Q: Jason, We have a fireplace upstairs and one directly below it downstairs. Sometimes when we use one fireplace the other one will be smoky. Do we need more than one chimney pillow, or can we even use a chimney pillow in this situation? - LK
A: LK, When you have a fireplace that is one above another the two hearths usually have separate flues and dampers and they run to the roof in the same chimney structure. See the diagram of the two fireplaces sharing a chimney structure.
When you are using one fireplace and chimney your home is drinking in air from other places to allow enough airflow to feed the fire and to draft the smoke and heat upwards out the flue. The home will suck in air from the path of least resistance which is usually the other unused chimney (even if the damper is closed). This draw of air into the house is known as the "stack effect".
So when the other chimney sucks in air from the roof it also sucks back in some smoke from the other fireplace chimney.
The solution to this problem is to use 2 separate Chimney Balloons, one for each flue. When you have a fire in one of the fireplaces then remove that Chimney Balloon in the used fireplace and leave the other one in place in the unused fireplace. This will force your home to find another less-smoky place to draw from, like doors or windows. - Jason