My house has a gas furnace and a gas hot water tank. Each of them occupies its very own closet in the hallway. The thing about gas appliances is they need to breathe--they need a steady supply of oxygen to feed the flame. Without a vent of some kind, the flame will either starve or--worse--it will "flash" out toward any oxygen source it can find--such as the oxygen coming in through the cracks around the closet door!
To mitigate this, a vent tube leading to the attic is installed to provide a steady supply to the appliance. However, for our purposes, this becomes a liability in the pursuit of running our HVAC systems efficiently because that vital attic-air in the closet will exchange with the air inside the rest of the house. Best if we can keep that attic-air tightly quarantined in the closet so our HVAC system won't have to work to treat that untreated air (not to mention the potential for particulate to wander down from the attic).
The good news is that we can seal the closets very easily and inexpensively. I used a combination of two products--the "Draft Dodger" (often As Seen on TV) and foam tape--the same kind often used to weather-strip an exterior door.
 |  |
| "Draft Dodger" seals the gap at the bottom of the door. | Foam tape seals the door shut. |
--
Happy Greening!
Andy
