How much space should I leave between my marijuana plants and my 1,000-watt and 400-watt high pressure sodium lamps?
When using a 1,000-watt horizontal HPS lamp at a distance of 24 inches above the marijuana plants, you’ll get about half the light on a cloudless day in early July at the 38thparallel. This is basically equal to the light produced in early September at noon at the 38thparallel. Most people run into heat problems when they use high-wattage bulbs. The heat produced by the lights makes the buds “run.” This essentially means that the branches stretch and new growth will start to appear instead of the marijuana buds bunching up.
The ideal height for a 400-watt lamp depends on the light itself and how it is used. Lamps that have air-cooled reflectors can get closer to the canopy than reflectors without a heat barrier. You can place moving lights closer than stationary ones, because the light is distributed more evenly. A stationary, unprotected 400-watt lamp reflector needs a minimum distance of 20 inches from the top of the canopy to adequately diffuse the heat produced by the lamp. A 1,000-watt stationary lamp needs to be placed around 30 inches away.
The marijuana grows lights produce heat in two different ways. The first way is when opaque objects absorb (mostly infrared) light, they produce heat. Infrared light is emitted by heat lamps and, when it hits a solid object (e.g. a plant leaf), it begins producing heat. The second source of heat is the air that comes into contact with the hot lamp, gets heated, and is forced down the sides of the reflector. As soon as it gets away from the sides of the reflector, some of the rises away from the garden, but a lot of it still reaches the marijuana plants. Its dissipation is dependent upon the size and shape of the reflector and the air current. You can set up swift circulation/ventilation systems to get heated air away from the canopy. This is ideal when you’ve got a constant source of cool air. In the Netherlands—an area with an ample supply of cool air—heated air is replaced with a constant flow of filtered street air. This will cause a low hum of white noise in large spaces. Air- and water-cooled reflectors enclose the heat to prevent it from reaching the canopy and affecting the ambient air temperature. You can place air-cooled lights about a third closer to the marijuana plants (14 inches for the 400-watt and 20 inches for the 1,000-watt lamp). Water-cooled lights only need to be put at about half the distance (10 and 15 inches respectively). A reflector and a bottom glass plate keep air-cooled lights enclosed.
Most of the lamp’s heat will still remain in the enclosed system. With 4-inch duct tubing powered by an inline fan, cool air is brought in from the outside to cool the light. Heated air is then pushed out through the exhaust duct on the other side of the reflector. Since this air has never actually been in contact with the garden, it contains no smell (just heat). You can use it to heat a living area or it can simply be vented outside. Water-cooled lights produce a thin stream of water that eliminates heat right out of the lamp. Despite the fact that the bulb touches water, these units are generally safe. Water is more effective at transferring heat than air, so these systems are quite efficient. To adequately use these lights, you’ll need plumbing or a consistent source of water to cool the water.
Stationary lights shine permanently on the leaf tissue. Lights on movers are just as intense, but give the leaf a rest and the tissue time to cool prior to receiving another burst of bright light and heat. You can put moving lights a few inches closer to the marijuana plants than the lights that don’t move.