Ventilation

I have a grow room which is currently not light sealed. Its a little room in the garage below a flight of stairs. This garage itself is far from light proof but so far this hasnt been a problem.

Since my plants are on vege, i havent needed to worry about light seeping in through the garage doors since i have had the grow light on 24/0. I have just left the door open with an oscillating fan blowing air around. The room is about 5 foot wide and 10 feet long (but it is not square - it has a flight of stairs above it). Unfortunately i dont have pictures but i will do at a later date...

Anyway, i have come up with a solution for 12/12;

I will make 2 holes in the door. One at the top for exhaust, and one at the bottom for intake. I will have 2 long ducting pipes which will transport the air into the outside world. This means no light can get in through the venting holes, and all i need to do is light proof the edges around the door (which should be easy).

Each of the ducting pipes will have to be around 7 meters long in order to reach outside and i plan on making them curl around slightly (in order to keep the light from getting up there).

My question is first of all can anyone see a problem so far or have any advice?

Does it sound like this will work?

Is it ok to have the intake/exhaust fans going constantly, or do i need to turn them off and let the air circulate for a while periodically?