Hey stoke man May be you can answer this Question when figuring out the specs on the 520 What is the B.T.U value that They are using for the coal per lb bTU's Because when looking in the manual under Specification Output Per Hr.
Gross Output is 214.690 BTUH At 25lb Per Hr with a 10 Teeth feed Setting Net Output BTUH is 186.690 BTUH When you do the math Just Taking The Coal at 25 lbs Per Hr With the coal burning at 13.500 BTU's Per lb That would be 337.500 BTUH so there is like a 122.810 BTUH loss with the 520 what i am I missing in you specs Other then Heat loss in the water & chimney Just wondering what BTU's They are using for coal Per lb With these Numbers you are looking at like a 60% efficient rating

what I am I missing here

Thanks