How HOT Do CFL Lights Get When You’re Growing Weed?

how hot do cfls getLow Operating Temperatures.

One of THE number one reasons to use compact fluorescent bulbs to grow Cannabis.

Powered on a mere fraction of the wattage used to illuminate their incandescent predecessors, the difference in heat generated is no small deal. These same bulbs used to save electricity in homes and offices also allows noticeably coolers temps while operating them–a big plus during the summer months

Now let’s not underestimate the heat that these little beauties can give off, as many have to their peril. Not being prepared for high possible temps, or not monitoring your grow to regulate temperatures can be a disaster waiting to happen. Remember: CFLs run cool–per bulb.

Touch one bulb up to 42 watts, and you can squeeze it while lit without getting burned. You’ll feel it getting warmer as you press it, but nothing major. Since the bulb surfaces don’t get too hot, you can typically unscrew a bulb barehanded, without injury.

However, do NOT underestimate the heat that multiple CFL bulbs can produce. You’re now playing on a different ball court. So, how hot do CFL lights get when they’re running for most of the day? HOT.


Here are some rules for keeping heat contained in your CFL grow:


Keep your lighting at a comfortable distance from foliage. The same distance as what is comfortable for the backside of your hand. If you place the back of your hand under the bulb, between the plant’s foliage and the bulb, you should get an idea of the temperature. If your hand feels too warm, so is your plant. Move the plant down or the light up, until your hand feels “cool” under the light.


With proper ventilation, your ambient temps will be 2-10°F above ambient room temperatures. Ambient room temp
is the temperature of the room where the plants reside, or where the grow cab/tent, etc resides. Depending on space, closed grow areas can easily exceed 100°F and cook plants if ambient room temperatures are too high, or if ventilation isn’t “up to snuff.”


Used correctly, CFLs don’t generate enough HEAT to become a concern for safety. They don’t, unless they weren’t properly wired or too many bulbs are wired incorrectly and the wires heat up beyond their normal operating temperatures. If you have doubts or can’t wire a fixture with 100% assurance that it will be safely constructed, contact a professional and DON’T do any electrical work on your own–at all!!


If the ambient temps are too high when using CFLs, ventilation is a problem. CFLs don’t just magically give their heat to the heat fairy. The heat has to go somewhere….and that somewhere is up, ideally. But that heat can also sit in once place and cause what’s called a “hotspot.” Bad for your plants. Keep your ambient temps contained and the temps with your lights on will be optimal.

If your ambient temps are too high, the heat from the bulbs will only make it higher, not lower. Your grow room or enclosure will NEVER have lower temperatures than the ambient temps, so work on getting those in order first and foremost, then get your ventilation in place to draw out the heat given by the CFLs.  Then it won’t matter how hot CFLs get because your plants won’t even ‘feel’ it!


Question: “So how hot do CFLs get when growing YOUR plants, after reading this post?”

Answer: Not hot at all!

By Robert Narley