Get HID heat out of the canopy
1000 W HID
about 3400 BTU of heat
Open reflector
all of that heat dumps into the room
Air-cooled hood
a glass bottom can trap about 66 to 80 percent of the heat in the duct
Duct size
6 or 8 in (15 or 20 cm) flex on both sides of the hood, pulled by an inline fan
Water-cooled lamp
nearly all heat leaves in a closed water loop to a radiator or reservoir, often holding the room under 80 F (27 C)
Exchange or cool the room air
Open door
mixing with the next room is often enough while you work
Closed door
vent out or add air conditioning so the canopy does not cook
Top and bottom
exhaust high, intake low, so hot air leaves and cool air replaces it
Fan size
empty the room in 5 minutes or less. A 4 by 4 by 8 ft space is 128 cu ft, so at least 25 cu ft per minute
Small AC
a 5000 BTU portable unit can hold a closet-scale room
Sensors
a thermostat-humidistat can switch fans, AC, or a dehumidifier
Breeze, humidity, and clean air
Oscillating fans
one or two, aimed so air rises through the plants
Why
the breeze strips heat from the canopy and replaces CO2-poor air
Humidity cap
keep RH at or below 55 percent to cut most fungal pressure
HEPA
a filter on intake air can block a large share of mold, insects, and bacteria
Floor
a waterproof tarp under pots saves the closet
CO2 when the room is sealed
Open and vented
room air through the canopy is enough
Closed
use a tank and a meter that opens the valve only while lights are on
Dark
shut CO2 off at night
Tank
a 20 lb (9 kg) cylinder can sit outside the closet with a tube into the room
Placement
release gas low so it rises through the plants