how to cool yr grow room successfully

A 90 CFM with a 600 watt light will raise your temperature above the intake by 21°. The 240 CFM fan will raise it 8°. 90 CFM wouldn’t be enough. It’s not the size of the cabinet that you want to take into consideration; it’s the wattage being used.

(600 × 3.2) ÷ 90 = 21.33

(600 × 3.2) ÷ 240 = 8

To find how much the light will warm the room with a different size fan you can just use the formula. Just take the wattage of the light and multiply by 3.2. Then divide the CFM of the fan to find how much the room will warm in Fahrenheit degrees.

For instance:

If the temperature of the intake air is 75° and you’re using a 600 watt light, the temperature would go up 5° if you used a 384 CFM fan. So, the temperature would end up 80°.

(600 × 3.2) ÷ 384 = 5

5° + 75° = 80°

Peace

Interesting. That math works for my setup. 400watt with 465cfm Temp at around 74-75. Thermometer reads 77...

hey video, I asked on another thread but might as well ask here too ...

where'd you get that formula, in particular the 3.2 factor?

seems like the size of the room has to have an effect. picture a lamp in a cabinet vs same lamp in a garage or barn.

also, by the formula, if you had no fan at all the temp of the room (any size) would increase by 1920 degrees!

I have no idea how a room would increase by 1920 degrees. Assumably, with no exhaust at all, and the light kept on, the temp would rise substantially, how far, I dunno. Try it, and tell me yr results.

The size of the room is irrevelant as the room does not produce heat, the lights does.
(It would have to be a really large room to change the formula)

No matter where you place the light, the wattage used will determine how much heat is put out, if you placed the light in a larger room, it still will produce the same amout of heat.
Peace
(I must have missed your previous question, sorry.)

fnord said:

hey video, I asked on another thread but might as well ask here too ...

where'd you get that formula, in particular the 3.2 factor?

seems like the size of the room has to have an effect. picture a lamp in a cabinet vs same lamp in a garage or barn.

also, by the formula, if you had no fan at all the temp of the room (any size) would increase by 1920 degrees!

Click to expand...

videoman40 said:

I have no idea how a room would increase by 1920 degrees. Assumably, with no exhaust at all, and the light kept on, the temp would rise substantially, how far, I dunno. Try it, and tell me yr results.

The size of the room is irrevelant as the room does not produce heat, the lights does.
(It would have to be a really large room to change the formula)

No matter where you place the light, the wattage used will determine how much heat is put out, if you placed the light in a larger room, it still will produce the same amout of heat.
Peace
(I must have missed your previous question, sorry.)

Click to expand...

the size of the room does have a major effect. the heat will expand to fit the room, smaller room, less area to expand too. the more heat expands the cooler it becomes. a barn and a lamp compared to a box and a lamp, major diffrence; in the barn no heat increase, whereas the box would have a considerable increase.

the only thing you could possibly calculate by just the watts used [with a specific bulb/ bulb type; I might add] is the amount of heat it will produce.

Heat increase also has to take into consideration humidity, and other properties of the air. I dont remember everything from physics and chem, but it does play a role and could make a difference when calculating such a thing as heat increase.

From looking at the forumula it looks like bolgna [spell?]...

im not trying to be mean or anything, im just pointing it out and explaining myself.

that's why I'd like to find the source for the formula. very large and very small rooms (and very small fans) bust the whole equation. could be it works fine as an approximation for rooms of a certain size though, but without the source I don't know.

get some nice vents and a extractor fan youll be fine

Well for the purpose of this thread, we are talking abut a typical grow room, not a barn. Regardless of the size room the light still puts out the same heat.
I do understand your position, but before you displace it as balognia, why dont you try it and see if it works for you?

According to silk, "Interesting. That math works for my setup. 400watt with 465cfm Temp at around 74-75. Thermometer reads 77..."

