Heat/Cooling Questions? Ask me.

Do you want to know how much hotter a 400W HPS will make your cabinet? Or if going to CFLs will keep your temps under control? Ask away, and I'll walk you through the solution. (thermo engineer here ).

I'll start off with: "A watt is a watt is a watt."

Any light bulb of any kind will generate heat at a rate equivalent to its wattage. 400W of LEDs = 400W of fluoros = 400WHPS/MH. I know, some may jump on me about this because it seems counter intuitive, and I'll agree the light outputs are completely different (with LEDs being the champ in lumens per watt), but when you have an electrical device - ALL power consumed by that device is eventually converted to heat. Even cooling fans.

So, let's get the thermodynamics party started! Ask away!

inform me on the plasma bulbs if you will...

For starters, by personal experience leds are cooler to the touch per watt. A 90w led is barely warm to the touch. A 90w cfl will burn you.

My situation: I have a 400w hps in a closet. I only grow 3 seasons a year in Ohio because I don't want to hassle with heat. In the spring, like dow, days start getting warmer outside so the room the closet is in is warmer and so is the closet. I like to keep temps at the plat tops below 80. On warm days I am having to dim the hps to 250w. That's with lots of fans. I don't want to fool with venting air, and adding AC to the room is a problem because it has to vent outside and will smell. I also don't want to fool with filters. That's why I went to just 3 grows instead of 4. I'd like to get the heat down without losing lumens, but I don't think it is possible.

dankshizzle said:

inform me on the plasma bulbs if you will...

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The heat generated by a plasma bulb will be the same as it's wattage, just like with all electronic devices.

ok how much heat does a 600w hps give off after being on for say 12hours

zippythehippy said:

ok how much heat does a 600w hps give off after being on for say 12hours

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Well, if you just want to know how much heat, thats 600W * 12h = 7.2 kWh, or ~25,000 BTUs. If you wanted to know how much airflow you'd need to remove that heat and maintain the temp in your grow tent at 90°F (32.2°C) assuming an ambient air temperature of 70°F (21.1°C) and neglecting heat loss through the tent material:

600W = (32.2°-21.1°C)*(air flow rate, "V")*(air density)*(air specific heat)

Use average air temp: 26.5°C, so density is ~1.15 kg/m³, and specific heat is ~1 kJ/kg*°K

600W is the same as 600 Joule/s (by definition). In this case °K and °C are interchangeable.

So we have 600 J/s = (11.1°K)*V*1.15 kg/m³*1000 J/kg*°K

Solving for V, we get 0.047 m³/s, or ~ 100 cfm if your ambient temp is 70°F and you want to maintain a temp of 90°F in the grow tent. In practice a smaller fan would work due to heat loss via the walls of the grow tent.

Illegal Smile said:

For starters, by personal experience leds are cooler to the touch per watt. A 90w led is barely warm to the touch. A 90w cfl will burn you.

My situation: I have a 400w hps in a closet. I only grow 3 seasons a year in Ohio because I don't want to hassle with heat. In the spring, like dow, days start getting warmer outside so the room the closet is in is warmer and so is the closet. I like to keep temps at the plat tops below 80. On warm days I am having to dim the hps to 250w. That's with lots of fans. I don't want to fool with venting air, and adding AC to the room is a problem because it has to vent outside and will smell. I also don't want to fool with filters. That's why I went to just 3 grows instead of 4. I'd like to get the heat down without losing lumens, but I don't think it is possible.

Click to expand...

The CFL is a lot more dense and has no cooling fans... a 90w LED is really an array of probably 30 3W leds, so that's why it's cooler to the touch.

So, adding fans inside a closed system will really not cool it down, it will heat it up. The fans are preventing hot spots by moving the air around, but unless the fans are exhausting air, they are heating up the cabinet. (the increased circulation may *slightly* increase the heat transfer rate through the walls, but that effect is small and probably neglible).

What about freezing 2L soda bottles of water in your freezer? Do like two in at a time, and switch them each day for the two in the cabinet? This is essentially free, and will bring your temps down. Hmmm... let me tell you by how much, maximum one sec...

Hydroctopus said:

The CFL is a lot more dense and has no cooling fans... a 90w LED is really an array of probably 30 3W leds, so that's why it's cooler to the touch.

So, adding fans inside a closed system will really not cool it down, it will heat it up. The fans are preventing hot spots by moving the air around, but unless the fans are exhausting air, they are heating up the cabinet. (the increased circulation may *slightly* increase the heat transfer rate through the walls, but that effect is small and probably neglible).

What about freezing 2L soda bottles of water in your freezer? Do like two in at a time, and switch them each day for the two in the cabinet? This is essentially free, and will bring your temps down. Hmmm... let me tell you by how much, maximum one sec...

Click to expand...

Heat of fusion of water (ie, heat absorbed when it melts) is 334 kJ/kg. I'm ignoring the temp of the water because that effect is small compared to the melting of ice. A 2L bottle has 2kg of water in it... so that's ~700,000 J. 700,000J/(24h*60min/h*60s/min)= 8J/s, or 8W of cooling for two bottles over 24 hours. You'll need more like 10-20 frozen 2L bottles to get 80-160W of cooling... a portable A/C unit is probably your best bet unless you have a large freezer and a lot of time each day.