If you're putting your eggs in those plastic egg carton trays for incubation, you should cut out the bottom of each cup so that the eggs get maximum fresh air circulation. I'm not sure where would be the best place for you to make ventilation holes in your incubator. With a still air bator, air hole placement is vital cause you're relying on natural convection to move the air around. But with a fan, I don't think it's quite as important. One of my bators (a Lincu Avitec) has the in-hole on the side and the out-hole on the front. The air is drawn in through a vent, goes through a plastic pipe directly underneath the heating element so it gets pre-warmed, with the other end of the pipe being right underneath the fan so that the fresh air gets blown down over the eggs. The out-hole is just a plain old hole, no pipes or anything. My other bator (Brinsea Octagon) only has one air vent on the front at the top. It also has four small holes in the base but I think that's more to drain away water than to assist with air flow.
You want a good supply of fresh air, but you don't want to be sucking in cold air and blowing it directly over your eggs. You want the fresh air to be warmed up before it hits the eggs. That's what to think of when planning your air holes. And please bear in mind that that's all just theory and I've never built a home-made bator so if you get different advice from folk who actually have, I'd say go with them!
You want a good supply of fresh air, but you don't want to be sucking in cold air and blowing it directly over your eggs. You want the fresh air to be warmed up before it hits the eggs. That's what to think of when planning your air holes. And please bear in mind that that's all just theory and I've never built a home-made bator so if you get different advice from folk who actually have, I'd say go with them!
