Wednesday, June 6, 2012

The World's Most Sophisticated Whole-House Fan Controller™

One of the nice things about living in a high-altitude semi-arid climate is that, except for a small number of days per summer, it cools off nicely when the sun goes down.  So while it may be 95 at 2:00 in the afternoon, overnight it will drop down into the 60s.  As a result, it's possible to use a whole-house fan and opening and closing windows appropriately to maintain the internal temperature of the house within a relatively narrow range: say 66 to 76 °F for most of the summer.  For those not familiar with a whole-house fan, it's a large fan that sucks air from the house's living space into the attic (where it is vented to the outside), drawing relatively large volumes of outside air through open windows.

The usual control mechanism for a whole-house fan is an on-off timer and a high-low switch.  You set the speed to high or low, then twist the spring-driven timer to set the length of time the fan should run.  A certain amount of guesswork is involved in setting things before you go to bed.  Err to one side and you wake up with the house warmer than you would like: you ran the fan on low, but it should have been on high.  Err on the other and it can be a tad on the brisk side: a cold front went through and it's 56.  A year-and-a-half or so ago, my wife decided that she wanted more control over how the fan behaved.  It should be capable of switching speeds, shutting off if things got too cold, etc.  As no such controller appeared to exist commercially, I was charged with building one.

The result was The World's Most Sophisticated Whole-House Fan Controller™.  Yes, it appears crude, but with a touch screen, a temperature sensor, and everything controlled by software, it can do many different things: multiple operating modes, the ability to reconfigure the button layout if you're left-handed, and a wireless remote control to name some of them.  If you include my hours at a reasonable rate, it probably also qualifies as The World's Most Expensive Whole-House Fan Controller, but that's not the subject of this post.  So, if that's not the point, then what is?

First, I suppose, is that it works.  No more 56-degree mornings.  And in some modes, the software decides how long to run on high before shifting to low while still having a good probability of reaching the desired temperature.  Certainly I have a much better idea when I go to bed of what the temperature in the house will be in the morning than I did before.

Second is that the device is characteristic of the sea change that has happened with control applications over the last 15-20 years.  The device consists of a single-chip microcontroller, some sensors, and a few actuators.  Everything else is software.  While that provides a great deal of flexibility, it also means that the whole approach is dependent on the continued availability of large-scale integrated circuits.  In the near term, that's not a problem.  In the longer term, with constraints on energy availability, limits on the scope of trade, preserving the production capability may be more difficult.  Certainly if you believe in anything approaching the Mad Max scenarios, integrated circuit fabs will disappear quickly.  Are we building ourselves into a corner?

Third, and this is mostly whining, concerns the fact that it looks... tacky.  The problem of mounting the various parts in a good-looking enclosure is, in fact, one of the most challenging parts of hobbyist projects like this one.  What I would have liked to buy was the LCD panel and touch screen all mounted in a nice plastic box.  One where everything was properly centered and aligned, and with enough room behind the display for the small custom printed circuit board.  Granted that this type of hobbyist market is small — how many times was I told that I shouldn't be building a box to mount on the wall, I should be writing a smart-phone app to control the fan? — but it seems like it would be big enough to support such a widget.


  1. Michael,

    You may be interested in our product.
    We just shipped the first of our web enabled whole house fans today. Take a look:

    Since you have already built a pretty fancy controller, I'd like to get your opinion.

    - Neil

  2. When I was thinking about what I wanted to build, I did think about a web-enabled (or at least LAN-enabled) system. I knew I would need a relatively sophisticated UI to meet my wife's requirements, and that would be easier to build to run in a browser on a PC or smart phone where someone else had dealt with the graphics display and touch issues. In the end, I decided against it for two reasons: (1) it increased the level of other in-home tech necessary for the system to work and (2) it seemed to me that it would increase the odds that the fan would get turned on with no windows open, and no one around to open the windows. The contractor who installed our fan had several horror stories about fans running with no windows open.

    That's not to say a web-enabled fan is a bad idea; I just didn't think it met the primary goals I was trying to accomplish.

    1. So, how do you prevent the closed window/ fan running problem ?

  3. "Prevent" is a strong word. Let's say that I tried not to make it any worse than a simple mechanical control. The software in my controller won't switch the fan from off to on except in response to a screen touch (although it may switch speeds). In the event of a power failure, the controller always comes up in a state with the fan off. Individual screens presented by the controller all have some way to immediately turn the fan off.

    If I were building a complete system from scratch, I can think of several things I would at least look at that might determine whether sufficient windows are open. Measuring the AC voltage, the motor's current draw, the fan's RPM, and overall air flow are all feasible in one fashion or another. Various combinations of those values would indicate insufficient flow and the controller could shut the fan off (along with displaying a message about what it had done).

  4. Michael, you may want to try a low pressure air switch like Granger model # 6XPX7. About $20, and adjustable from 0.1" to 10" H2O. Just put a small hose to the outside, and when the DP gets above your set point it signals you controller to shut off the fan.

    Hope this helps

    Mike Swift aka tomswift

  5. Great post! Interesting! Thanks for sharing this. And I agree that living in a high altitude is a good idea.

  6. Cool! It's definitely state of the art with touch screen and it's temperature sensor features and i totally agree this provides a great deal of flexibility. Thanks for this post.


  7. this is amazing and i really appreciate you taking the time to share your awesome ideas.But As a reputed industrial air conditioning and ventilation parts supplier, we are now supplying industrial fan controllers to worldwide clients.
    Wow, nice work, congrats.