The condition described can be due to two reasons:

  1. Incomplete combustion
  2. Reduced heat transfer

To check for incomplete combustion, check the stack for O2 and CO emissions. If the problem is not from incomplete combustion, then the problem is due to reduced heat transfer. Reduced heat transfer can be due to fouling either inside or outside the tubes. If the assumption can be made that fouling and has not occurred inside the tubes, it would be safe to assume that fouling has occurred on the outside of the tubes.