Here is a pic of the one I built last year.

Apparently refrigerators are quite well insulated and with this medium sized one in a rather cold basement only a little more than a foot of heat tape between two 15" tiles (sitting on oak squares to allow air flow underneath) was enough to heat it just fine.

I eventually moved the proportional (recommended over simple on/off for incubation) thermostat probe to the middle shelf and not in an egg box. I'm thinking you want to control the background rather than overshoot trying to warm up one box.

I used a 12 volt AC/DC converter since the PC fans run on 12 volts in the computer case but if the 16 volt works and are easier to find I guess that's ok. I did make the mistake of getting the polarity wrong the first time causing the fan to spin backwards. While you can flip it to make the air flow which ever way you want it was a problem because of the curved blades on this fan and the incorrect spin caused them to lengthen and rub on the housing and burned my first fan out (causing bad smelling smoke and hence the baking soda).

I used the PVC tube seen along the left edge of the pic with the fan at the top end pushing the air down as I thought that would go with the natural air flow at the edges. I thought about putting a 2nd fan at the bottom to help along as they don't seem to move much but didn't get it done. Actually I think it was doing well as I actually had my bottom shelf hotter than my top shelf so figure the fan was overcoming heat build up from convection and the bottom shelf was being heated radiantly. I ended up putting a rather thick sheet of insulation big enough to cover the egg boxes but still with several inches around the edge for air flow on the bottom shelf under the boxes to help shield them from radiant heat and the probes in the bottom boxes then where only slightly higher than the top. I put one ventilation hole at the bottom into the down air pipe (using some large fish tank hose) thinking that the Bernoulli effect of the air moving down would cause fresh air to be sucked in and then another hole at the bottom near the door for the presumed outflow. I still may try a smoke test to try to get an idea how the air flow is really working this year before I put eggs in. I'm also going to work on isolating the air pipe from the shelving to be sure the fan isn't directly vibrating the eggs any more than I can help it.

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