What indicates that a gas appliance is underfired?

A gas appliance is considered underfired when it operates at lower than optimal combustion efficiency, which can be apparent through specific signs. Notably, the combination of low heat rise and low vent temperature indicates that the appliance is not producing sufficient heat. Low heat rise reflects that the appliance is not generating the expected amount of hot air or heat transfer, while low vent temperature signifies that the exhaust gases are cooler than normal due to incomplete combustion.

When an appliance is underfired, the combustion process does not reach ideal temperatures, leading to a lack of effective heating and potentially inefficient operation. This can result in safety hazards over time, such as the accumulation of carbon monoxide or other harmful gases due to improper venting. Recognizing these indicators is crucial for maintenance and ensuring the appliance operates safely and effectively.

Thus, the combination of low heat rise and low vent temperature truly captures the concept of an underfired appliance, making it the correct answer.