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You should never block both the top and bottom of your chimney. Proper airflow is essential because the temperature inside the chimney will still fluctuate as your house warms and cools. Moisture can build up without ventilation, leading to dampness in your walls over time. Blocking the top prevents debris and animals from entering, but the chimney still needs air circulation to avoid condensation issues. Allowing the chimney to breathe maintains the health of your home and prevents long-term damage.

Chimneys should either be sealed or waterproofed, which is different from sealing off—closing at both ends—a chimney. Waterproofing and sealing are both crucial steps to take to prevent water damage and the chimney leaking inside the house. While waterproofing is more comprehensive, both help safeguard your chimney, extending its life span and preventing expensive repairs. 

The damper, located just above the firebox, can open or close the chimney flue. The damper should remain closed unless there’s a fire burning in the fireplace. Closing the damper prevents energy loss through the chimney and cold air from seeping into the home during the winter. Opening the damper helps maintain airflow when there is a fire and prevents smoke and gases from venting into the house.

While you have some flexibility in the types of wood you can use in your outdoor furnace, it's important to make wise choices for optimal performance and efficiency. Hardwoods, such as oak, maple, and cherry, are generally recommended for their higher energy content and cleaner burn. These hardwoods produce more heat per volume and tend to produce fewer creosote deposits, which can accumulate and cause a chimney fire, according to the U.S. Environmental Protection Agency.