The White Mountain Hearth VFA2 is a fresh air kit, designed to allow a mix of outside/fresh air to be incorporated into the combustion process. This helps keep the inside air fresher when the fireplace is in operation. It is an optional upgrade that we recommend for units on an outside wall, or adjacent to a ventilated crawlspace, that are going to get more than occasional use. It includes a damper to inhibit airflow when not in use. A “P-Trap” is recommended on straight duct installations to eliminate cold air draft, and slow air velocity.