It is no coinsidense that it works for him, as it works for thousands, the formual has been circulating around for a long time, It originated from cannabis culture magazine fnord. In fact there is another twist to this that converts it to celcius too. I don't have that handy though, sorry.
Peace

Brokenhope420 said:

the size of the room does have a major effect. the heat will expand to fit the room, smaller room, less area to expand too. the more heat expands the cooler it becomes. a barn and a lamp compared to a box and a lamp, major diffrence; in the barn no heat increase, whereas the box would have a considerable increase.

the only thing you could possibly calculate by just the watts used [with a specific bulb/ bulb type; I might add] is the amount of heat it will produce.

Heat increase also has to take into consideration humidity, and other properties of the air. I dont remember everything from physics and chem, but it does play a role and could make a difference when calculating such a thing as heat increase.

From looking at the forumula it looks like bolgna [spell?]...

im not trying to be mean or anything, im just pointing it out and explaining myself.

Click to expand...

Alright, seeing what CFM stands for. the diffuse temp variable would actually make a calculation of this seemingly possible. I think I can see why it works... but I cant see it possible unless your using a CFM fan...

thanks for the additional info.

in a few weeks I should have a couple of different rooms available to check it out. I still think there are too many variables not considered in the formula, and I was looking at it with my math glasses on expecting a formula to be precise. I see now it is meant as an approximation under average conditions, and it must work if it has been around a while.

but to clarify: where in the room are you measuring the temp? at plant top? at a fixed distance from the light?

thanks again

cfm stands for cubic feet per meter, I didnt know that, so thats why it didnt make sense. Still this depends on other factors, smaller, but they can make a difference. Air composition is a big one, more humid rooms will heat diffrently then a dry room. Another big error is when your calculating the heat by watts, you didnt factor in the fact diffrent lights produce more/ less heat than each other. that might work for the hps? lights... but I am doubting its accuracy with cfls, and without a doubt doubting it could be accurate with incandescents [which you wouldnt be using, but they utilize 9/10 of the power on heat...].

the temp of the room you measure should just be one spot, where you think the average is, the average is the best number you can use. but this also is just a ball park average, getting an exact temperature measure is not critically important. Or if you would like to think of it diffrently, where you took the original temperature reading, is what the calculated temperature change would be at that spot. you also measure air intake, the heat of the air the fans blowing on it, measure it right behind the fan.

this formula does work, but only for one type of lights [hps?], other lights could be close, but this is definitly only accurate with one type of light.

Broken, if it works for an HPS light, it'll work for most other types of grow lights. Before you try your best to rip the equation apart, why don't you see if it even comes close to working? Appearantly it does work. Obviously it isnt geared towards incandescents, as there not used in a grow room.
Actually, if I searched, I do believe that there is a chart floating around based on this formula, and taking into account different lights, like HPS and MH, I don't think they actually took into consideration CFL's when they did this. sorry.
One watt if light will displace a certain amount of heat brokenhope420, no matter the humidity in the room. You talk of chemistry but have no idea what cfm is? Than you say IF your using a cfm fan, all fans are measured in cfm buddy.
Maybe this will help yo see it clearly.
http://www.cannabisculture.com/forums/uploads/937331-CFM%20Chart%20558W.jpg

Peace

Brokenhope420 said:

cfm stands for cubic feet per meter, I didnt know that, so thats why it didnt make sense. Still this depends on other factors, smaller, but they can make a difference. Air composition is a big one, more humid rooms will heat diffrently then a dry room. Another big error is when your calculating the heat by watts, you didnt factor in the fact diffrent lights produce more/ less heat than each other. that might work for the hps? lights... but I am doubting its accuracy with cfls, and without a doubt doubting it could be accurate with incandescents [which you wouldnt be using, but they utilize 9/10 of the power on heat...].

the temp of the room you measure should just be one spot, where you think the average is, the average is the best number you can use. but this also is just a ball park average, getting an exact temperature measure is not critically important. Or if you would like to think of it diffrently, where you took the original temperature reading, is what the calculated temperature change would be at that spot. you also measure air intake, the heat of the air the fans blowing on it, measure it right behind the fan.

this formula does work, but only for one type of lights [hps?], other lights could be close, but this is definitly only accurate with one type of light.

