In what scenario would a 5" diameter vent be required for a heating appliance?

A 5" diameter vent is required for a 150,000 BTU/h non-condensing furnace because such appliances generate a significant amount of flue gases that need to be vented effectively to ensure safe operation and proper draft. The sizing of the vent must match the appliance's BTU input to handle the volume of combustion gases produced during operation.

Non-condensing furnaces typically operate at lower efficiencies compared to high-efficiency models, meaning they don't utilize combustion gases as completely. As a result, they produce more exhaust that must be effectively expelled through a venting system sized appropriately to prevent any backdraft or dangerous build-up of gases. Therefore, a 5" diameter vent is necessary to accommodate the higher volume of flue gases associated with this type and size of the furnace.

In contrast, other appliance types, such as high-efficiency boilers or different water heating systems, may not require a vent of this size due to their more efficient operation and lower volume of exhaust gases. They often use different venting materials and configurations that are not compatible with a 5" diameter vent.