Heat loss through large air ducting

Hello everyone. I'm a student engineer on a work term and I've been running some very rough calculations on the exhaust air of a client of ours. I'm looking for the heat loss that's being pumped to the exterior of the facility through an exit duct. What I have for information is:

-flow rate: 40 000 CFM

-exit air: 20 degrees Celsius, standard indoor humidity humidity

-inlet air: -20, -10, 0, and 10 degrees Celsius, standard indoor humidity (the different temperatures reflect seasonal temperature changes)

-The degree days for our example location are 5132 (value taken from Environment Canada's website)

-Exit and inlet will have the same cross sectional area of 80 square feet

I have applied the first law of thermodynamics to determine what the heat loss is in btus/hour but I don't know if the number is correct.

I'm assuming that the degree days were in the Celsius scale and so the average daily temperature for a year would be 5132/365 = 14.1 degrees Celsius = 57.38 degrees Fahrenheit.

Here is what my original equation looks like:

Heat loss = (40 000 CFM) x (1.08 (BTU*min)/(degree Fahrenheit*ft^3*hour)) x (68 degree F - 57.38 degree F) = 458 784 BTU/hour

My question is whether or not my chain of reasoning makes sense here? Any suggestions would be really appreciated!! Thanks!