Factory-built chimneys for medium-heat appliances must comply with UL 959 if flue gases exceed __________.

Factory-built chimneys for medium-heat appliances are designed to safely vent the flue gases produced during the combustion process. The compliance with UL 959 is crucial for ensuring that these chimneys can handle the high temperatures generated by certain appliances.

Specifically, when flue gases exceed 1,000°F (538°C), it indicates that the potential for thermal stress and structural failure within the chimney increases significantly. UL 959 outlines the performance requirements that factory-built chimneys must meet to ensure safety and effectiveness at these elevated temperatures. This standard addresses factors like material durability, fire resistance, and overall structural integrity under extreme conditions, which are essential for preventing incidents such as chimney fires and ensuring the safe operation of the heating appliance.

Since this temperature threshold is critical for compliance, it is necessary to adhere to UL 959 at this level to mitigate risks associated with high heat exhaust.