Corrosion caused by low input, running with natural gas. Exhaust temperature below dewpoint caused corrosion

The heat exchanger made of steel sheet was designed for a specific heat load with hot exhaust gases from 1706300 BTU/h (500 kW). It was actually powered with a heat load of approximately 68250 BTU/h to just under 682500 BTU/h (20 kW to just under 200 kW). So it was with the exhaust gas not possible to go through the dew point.
This caused corrosion and the only way to stop the corrosion was by exchanging the system and replacing the sheet steel with another material, e.g. stainless steel which happened and it was successful.
This solution was contained in my report to the court.
In addition I found the combustion air to be aggressive and I advised that the air should come from a different source namely where clean air could be provided. My advice was accepted and corrosion eradicated.

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