DEAR TIM: After a few minutes of starting a fire in our living room fireplace, we can smell and eventually see some smoke in our downstairs basement. Do you know why this is happening? We have a cap on our chimney, and although our fireplace and gas exhaust do share the chimney they are separate. Rich Peters, Brighton, MI

DEAR RICH: I know exactly what might be happening. There are multiple possibilities. My first inclination is that your living room fireplace does not have a proper separate combustion air intake. When you burn a fire in the fireplace, you can see smoke issue forth from the chimney. The draft from the heat might be producing an air flow of hundreds of cubic feet of air per minute up the chimney.

An equal amount of air must enter the firebox to satisfy this partial vacuum created by the fire. The air will enter your home at the point of least resistance. Since you have another hole in your house immediately next to the fireplace flue - the opening of the gas exhaust from your furnace and/or water heater - the fireplace may pull the needed air back down this handy conduit.

In older homes with traditional masonry fireplaces, it is not easy to retrofit combustion air. If the fireplace is on an exterior wall and the outside of the chimney structure is brick or stone, an experienced mason can install a combustion air intake vent that feeds air directly from the outside into the firebox.

The smoke can also be drawn back down into the basement by any other fuel burning appliance that is operating. Remember, all of these devices need air to burn the gas, propane, wood or oil. If your water heater fires up and needs air while a fire is burning in the fireplace, then smoke might be drawn back down a third adjacent flue that is not being used. This back drafting is very serious as it can draw carbon monoxide into a home.