Using Your Attic to Heat Your Home – Part 2: Automation

So, last we left off, I had a box fan in my attic opening blowing the warm air down into my house last fall. It worked great as a proof of concept, but definitely has issues. The biggest pain is that it had to be manually setup and taken down every evening. It also blew heat into my second floor which is normally warmer than the downstairs. I needed a way to get the air downstairs and it would be best if it was completely automated so it would work all day long while I was at work.

The solution ended up being quite simple. I have an old chimney that is cutoff in the attic. There is even a hole cutout in my kitchen that I had previously plugged up with some rigid foam. I could plop a fan on the cutoff chimney in the attic and blow hot air down into my kitchen. The chimney provide a perfect duct to get the hot air down to the first floor.

To get the air down to the kitchen I needed a fan of some sort. I ended up getting some used bathroom vent fans from my father-in-law. They are squirrel cage fans that are about 6 inches in diameter. They each pull about 50W of power.

I mounted the two fans on a small piece of plywood cut to fit over the chimney. I also made a 2×4 frame and glued it to the top of the chimney bricks so I could screw the plywood down to it so it sealed up.

After getting that all setup, I had to make a thermal differential controller to control the fans automatically. What the thermal differential controller does is it watches the temperature of the kitchen and the temperature of the attic. Once the attic gets a few degrees warmer than the kitchen, the fans turn on. Once the attic cools off, the fans turn back off. Its the same type of controller used for solar hot water panels.

The last part needed was a damper of some sort. This will prevent the warm kitchen air from going right back up the chimney once the fans have stopped blowing. I had picked up some nice dryer exhaust duct dampers, but ended up just using the piece of foam I cut out. It is hinged with some plastic bag material and some pins. It works quite well, gives a visual indication that the fans are on, and was quick and easy to make up. Eventually, the setup will get the nicer dampers and a filter in the attic.