Click to expand...

600 wats and a 2x3 closet goes over 100 degrees. about mid way up at the top it must be scary where the light is.

Thats a little big for that space anyway.
A 400 watt light cover a 3x3 space.

bgift1 said:

600 wats and a 2x3 closet goes over 100 degrees. about mid way up at the top it must be scary where the light is.

Click to expand...

one person is talking about heat generated by the light itself. it will always be the same. i think most everyone else is misunderstanding it to be the room temperature.
ie... let say a 1000 watt light gives off 90 degrees of heat. regardless of the size of the room, the 1000 watt light is still going to give off 90 degrees of heat. now, of course a closet with a 1000 watt light is going to be hot as hell compared to a 1000 watt light in a barn, but that is comparing room temps.
i know saying "1000 watt gives off 90 degrees of heat" doesn't make any sense at all, but i was using it at an example to show that it is a constant unlike the temperature of the grow area which can easily fluctuate.

Brokenhope420 said:

cfm stands for cubic feet per meter, I didnt know that, so thats why it didnt make sense.

Click to expand...

It actually stands for Cubic Feet per Minute.

Brokenhope420 said:

but I am doubting its accuracy with cfls

Click to expand...

I think you're right to doubt it's accuracy with CFL's.

I think Videoman's formula is a simplified version of the more complex one which would take into consideration other types of light sources other than HPS and also ambient air temperature and room volume.

Here's the fuller and more complex explanation and formula - (taken from another site as I can't find a thing in our own GrowFAQ's) perhaps you can work out from this where the 3.2 in Videomans formula comes from.

What temperature will my growspace get and how much ventilation will I need?
------------------------------------------------------------------------
Overheating is a common problem encountered by growers, especially in closet or 'box' setups. High temperatures cause whispy buds and is often a sign of inadequate ventilation, which brings a whole host of other problems.

Whether at the design stage or struggling with an existing problem, the following formula can be useful in assessing your situation. Its pretty basic in terms of heat transfer but from experience has proved to be pretty accurate for our purposes.

The formula is:

Q = V x P x C x dT

where:
Q = Amount of lighting (kW)
V = Volume of air being ventilated (m3/s)
P = Density of air (assume 1.2 kg/m3)
C = Specific heat capacity of air (assume 1.02 kJ/kgK)
dT = Temperature difference between ambient and growspace air in degC

You can use this to determine what the temperature rise in your space will be (dT), or given a desired temperature rise you can use it to work out how much ventilation you will require (V)

To get from CFM to m3/s divide the CFM by 2119.

Examples
Here are some examples of how you could use the formula in three different ways, each using the same basic figures for clarity.

What temperature am I likely to get in my growspace?
Assume: Lighting = 400W (0.4kW), ventilation = 240m3/hr (0.067m3/s) and temperature of air entering room = 21degC

Q = V x P x C x dT
=> dT = Q / (V x P x C)
=> dT = 0.4 / (0.067 x 1.2 x 1.02)
=> dT = 4.87, i.e. 21 + 5 = 26degC in growspace

How much ventilation am I likely to need?
Assume: Lighting = 400W (0.4kW), temperature of air entering room = 21degC and temperature of growspace to be no more that 26degC

Q = V x P x C x dT
=> V = Q / (P x C x dT)
=> V = 0.4 / (1.2 x 1.02 x (26-21))
=> V = 0.065 m3/s i.e. 240 m3/hr

What is the most lighting I can put into my growspace?
Assume: Ventilation = 240m3/hr (0.067m3/s), temperature of air entering room = 21degC, temperature of growspace to be no more that 26degC

Q = V x P x C x dT
Q = 0.067 x 1.2 x 1.02 x (26-21)
Q = 0.41 kW i.e. 400 W

So that’s it, once you get used to using it its very simple really, Just stuff the formula and figures in a spreadsheet and let it do the work.

Authors: Podge and snoofer
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