I have a fan kit on my wood-burning fireplace but it doesn't seem to blow much air.

Our fans are purposely designed at lower CFMs (cubic feet per minute). There are two reasons for this. This first is to minimize fan noise. Second, a high velocity would tend to cool the air temperature coming out of the top louver of the fireplace. The fan is designed to move warm air into the room, not to move air clear across the room.