What could explain white and yellowish deposits below a draft inducer fan?

The presence of white and yellowish deposits below a draft inducer fan is most likely attributed to moisture condensation in the venting system, which can run back and collect in this area. This condensation may occur due to a variety of factors, such as improper vent sizing, inadequate insulation, or humidity levels within the system. When moisture vapor condenses within the vent, it can result in mineral deposits forming, often appearing as white or yellowish residue.

The other potential explanations do not directly account for the appearance of these specific deposits. A failing seal between the fan and the furnace may lead to air leaks or reduced efficiency, but it wouldn't typically produce the described deposit build-up. If the fan is not operating properly, it would likely impact the furnace's performance more broadly rather than leading to localized condensation issues. Lastly, although a failed furnace can pose safety concerns, it does not specifically relate to the presence of deposits under the inducer fan. Understanding these distinctions helps in effectively diagnosing issues related to draft inducer fans and venting systems.