I've been starting seeds under lights indoors forever and all I need is a window cracked open an inch, 24 hours. Never had a problem till I shut the window one day. This is not rocket science although I can see how people like the specific technical breakdown and numbers as a hobby.

I've been starting seeds and propagating plants from cuttings hydroponically under a couple of flourescent tubes for many years myself. But growing vegtables to maturety in a Small storage room under a 400 watt H.I.D. lamp is a bit more demanding to say the least.
I have installed a salvaged HVAC blower throttled down via a variable speed control.
The long exaust run (25 feet X 4 inch) seems to be a problem. Now I basically have a pressurized room. It takes a bit of effort to close the door and then my ears pop as if flying on an airplane.
I placed a portable1500 watt 5200 BTU heater in the room for testing and more problems arose. I had to change a breaker from 15 amp to a 20 amp. I turned the heater down to the 600 watt setting which should equal approximately 2,060 BTU's for the sake of comparison. With the inlet air at 65.9 Deg.F the temp inside the room rises to 79.8 Deg.F The blower just strains harder as more power is applied. In the summer when the ambient temperature is much higher this will be a problem. It's quite simple. Not rocket science at all. I just can't push enough air fast enough through that 25 foot X 4 inch tube that takes the hot air from the 400 watt HID bulb in the room to the outside of the house. This part of the house has no windows to crack and I don't want to chop another 4 inch hole in the side of it. So you see, Susan, this is about more than specific technical breakdown and numbers as a hobby. Its a real problem.
~Ken~

Ken, Is your exhaust run flexible or solid pipe? Instead of chopping a new hole in your house could you make the one you have bigger? Going to a 6" pipe will increase flow capacity by over 100%.

Even though I have windows in my indoor setup I just finished installing a cheap A/C unit incase ambient temp gets to high. Not sure if a portable unit would work for you. Again 25' is a long way to vent it.

Hey,Thanks S.C.G.,
You are totally correct. A 6" pipe will out volume 2 or maybe even 3-4" pipes.
I have Since corrected the problem by adding a 4" booster fan (rated at 100cfm) close to the end of the 25' exaust run.
I went with a 6"("cool tube") type lamp fixture with a 6" to 4" reducer thru which all exaust air much pass, thereby cooling the bulb and and removing high temps in continum.
It keeps the room stable at 4 Deg. F.(measured at the ceiling) above ambient incoming bassment air going in.
It's not what i would have prefered, and It's far from perfect, but it's working.
I was a bit surprised when I hooked it all up and the whole thing went kapluei. A 20 amp G.F.I. receptical popped!
Yet one more lesson learned!!!
Thanks,
~Ken~