2500
Btu/h
2000
Btu/h
1500
Btu/h
5 deg in/out
10 deg. In/out
15 deg. In/out
20 deg. In/out
25 deg. In/out
30 deg. In/out
1000
Btu/h
500
Btu/h
Btu/h
Heat loss through a typical open letter box 8” x 2” = 16 sq. Inches (100 sq. cms) with a temperature difference internal to external of 5
to 30 degrees Fahrenheit and with a breeze of 1mph to 10mph indicate BTU/h needed to replace lost heat. Damp air could cost more
than 4 times this energy.