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What temperature will my growspace get and how much ventilation will I need?

Smokin Moose

Fallen Cannabis Warrior
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.
 

FatRob

New Member
Something I've found that greatly diminishes temperature swings in my grow closet is the addition of several 5 gallon buckets full of water. I add a quarter cup of bleach to each and keep a lid on them, they don't need to grow anything or add any humidity.

The added thermal mass really calmed the temperature swings.
 

TckCliff

New Member
never heard about this before, i think i understand the concept but i dont know how practical this would be for a large garden.
 

FatRob

New Member
never heard about this before, i think i understand the concept but i dont know how practical this would be for a large garden.

I've seen it done with some pretty large spaces, using larger water containers. I had a friend who used 55 gallon drums to keep his large house warmer than it otherwise would have been. It really doesn't take as much water as I thought it would. It basically stores heat from the warm parts of the day and releases it when cool, no muss no fuss. Since I have a bunch of fans moving air around anyway it's basically free.
 

TckCliff

New Member
Ya i but ill keep that in mind when the lights off if i have some winter temps that lower their sleepy time.
 

steeders

New Member
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.
thats is about as handy as two left feet you have to be a physics expert to figure that out (simple formula my hole)
 

TheStudent

New Member
thats is about as handy as two left feet you have to be a physics expert to figure that out (simple formula my hole)

Hi, I'm new here and I am very pleased to see that I'm not the only person utilizing thermodynamics to optimize growing!
With another simple thermodynamics equation using Ideal Gas law one can calculate the amount of CO2 necessary for a grow space given certain grow space parameters.
It is even possible to calculate how much CO2 a simple dry ice setup will produce and if that's enough for the intended space.
Using a little quantum mechanics and classical physics it is also possible to calculate and account for every aspect of lighting!

Glad to be here,

Student
 
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