I'm looking for possible causes for the carbon build up at the furnace draft hood. Approximately 15 year old 2 burner gas unit.

Thanks,

Eric

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Deposition of combustion byproducts is a common occurence and expected after 15 years of use. I would be shocked if the draft hood and flue piping were free of such.

In your photo though I am seeing mosty rust stains and not really much build up at all in the grand scheme of things.

Thanks Philip,

I explained this to my client but they weren't convinced. They had the housepretty much "inspected" before I got there.

Eric

Oh well, it always tough to change someone's mind when its already made up. At 15 years old though its probably best if they get the unit check closely by an HVAC contractor anyway. He may be in a better position to calm their worries or validate their fears as the case may be.

That furnace needs to have a full combustion test & draft interference test run on it.

Rust is not a normal thing in any furnace regardless of age, if you have rust you have a venting problem.

Until any combustion readings are taken the determination of that furnace being safe are strictly a guess.

The signs of overheating on the sides of the drafthood would be enough to cause me a good deal of concern.

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Measured Performance more than just a buzzword

The markings tell a story of impaired performance that should be followed up as Davidr posted. They also warrant a closer inspection of the heat exchanger, burners, orifices, manifold and inlet gas pressures, primary aeration, etc. I disagree that in a non-luminous flame burner, you should not be getting soot. If you enlarge those pics, it appears there may be a crack along the top of the HX. Regardless, these components are made of materials that have a corrosion resistance suitable for the class of service. If you are getting corrosion here, it is probably an indicator of excessive condensation. Even if this unit tests up ok, its long term durability is certainly in question. In such cases, I'm not shy at all about sending pics to the mfr. for written comment. While they may not have sufficient information to comment fully on this, they can comment on the warranty, durability, replacement parts, etc.

Full inspection combustion analysis and Worst Case Depressurization testing are in order.
Bob

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Keep the fire in the fireplace.