Hey everyone. I'm almost done with my mini fridge grow box an have a question on the type of light I should use. I plan on scrogging 2 plants using a 400watt MH grow light. My question is, will this burn my plants in the little space they will be growing? The mini fridge is 35" tall, 19" wide, and 16" deep. If I just put a fan in there blowing on the plants, should they be ok? Don't know if it matters or not but the fridge is lined with an emergency blanket to reflect the light. Also, do MH lights drive up the electricity bill? I'm in an apartment where electricity is paid for so I don't want my landlord to become suspicious. Thanks for the help!
Heat control's your worry, not light, really. You're never going to match sunlight intensity, shy of burning down your neighborhood instantly. If your heat at the plants, directly under the bulb, remains a constant 70-80 (F), you're OK. If it cycles between as low as 50 and as high as 100 (F), you'll be OK. The peak and trough I just named are natural conditions where they grow well with minimal or no attention. As long as they don't consistently STAY in one side or another, they'll cope just fine. Opinion here, and a reason for it. NEVER blow directly on plants. What volume of air passes through what surface of opening to pass directly over a plant? What's this equivalent to in wind speed? Even a standing 14" oscillating fan is producing "windstorm" speeds in its direct cone. Throw in something bigger...a windking or wind tunnel, you're talking anywhere from tropical strom winds and F3 tornado winds. How do plants outside fare when exposed to these at a constant rate for any length of time? And you want to do it for their whole lives? Suck hot air out. Works better. Mount fan at/near the top of your grow space. If possible, run a duct from the fan intake to where it sucks air across the light. Duct fan exhaust so it blows the warm air directly out of the space. Use negative pressure to force fresh, cooler air in far more gently (since it finds multiple points of entry totaling more space, the intensity of the "wind" isn't as strong). Explanation of principles here....lower pressure air means there's less actual air at a given point...which means less air to transfer heat energy. Plus reducing pressure automatically cools...this is why a can of compressed air gets cold as hell as you discharge it, how a car radiator (or any radiator) works, etcetera...you compress, it gets hotter, you reduce pressure, it gets colder. Blowing air in, you're compressing it. Sucking air over the lights instead of blowing it means the air, and lots of it, is moving across the bulb, taking the heat. Since that air is at lower pressure, it's also colder...more efficient at cooling the source, which means the bulb(s) last longer. Sucking hot air out of the tent cools the tent better than blowing in. period. Doing it so you remove as much heat as possible from the light source is just "good sense". Also, moving hot air out of the tent means the inside of the tent is lower pressure than outside...so as outside air is forced in due to pressure differences, it expands...so it cools even more than if you were growing in an open room.
Another thing, 400 watt bulbs are all the same concerning energy usage...they are just that, 400 watts...The electric bill on a 400 watt is neither alarming to anyone nor very costly to you...You should get a HPS when you flower too
Technically not quite true...inefficiencies in the ballast with HIDs. Top of the line digital ballast pulls a 4% inefficiency constant and 8% on startup (meaning a 1000 watt system on a top of the line, most efficient digital ballast requires 10.4 amps constant or 10.8 on startup, on a 110 circuit...which translates into the light using 1040 watts running and 1080 starting). Aside from that, right idea. When doing calculations, always pad for 10% inefficiencies. a 110 circuit, use 100 volts, not 110. 2020 volt circuit, use 200 volts. A 100 watt ANYTHING except an incandescent lightbulb should be figured as 110 watts, a 1.4 amp piece of equipment should be figures as a 1.5 or 1.6 amp (I always round up, for safety). Look up an ohm's law calculator if you ever need to do such calculations...there are several online where all you do is plug in the values...make the padding for inefficiencies I just suggested, and your system will work if the numbers from such a calculation say they will. No ifs, ands, or buts.
That's a "rule of thumb" on the 10%...generally, they are lower, but...always be generous, in case it's under at some point, you know? Same reason you buy 200 solo cups for a party if you expect 150 people to show up, rather than 175. MAYBE only 25 people will bring a single other person who wasn't directly invited. Maybe some asshole will invite his whole fraternity, though.