Final answer:
Fireplace flues should be straight, while furnace flues can change direction within code limitations.
Step-by-step explanation:
Fireplace flues should be kept straight as possible to ensure proper airflow and efficient functioning of the fireplace. This helps to prevent smoke and gases from escaping into the living space and directs them up and out of the chimney. However, the direction of furnace flues can change within code limitations. The furnace flues are designed to expel the byproducts of combustion from the heating system, and their direction may need to be adjusted to accommodate the layout of the building or to meet building codes.