We've recently moved into a two-story home that's 5 years old. The living room (which has a gas fireplace on the main floor) is COLD! The thermostat to the house is in that room, so our furnace continues to run, which is making our upstairs at least 10 degrees warmer. We thought the gas fireplace was bringing in all the cold air, or pulling out all the hot air so we looked for a flue or damper to close - no luck. We have it sealed off with plastic right now, which has helped some, but our main floor is still cold. Any ideas? We've checked the windows and doors and everything seems to be sealed up. It's noticeably colder next to the cold air return duct, is that normal?