I just searched this forum for "Whole House Fan", but I didn't find much about them (except this and this).

I'm curious to hear what experiences you all may have with these simple and relatively inexpensive devices out in the real world.

Here are some pictures and a basic definition of the type of fans I'm talking about:

4.7.12 Alternative Systems - Ventilation Cooling (2013 CA Energy Efficiency Standards)

The CEC also maintains a list of models approved for sale in California. Does this list adequately represent the state of the art?

Or are there other emerging whole-house ventilation technologies that home professionals ought to know about?

For this discussion, let's try to not confuse an old school "Attic Fan" (which isn't designed to ventilate a conditioned space) with a "Whole House Fan" (which is).

Don't hold back!

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Whoops, looks like I got my links mixed up. Thanks for pointing that out. Here's the right one: https://www.youtube.com/watch?v=c3cN0kPqGQI

Cool attic box. This looks like appropriate technology to me. Manufacturers would be smart to include something like this as an optional accessory.

I do not like them and I am in Southern California. I think the whole house fan in prescriptive measures for title 24 3013 is a colossal mistake. 

My blog regarding whole house fans here http://www.rede3.com/Whole_House_fans.html


I read your blog with some interest.  You say 

" Per the Home Energy Magazine article referenced for this page lets go with a 4,800 cfm fan. We need a minimum of 6.4 square feet of free air for the unit to run proper. If there is a screen we need to multiply the number by three to accommodate the resistance or 19.2 feet of screened opening."

I think you are saying that 6.4 SF of open windows is required? What caused you to say that?

How do you define "run properly"?

Where did you get the x3 for a screen?

I have sold and installed dozens of whole house fans and have one myself. I live in Temecula where it gets very hot in the day and very cool at night. There is no more energy efficient way to cool MY home. 

About T24, the recommendation for whole house fans is not a mistake, but it could be argued that their new ventilation spec of 1 square foot per 375 CFM IS a mistake. I am sure you already know that 1000 CFM going through 1 square foot of opening is 10 miles per hour. And the industry standard has been 1 SF per 750-1000 CFM for years. 

I no longer have my subscription to the Home Power Magazine but these figures were pulled from the article that was once linked including that a screen requires increasing size for flow. The resistance of the screen reduces flow.  It is in the archives of the issues.

Proper operation would be  a sufficient opening to not create negative pressure or as little as possible in the home. Proper design would be to account proper exhaust including resistance from mesh screen in gables and soffits and matching the cfm to not exceed that number

As I mention a properly designed and installed fan with an educated user makes these systems worth while.

I have seen these many times in homes which were not designed well and simply pulled of the shelf and installed and the customer has no idea of its proper operation. uninsulated with a infiltration super highway connected directly to the home.

I think the code is a mistake myself. I can appreciate that not all will agree with my point of view.

I like the Tamrack unit but never see that installed on an assesment


I went to Homepower.com and did a search for whole house fan. Could not find that article. I would appreciate your finding it for me. What do you like about Tamarack?

Tamarack has lower CFM than typical fans and has a insulated lid that automatically closes when fan is not in operation. Providing a thermal and air barrier when not in operation.


 As far as screen reducing CFM that is pretty basic principles. A free opening will not resist flow while a screen on will. I did not come up with the 3x number but used it based on the article. I think it is based on the reduction of Net Free Air and the resistance of the screen. I do not have the formula of how it was deduced

Link is here but it does not work which is why it is no longer on my page. It is somewhere in the archives. No longer being a subscriber I do not have access to the articles. 



You are the first person who has ever told me they prefer lower CFM. What you left out is that their price in dollars per CFM is the highest in the industry. 

You will be interested to know that Invisco has R50 insulated dampers that automatically close. Price per CFM is the lowest for rafter mounted systems. 


I think cost of operation is always a factor but I am not sure I would leave it just cfm. What about watts to operate? What about the heat/cold transfer from the attic? What about the perfect connection of conditioned to unconditioned space I see in many systems

Looking at the Invisco I like that it has a double damper. But the dampened area is not insulated. It is a huge leaps and bounds improvement over the gravity louvered units I have seen the most in the field.

I think if you look at the Tamrack you can see some faults as well. How well air sealed is the cover? I do like its design as a turnkey set it and forget it. While not perfect it does address air and insulation barrier.

As far as wanting less CFM it is based on at 1000 CFM almost any attic will have sufficient NFA to exhaust the air with no retrofit. I risk less having the house under negative pressure from large CFM Fans. With a 1000 CFM I am turning the air over more than 4 times per hour in a 1600 foot home.

Like anything else I think we take allot of information based on our training and experience and formulate opinions. I have seen a 4000 CFM fan in a home with the water heater in conditioned space. The louvers were so dirty from transfer air I had to wipe them with a cleaning product before I could get tape to stick to them for my blower door test.

I think any system when designed right and operated correctly will have a benefit. I have often mused that a magnetic reed switch on the back patio door should be installed on a whole house fan project to ensure sufficient opening during operation. I got this idea when on vacation and the air conditioner turned off as my wife opened the window at the hotel. I thought that was brilliant.


Reading the Invisco description you will find that it is designed to be surrounded by cellulose to cover the bottom damper entirely. The thermally broken assembly allows the top damper to be freezing while the bottom one is at room temp. 

I agree with you that almost any home can vent 1000 CFM from the attic. But the industry standard is to install about 2-3 times the square feet in CFM. Anything less and homeowners are disappointed. QuietCool recommends their QC1500 for one bedroom. 

Your door switch is right on. Airscape released their fancy 7 speed remote controlled fans in 2012. In 2013 they released a pressure sensor for the home that disables their fan if no windows are open. 

CA Title 24 requires 2 CFM per square foot.

@ Kurt and @ Glen:

If you or anyone else have strong arguments in favor of changing the CEC's current ventilation area requirements, fan sizes, efficiencies, or anything else, now is to time to speak up. They just extended the deadline for comments to Wednesday (8/6/2014):


Title 24 requires whole house fans in zones 8-14 in the prescriptive path only. With many restrictions including glazing limits being the most pertinent it is thought that most builders will opt for the performance path. In the performance path no whole house fan will be required.

Traditionally builders opted for the prescriptive path because it was easier and they could eliminate HERS verification. In the new code there is no way to avoid HERS verification. Even prescriptive approach will have Raters verifying systems.

It remains to be seen how this will play out as the new code started on July 1st of this year.

I can see the duct is buried and in fact in the picture the use an example of an 18 inch duct. While the cellulose fills around the outside of the duct. They use terms such as thermally broken and claim heat cannot transfer.  However the 18 inch damper or the area of 1.77 feet would have an R Value of 0 as shown in the picture.

I am sure two magnetically closed dampers will provide a good air barrier but I fail to see the thermal barrier


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