During hot days, the air in my attic can reach temperatures much higher than the outside.  This causes the rest of the house to warm up.  As a result, cooling the living space requires more energy.  To prevent this, I use gable fans to force the hot air from the attic to the outside.  I have two fans, each located either on the east or west side of the house.  Each fan has an independent thermostat that engaged the fan when a set temperature was reached.  The fans would turn off once the temperature dropped below the set point.

The problem I noticed was on extremely hot days.  The east side fan would turn on mid morning while the west side of the house was still cool.  This was fine, because cool air was drawn in from the west side and vented out the east side.  Later in the day, the west side fan would engage.  Now I was operating both fans and the air was being drawn from the roof vents and not the lower house.  This just introduced more hot air into the attic.

To solve this problem I installed another fan to draw air from the basement.  I only wanted to operate this fan when both attic fans were operating.  To do this I installed micro-controllers to operate all of the fans from a central controller.  Much of the work I did earlier with my space heaters was portable to this project.

Instead of using solid state relays, I decided to use 5v relay modules that would easily fit inside the thermostat housing of the fan units.  My micro-controller boards were small enough that they fit inside as well.  Ninety percent of the hardware was identical to the space heater project, the only real difference was the logic I programmed to operate the fans.

Each fan micro-controller would detect its thermostat state.  Based on that, it would operate the relay module to control power to its fan.  I had the master controller check each of the fan micro-controllers for their operating states.  If both attic fans were engaged, the master would instruct the basement fan micro-controller to turn on its fan.

I also track the operation of the fans with my Raspberry Pi by querying the master controller.  Just like my space heater project, I track these with Cacti and monitor my usage trends.  This lets me tweak the system for better results.  I also have notifications setup so I know that things are working if I’m away from the house.  Again, this is largely based off of the same principles I used in my space heater project.  It’s nice to keep cool without sweating about my energy bill.  I hope you have enjoyed this and look forward to covering more topics about home automation in future posts.