To calculate the correct drill bit size for a propane orifice to produce 15,000 BTU at 11 WC, you can use the following formula:
Orifice Diameter (inches) = √(BTU Input / (6500 × √WC))
Orifice Diameter (inches) = √(15,000 / (6500 × √11))
Orifice Diameter (inches) = 0.062 inches
To convert the orifice diameter in inches to a drill bit size, you can refer to a drill bit size chart. The closest drill bit size to 0.062 inches is a #51 drill bit.
Therefore, you will need a #51 drill bit to produce 15,000 BTU at 11 WC.