using a fan in a fridge incubator

1. is fan always on or only on when heat source is on?
2. any holes in the fridge for air circulation or is it completely sealed.
3. how to keep temps the same through out the incubator consistent

any other info would greatly help thanks

I built an incubator from an old fridge and have a few eggs in it now.It seem to be working great and the temps are steady.The fan runs all the time.It is completely sealed or it would be harder to have consistence temps.I am using a Helix thermostat.You have to use a proportional thermostat for it to stay steady.

I also use old fridges. I had to get a second this year. I have no holes in mine. I also use the Helix dbs1000, and have two fans installed (just in case) but have not had to turn them on all year and I keep the same temps throughout. If you cant keep the temps the same throughout then I would put a low voltage fan in just to circulate. But make sure there are no holes in the incubator to draw in new colder air. You want to keep the same warm air in just move it around. I know the Helix may not be the cheapest but I have 4 of them and I think it was the best investment I have ever made.