Quote Originally Posted by stinkyattic
I've got another couple techniques for ya:
-Reduce the 'swing' between night and day temps so that they are almost identical
-During the first 3 weeks of flower, use BLUE light (halide, daylight flouros, whatever) instead of RED light. Then, when the stretch period is over, go back to HPS to finish. This will make them stretch a LOT less.

This will help with stretching.

As for the ventelation. I am currently using a 250w HPS in a cabinent that is 21"x18"x56" So mine is a lil smaller than yours but I have the same heat issues. You seem to have a large enough passive intake though. If your house stays around 70 and you have a 7" exhaust fan mounted on top... I am pretty sure you'll be fine. If its of any concern you can always put your cab right in front of or behind your air duct. That should help suck some of that cool air in.

Also if you can or have one, a digital thermometer mounted about the same distance you would have the top of your plants from the light would get the hottest temps at the maximum height. And as long as those temps dont get above 88. You should be fine. Just watch for heat stress.