I have a habit of thinking my projects threw till I think I have everything perfect before I actually start building something. I'm pretty sure it has something to do with being to lazy to do things twice if I can help it.
When I desided I needed to build a big incubator I had a fridge in mind from the start. What I couldn't figure out is how to keep the air moveing in the fridge so that you dont get any thermal layering. In an empty fridge its not that hard but when you add shelves of eggs you disrupt any air flow you can get from big fans. The only real fix to to duct the air from the top to the bottom with a strong fan. Pulling the hot air from the top an putting it on the bottom effectivly stops thermal layering. Actually doing that in a standard fridge gets complicated.
The fix I came up with was a side by side fridge. It has everything you need in a box for an incubator. Its big enough, its well insulated an in has two seperate chambers that run top to bottom. Thing of the main fridge as the incubator an the freezer as the duct work. Finding a ok looking side by side that does not work took more doing than I expected. By the time I found one I had over a year of brain storming in this build.
The first thing I did was wire an outlet in each side. Then I cut a 3 inch hole threw from the fridge side to the freezer side right at the very top. I mounted the factory fridge fan in that hole blowing in to the freezer. On the bottom just above the bottom drawer I drilled a row of 1 inch holes all the way from the front to the back. That completed the circulation system. Closing the freezer an you can feal how well the air is forced from the top to the bottom out the small holes.
Next was heat. I tryed the guts out of a foam incubator but it failed to get that large of space up to temp. I then built a thermostat out of an old home thermostat an a relay. I wired the thermostat up to control another outlet on the freezer side. For heat I use a hair dryer. I just plug it in an set it in the freezer. If it ever fails it can be swapped for a new one in seconds. An there at any department store for about $10.
For turning the eggs I got lazy, I used turners for foam bators. I took the glass out of the shelves an tied wire across them to hold up the turners so air would flow up threw the shelves an eggs an not around the shelves. The bottom drawer became the hatching tray.
Ventilation is just the ice dispenser door propped open . A square 5 gallon bucket fit perfectly in the freezer side to hold water. I used a frog habitat foggier hooked to a humidistat for humidity control . I filled the shelves in the doors with jars of water an the extra spaces with blocks all for extra thermal mass.
After running it threw the winter out in the weather it has proven to keep stable temps with the vent full open down to the 30s outside temp. And with the vent closed it keeps stable in the single digits (the coldest we see). I have had pretty good hatches even when the vent was closed. Also it keeps temp pretty well (vents closed) even threw a 6 hour power outage with weather in the 30s.