Which of the following does NOT cause flashback in a gas-fired heating unit?

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The occurrence of flashback in a gas-fired heating unit is typically associated with situations where there is an imbalance in the combustion process, leading to a dangerous return of the flame to the burner assembly. Delayed ignition can lead to a buildup of gas in the combustion chamber, which may ignite suddenly and potentially cause a flashback, but it is not a direct or standalone cause of flashback.

In contrast, a faulty gas valve can improperly regulate gas flow or fail to shut off gas entirely, contributing directly to conditions that can lead to flashback. Similarly, a contaminated burner can impede proper combustion, causing incomplete burning of the gas and creating conditions conducive to flashback. Low gas pressure can lead to inefficient combustion and may also contribute to flashback scenarios due to an improper gas-air mixture.

Therefore, while delayed ignition can create hazardous conditions when it occurs, it is not categorized as a direct cause of flashback in the same manner as the other listed factors.

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