Sunday, August 26, 2018

Building a Wood Stove Eco Fan

Towards the end of last winter, the squirrel cage fan in my wood stove gave up the ghost.

Pricing out a replacement fan for my type of wood stove almost made me wonder if they were also tossing in a new stove with the price of the fan. Looking for alternative ideas, I didn't have to go no further than to my parents house.

My parents have heated their home with a couple of wood stoves for many years, with the heat from these stoves being distributed by fans that get their power from the heat generated by the woodstoves themselves. Though much cheaper than the replacement fan for my stove, I felt that even those fans were a bit too pricy for my tastes.

Looking at my parent's fan a bit more closely, I noticed that there seemed to be some sort of magic wafer that was mounted in the middle of the metal framework of the fan. That wafer was wired up to the fan motor and was providing the motor its power. It all looked pretty simple - assuming I could get one of those magic wafers.

A quick search on Instructables and YouTube, gave me the answer, since there are quite a few people on those sites that have built these gadgets. The magic wafer is in fact a  thermoelectric cooling (TEC) module.  These modules work by generating electricity if one side of the wafer is cooler than the other side. In the case of the stove fan, the electricity is generated by having a piece of metal pull heat into one side of the wafer, while a metal heat sink pulls heat away on the other side of the wafer.

A quick look on ebay showed that I could have one of those TEC wafers for less than $3.

An idea began to form....

With a day job in IT, I have ready access to a number of old, obsolete desktop computers. The key thing to remember about older desk top computers is that they tend to have fairly large heat sinks attached to their CPU's, not to mention other very useful parts like low voltage motors in their CD ROM drives.

A quick scrounge of the e-waste bin and I had the parts that I needed:
  • 2 large aluminum CPU heat sinks - I ended up with one heat sink being larger than the other
  • a metal retaining clip - it was used to secure one of the heat sinks to the CPU, but I thought it would make a good motor mount for my fan. 
  • A small DC motor - in a former life it was used to run a CD ROM drive
After getting a  quick delivery from ebay for my TEC wafer, I was ready to go. 

Two CPU Heat Sinks and the TEC wafer

I started by determining that the best approach for my fan was to have the bigger heat sink act as the cooling part of the fan, and the small heat sink serving as the heating part of the fan. To test my theory, I put the TEC wafer between the 2 heat sinks, and put the whole amalgamation top of my kitchen stove and measured the voltage with a multi meter.

Cooking the TEC on the Stove

After setting the stove to a fairly low heat setting, the TEC wafer was putting out over half a volt. Quickly hooking up the CD ROM motor to the TEC, I could see that the motor was turning at a fairly decent rate - it looks promising! 

Getting some JB Weld putty I attached to 2 heat sinks together, with the TEC wafer sandwiched between them. 

Getting ready to join the heat sinks and TEC together 
JB Weld Applied
Putting them all together

After the JB Weld cured, I then attached the CPU retaining clip to the top of the cooling heat sink, again with some more JB Weld - I made sure that a part of the clip over hanged off the end of the heat sink in order to provide a mount for the CD ROM motor later on. I secured the clip in place while the JB Weld cured.

CPU mount and heat sink assembly
CPU mount attached

While I was waiting for the JB Weld to cure, I focussed on build a prototype fan blade. I went for a very simple and light weight  fan using an aluminum can that I had cut apart and flattened out. The fan is basically a circle with 2 lines drawn at 90 degree angles from each other drawn on it. I then cut along the lines for a couple of inches and curled the cut edges until I got a fan shape.  I also marked the center of the circle in order to make mounting the fan on the motor easier. Just to make it pretty, I gave the fan a quick code of bronze paint.

With the motor mount now attached to the heat sink, I can now attach the motor itself. Attaching the motor is actually a pretty simple affair - I grabbed a 2 inch hose clamp that I had in my junk draw and attached the motor to the mount with the hose clamp.

Attaching the motor to the fan assembly

The next step was to make the physical electrical connections from the TEC to the motor. It's a pretty simple process - just solder the leads of the TEC to the leads on the motor

Soldering the motor to the TEC wafer

Finally the last step is to install the fan blades Again, it's a fairly simple process too. Poke the shaft of the motor through the center hole of the fan blade. A little bit of JB Weld holds it in place.

And with that - the fan is complete. The only thing left to do is to give it a quick test with the kitchen stove acting as my wood stove test bed.

The verdict - it seems to work pretty well. The only complaint that I have is that it doesn't seem to move much air, but I consider that more of a flaw in the fan blade design than anything else.

The next step may be to try it with a better designed blade.

When the weather starts to get colder and the stove is starting to be used again, I provide a bit of an update on how things are working

Stay cool!

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