Just to let you know, I am not an expert on boilers. I just have some formulas due to us custom building our equipment to high specs than whats out there in order for us to do specialized cleaning in Parking Garage Cleaning.
At the time we built our equipment, there was really nothing out there that would hit 300 degrees under a full load of 6 to 12 GPMs for Parking Garage Cleaning.
Here is the formula that we used:
First, convert GPM to GPH
Next, Determine the pounds of water per hour by multiplying (XXXXX) GPH x 8.35 LBS. per Gallon
Remember, It takes 1 BTU to raise 1 pound of water , one degree F.
Most Pressure washers operate 75% efficiency. Therefore you must divide the BTU by 75%. I actually felt 68% is a more true number. ( 32% heat loss). and if your burners are a few years old or have more than 1,000 hours on them, then lower the percentage by 5 to 7% more.
There is the formula.
Glad to help.