How much heat
Rule of thumb
Each 1,000 W of light input makes about 3,414 BTU of heat
Three lamps
Three 1,000 W lamps match about two electric space heaters
If it stays in the room
You must vent or air-condition that heat out of a room-temperature garden
Canopy first
Hot air sitting on the tops is the burn and stretch risk, not the BTU number alone
Sealed air-cooled hoods
Path
Cool air enters one duct, crosses the enclosed lamp, and leaves the other duct without mixing with garden air
Duct size
Commonly 6 or 8 inch inline fan and hose
Odor
Exhaust has lamp heat and no room smell, so it can leave the building or heat another space
Capture
A good sealed hood removes about 60-90% of lamp heat from the room
Glass cost
The bottom pane absorbs UVB and about 8% of other wavelengths, which cancels the UV advantage of MH
Open air-cooled hoods
Pull from the room
Air is drawn up through the reflector, taking canopy heat with it
Odor control
Clean that air with an in-line carbon filter before exhaust or reuse
Why use it
Keeps tops cooler without a UV-blocking pane
Trade
You are now moving garden air, so filtration and negative pressure matter
Water-cooled hoods
Loop
Water crosses the hot glass of the bulb, then dumps heat in a reservoir or exchanger outside the room
Contact
Water touches glass only, not live contacts. Commercial units have a long safety record in the book
Heat grab
Water can take virtually all lamp heat. Large reservoirs (the book mentions a hot-tub-sized tank) can serve many lamps
Light cost
Water absorbs about 10% of the light, and more than 10% of the red because red is absorbed first
UV path
UVB passes water but not glass, so a glass jacket still blocks UVB