you could use aluminum foil tape or butyl tape to seal the corners and the collar. 

Flex duct has a core with wire on the inside and insulation and a jacket on the outside. Fold back the jacket and insulation then slide the core over the c-can collar. Tape the core to the collar using a clean duct tape. You could also use zip ties over the duct tape to keep the flex from sliding out. Then pull jacket and insulation over the core and tape it with duct tape. 

Step 5 – Attach C-can Box to the Ceiling 

When attaching the c-can to the ceiling you need to have at least one wooded truss to attach the box to. Technically you should have the framing all the way around the c-can box. The framing is to hold the c-can and the register on the ceiling. You cannot screw-in the register in the sheetrock by itself because the sheetrock will not hold the register. But you could get away with just one truss framing by making a sheet metal flap on the side that does not have the framing. This metal flap needs to be about 5/8″ from the bottom of the can. When inserting the c-can in the opening, the flap will be above sheetrock. The register screw will be screwed-in through the sheetrock and into the metal flat. Attach the c-can to the truss with three wood screws. 

Step 6 – Tape Around the C-can with Butyl Tape

To prevent any air leak between the register and the sheetrock, use a Butyl tape to tape the c-can to the sheetrock. First, place the 1/2″ tape on the sheetrock and then fold it inside the c-can. Do this on all four sides of the can. When attaching the register, it will cover the butyl tape so it will not be visible.

Step 7 – Connect the Duct Flex to Wye Branches

Now that the c-can is attaching in the ceiling with flex duct, connect the duct to the wye fitting. Connect the duct using the same method as shown in step 4. The duct should have a smooth radius running from the wye fitting to the register c-can. By code, you should have the duct strapped every 4 feet. So use 1” metal strap to go around the duct and screw it into the wooden truss. 

Step 8 – Install a Backdraft Damper

Now install the backdraft damper in front of the inline fan. The backdraft damper opens when the fan is on and closes when the fan is off. This damper has two metal flappers on a rod with a spring. The spring holds the damper closed by default, but when air pushes on the flappers they open. Typically your central air conditioner has a negative pressure. That means AC is sucking more air through return register then releasing it through the supply register. If you don’t have this backdraft damper installed, when running your central AC, you’ll have hot air from the attic sucked back into the house. When installing this damper place the damper’s shaft vertically. This will keep the flappers closed at its default position. Use aluminum foil tape for attaching the damper to the fan. 

Step 9 – Install the Registers

When installing the register first screw-in the side without the framing. Make sure the resister screw goes thru the metal flap. Then screw-in the other side of the register. Make sure that the register is squared with the wall. 

Step 10 – Wire the Switch to the Multi-Room Whole House Fan 

The fan that I used for this project comes with two speeds. So when I wired the fan, I made two switches. The first switch is to turn the fan on or off and the second switch to control high or low speed. Instead of a single on/off switch, I installed a fan timer. This timer could be used just as an on/off switch or you can set a time for how long you want the fan to be on. It’s very useful to have the timer on the fan especially if you don’t want to manually turn off the fan.  You’re done with this multi-room whole house fan.