How to keep my basement marijuana garden cool?

I’m in the process of designing my own marijuana grow room. It’s going to be in a basement, but the actual garden space will only have dimensions of 2’ x 5’ x 6’ (10 square feet) under a 400-watt high pressure sodium (HPS) lamp.

I’ve got the technical stuff worked out aside from two things:

1. I need a way to control the temperature (particularly in the summer).
2. I want to augment the space with CO2.

Would it be possible to use the exhaust from the water heater if I used a portion of it for the marijuana growing space?

There are a few things you can do to maintain a range of 70 to 85*F. The main heat contributor is the lamp itself, and an air-cooled reflector gets rid of most of the heat before it even enters the room. Even so, the marijuana garden will still gather heat. The simplest way to cool the space is by installing a window or portable air conditioner. These units exchange heat, not air and help keep CO2 in the space. You can use the exhaust from the water heater to enrich the air with CO2. The marijuana plants will grow considerably faster if you keep the air constantly enriched during the light period. This is easily accomplished with a CO2 tank with a meter and regulator. This unit will measure the amount of CO2 in the air and adjust the flow valve on and off as needed.