BTU Question

[Aheadatime]

Hey all. From what I understand, BTU calculations work like this

Room BTU (8ft ceiling) = Length x Width x 40
Lighting BTU = Wattage x 4
Equipment BTU = Wattage x 3.5

Add em all up and you get your BTU requirement. My situations a bit different though, and it requires some explaining.

I'll be using my basement. 15X13 area for flower, 7x13 for veg. These two rooms together form a perfect rectangle (notice they share 13ft width). I'll be going with a "sealed room" method, but the two rooms will share the same air.

There will be two 6inch fans, one in veg room and one in flower. They will each be pulling air from one room and moving it into the other (via ducting with a 90degree bend or two to avoid light sharing).

Using the BTU calculations, my BTU requirement comes out to ~34k. I plan on going with a 36k BTU unit for the current situation, which should suffice. My concern is that I may add two additional 1k lights in the flower room in the future, pushing my BTUs up to 42k.

My ceiling height is 6'9" (1'3" shorter than the calculation), the room is inherently chilly, and ~800 watts of my lighting comes from T5s. Do these factors lower my BTU requirement enough to not worry about the two additional lights in the future?

 

[Aheadatime]

Forgot to mention in case this is relevant information - I'll be using a mini-split AC.

 

[Aheadatime]

Also forgot to mention that I won't be using air-cooled hoods. I've read that the glass reduces PAR output by 5-10%, and the cool air touching the bulbs takes another ~5% away.

 

[Cork144]

Lumens deplete at around the same rate per inch as what you lose from running air cooled setups from some rudimentary calculations, so to me it seems gaining even 2 or 3 inches closer to the plants may infact be beneficial, and you get to half the BTU output of the lights

 

[Cork144]

at 20inches, a 1000W bulb is rated to be 16043 lumens
at 18 inches, a 1000w bulb is rated to be 19806,

so we take 15% which is what your calling the worse case scenario, from 19806 and we are left with 16835 lumens, just a 2 inch difference but even with a 15% loss due to setup, gives you a 792 lumen gain, closer light counts more it would seem on paper.

 

[Aheadatime]

Yeah I've been hearing alot from close friends and members of other forums that air cooling the lights is probably most beneficial. It would also, as you mentioned, cut the required BTU in half from the lights. Still thinking everything over, lots of food for thought.

 

[Cork144]

likewise mate I am currently planning one so been doing the research over the last few days, you can get hoods designed to prevent direct airflow on the bulb, meaning the lumen count will be even higher.

 

[queequeg152]

and the cool air touching the bulbs takes another ~5% away.

that might be the case for florescent bulbs, but HID's have an outer envelope with a partial vacuum... they are far less sensitive to changes in environmental temperature.

you can test this with a generic light meter though.