Our crude fired heater is having trouble with positive draft. The heater is the balanced type. Our initial suspicion was fouling/leaks inside the air preheater. But when the ID fan was stopped, the draft inside the furnace increased even further. Chemical cleaning of the heater radiation zone was performed. The convection bank manway was opened and inspected. No significant clogging was observed during the inspection. Any clues as to what could be the possible reason for this?