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

Contributed by: Podge

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

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.

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 thats 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.

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Last update:
2002-12-12 15:28
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Comment of kidhidro:
k plus buddy. i will use that spreadsheet on my trip to denver.
Added at: 2015-06-13 23:57