the software wouldn’t allow me to use any numbers for outside lower than 68 degrees F, so I rolled old school with the following calculation: Had to do come unit conversion so hopefully you can follow me.
Q h = A x k x (Ti – Tu) where: A is surface area 8.44 sqft = 0.784 m2 , k = heat transfer coefficient for carbon steel at 5.5 W/ m2 , in this case you have 0.784 m2 x 5.5 = 4.312W of thermal transfer. You have lowest ambient of -33 F and lowest internal at -5 F
Q h = 0.784 x 4.312 x (-5 – (-33))
Q h = 0.784 x 4.312 x (28)
Q h = 94.65 W
This says your minimum heat requirement is a minimum of 94.65 W. Now we add solar loading . For a gray box, you add 61-300W/ m2 in direct sunlight but this depends on factors like latitude, time of day and year, use of solar shields or double-wall constructions. Since it is so cold outside in this application, I would not count on any solar gain for this calculation in my opinion. If you want to take the 2W of load given in the info, you’re required heating would be a minimum of 92.65W.
By this calculation, I would recommend a 3105.360 (86-100w) or 3105.370 (130-150w) heater. A thermostat is recommended if you will have temperature swings or a hygrostat to maintain a specific humidity within the enclosure.
