You can have multiple appliances served by one chimney, but only __________ per appliance or fireplace.

The correct answer addresses the requirements for venting appliances connected to a chimney system. In the context of mechanical code, each appliance or fireplace must be equipped with a dedicated vent to safely convey exhaust gases to the outside without allowing combustion byproducts to enter living spaces. The requirement for one vent per appliance ensures that each unit operates efficiently and that there is no backdraft or contamination from other devices' exhaust.

Having multiple appliances served by one chimney can pose risks if the venting is not done correctly. Multiple appliances sharing a single vent could lead to improper drafting and might result in hazardous conditions. Therefore, it is essential to ensure that each appliance has its own vent to maintain safety and compliance with code requirements. This regulation helps in managing exhaust flow effectively and ensuring that each appliance operates properly without interference from others, which is crucial for both safety and performance.