I have a 2x2 cabinet with 2 intake fans and an exhaust fan with an air cooled 400 watt light (I know 400 is overkill, but it was free). I can't get the damn temp down though. Now I've heard if you can put your hand beneath the light and if it isn't too hot for you, then it's not too hot for the plants. But my temp gauge says it's like 90-95 degrees and putting a fan in seems to only dry out the plants and kills them.

Is this a reliable rule of thumb or BS? Any suggestions for heat reduction?

The hood should be cooled on a separate vent, pulling air from outside the box and pushing it out the other side, preferably out the room altogether.
If you don't have a good inline fan for that get one.
I would have the two fans for exhaust and just use passive intact, if you have another fan use all three for exhaust.

Using a fan for intake is a waste unless the vent for the intake is so long it needs some help to get cold air in there, but with your box that's not going to be a problem.
Remember, if you use a 100cfm fan for intake and one for exhaust you have 100cfm of movement, if you make them both exhaust you now have 200cfm of movement.

I agree with enojadoperro. I would even suggest putting a piece of glass between the light and the plants. You can seal it off so that your air movement that cools the light doesn't enter the same space as the plants.

The only disagreement is about two fans moving 200 cfm. That is true if the fans are in parallel, in other words, they have to be separate. They can't just be one after the other. That should be common sense but you know how sometimes everyone doesn't have it. I am pretty sure that's what you mean, enojadoperro, but just wanted to make sure anyone reading this didn't misinterpret.

Yes, that's what I meant, thanks for clarifying it.
Some people might have thought that two 100cfm fans hooked up to the same exhaust vent would make 200cfm when it doesn't.

im not sure but is it possible that the piece of glass pothead mentioned could get too hot from the light, and possibly bust if got wet? such as from foliar feeding or something? the space is pretty small. i was just thinkin just in case...thatd be a pretty shitty scenario.

Sure, it is possible. You could use tempered glass. However, I think if you got the glass hot enough that it might shatter you would already know that you didn't have nearly enough air flow above the glass by the light. The ventilation for the light would be for cooling. Think about how hot a residential dormer window say in an attic might get during a hot summer in Arizona or somewhere similar. Hot enough to burn your skin, or at least be very uncomfortable to the touch. I would say if you used the glass make sure there is enough air flow around the light fixture to keep the glass cool to the touch. If it isn't you are going to be conducting/convecting a lot of heat onto your plants. If it gets the glass hot enough to shatter, your plants would already have been fried.

Another benefit is that you could keep your "box" cooler and have a slower air flow rate (cfm) through you growing chamber. This could help with supplementing CO2 later on as well.

Either way your light is going to create the same amount of heat. The question is if you have enough air flow to transport that heat away fast enough to minimize the effect on the temp.

Do you have a hood for your light? If so, what shape is it and does it have ports for cooling?

Your light is your problem. Your fans are fine for exhausting the room, but you need to cool the light somehow, and a regular fan blowing on it will not work in your space.

And yes, you would need to use tempered glass, or look into a light diffuser-they help with hot spots--not heat though.

IMO, you really don't need to supplement CO2 if you have proper air movement. Plants (all, not just mari) can only absorb a certain amount of C02 and if you allow your room/cab/tent to fill will fresh (CO2 filled air), the plant enough time to absorb the CO2, and then exhaust you will be meeting the plants needs sufficiently.

For example, you could use one fan to bring fresh into the room (filtered through a filter like what you use for your home AC) that runs constantly, one fan that vents your light that also runs constantly, and one to exhaust the room. The fan for your light needs to be ran independent of the room-exhaust fan and you will need a fan controller for the room-exhaust fan for this to work. Let the room fill with fresh air, say around 15 min (depending on the room-intake fan cfm); then vent (tot room vol. vent should occur in 3-5 min.)

Sorry, didn't mean to go off subject, but ya, your light is the source of your heat problem.

I agree.

(I don't supplement CO2 either, although some people swear by it.)

You absolutely have two ventilation concerns. The first is keeping a healthy supply of air to your plants. The second is to move the heat produced by your light away from the light and thus away from your plants. The lights get really hot so you need more airflow to cool them than your plants need to breathe, comparatively speaking. Many people perform both tasks with the same fan, however, a depending on the individual space separate ventilation to address the two issues may be necessary.

I have two 180 cfm fans on my air cooled hood, one for the intake and one for the exhaust. I also have the same for the grow box itself, a 180 cfm for intake and exhaust.
The hood is pretty basic in design. Two ports, opposite sides, unparallel to each other.

Does having a cooltube make that much of a difference?
If so, I've heard of people making cooltubes themselves. Anyone know how it's done? A cool tube that fits for a 400 watt with 4" fans? And know how to wire the capacitor and all that?

If so, please let me know.

A cool tube is not going to help you alot, a 400 watt light needs to be 12"-18" away from the canopy. I hate to tell you this but you really need to get the light off the tops of your plants, by the time you add the pot to the height of your plants it does not leave much for growth. Can you raise the top of your cabinet?