Three Chimneys
Q: Jason – I have two fireplaces in my home, one in the living room and one in the basement. When I light one fireplace the other one gets smoky. My fireplace shop said to crack a window so this wouldn’t happen, but it is -1 degrees F out, who wants to open a window when it is so cold?! Why would that help keep the smoke out? – KM
A: KM – You are experiencing a very common problem among new homes with multiple fireplaces. Let’s look at this in sections
1) Your fireplace sucks air when lit.
When you light one fireplace that fire starts drawing air from the rest of the house like a vacuum for the combustion process. To replace that air the house will start drawing in outside air through the path of least resistance in order to equalize the pressure. This is called the “stack effect”. Your home could draw this air through worn weatherstripping, power outlets on outside walls, cracks, under insulated windows, or many other ways. One of the weakest points in a homes air envelope seal is the metal fireplace damper. In you case your home is pretty tight but your fireplace damper is the path of least resistance for cold air to gain entry. See the image below of 3 chimneys that are too close together, they may draw smoke from each other.2) Your fireplace damper is weak and is the easiest way for cold outside air to gain entrance.
When the air is drawn down the chimney to equalize the pressure in your home it also draws down the smoke that is wafting around your roof. This is why your basement gets smoky.3) Seal your basement damper.
If you seal up the basement damper and flue with a Chimney Balloon it will then be air tight and will not draw in smoke.4) Your home cannot be a vacuum.
When you burn in your fireplace your home will continue to draw in air from the outside. It just wont be drawing smoky air from your roof. But it will draw from somewhere. That is why burning in your fireplace does not save you on heat, it actually costs you heat in most situations.