1) Is there a rule of thumb or any way to establish how much ventilation is required for 2 plants in a 5" x 11" room using a 600watt HPS? 2) What are the symptoms of lack of ventilation?
As much ventilation as necessary to keep the temps below 85 degrees F. Not enough ventilation would be too high temperatures seen as the leaf edges curling over themselves.
i really hope you mean 5'x11' and not 5"x11" inches! that'd be some power for that small of an area!! i just built a biggish cabinet that measures 48" wide x 30" deep x 71" tall with a 400w hid. four feet wide and almost six feet tall so i hope to grow some monsters! but heat is an issue. i put a 300 cfm inline fan at the top to blow out hot air and cut an intake hole at the bottom to suck in cool air. looks like i'm going to have to get another fan (prolly same one) to assist the intake cause temps are beginning to be an issue.
Not only is it an issue of heat but your plant will deplete the co2 in your room without ventilation. That said, the rule of thumb is to remove all the air in the room every 3-5 minutes to reduce heat.
Check out my sig ISTANDALONE I just finished my 400 watt air cooled by my vortex 600 and up top to a carbon filter. I used insulated ducting... Yeah I knew it was a good idea to invest in a big fan, and I'm going to put 3 more darkroom louver passive intaked today....... on day 11 veg
Is that still true for a room where, even with the door closed, the temperature does not even reach 75(f) ? Heat is not an issue in the room i am using to grow because the heat seems to absorb into the roof. So if you eliminate heat from the equation, how much ventilation would a plant need? The answer is probably not easy to come up with (or is measured with equipment i do not have), which is why i was interested to know what signs to look for in a plant that is lacking in oxygen.
of course, for optimal growing you must replace the air every 3-5 minutes. So if your room is 5x11x8 then you would have 440 cu.ft. divide that by 3..5 and you need a fan that would have a rating between 88-143cfm.
In a room that big there shouldn't be much problem. I run a 600 in a 5 by 5 room, I think that is a good size for you to run. You'll need more light if you want to do the entire 11 feet.