What causes this?

[ahendrickson]

Furance is about 20 years old... this corrosion appeared right at the exit of the exchanger as it transitions to the flue. The picture isn't the best but any suggestion on what it is or how it is caused would be appreciated.

[David P. Valley]

Adam,

Hot combustion gasses do pass through the heat exchanger tubes depositing soot and scale on the heat exchanger walls. The heat transfer properties of the tubes are reduced as this layer of deposit builds up and the furnace must work harder to provide the required heat. This causes undue wear on motors, blowers and controls, resulting in higher operating costs and increased fuel consumption. Regular cleaning of the fireside surfaces of the heat exchanger yields big payoffs in lower operating costs and fuel savings. No matter what fuel is burned, exterior heat exchanger firetubes have to be cleaned.

Due to it’s age anyways, I‘d recommend a licensed HVAC professional evaluate the heat exchanger for scale build-up and to inspect for holes.