By the numbers
The portion of your heating bill that goes right up the chimney from an open damper.
from an open damper from an open damper
We all love a fire in the fireplace especially around the holidays, just remember, once the fire is out, close that damper! Keeping the damper open is like keeping a 48-inch window wide open during the winter; it allows warm air to go right up the chimney.
Fireplace tid bit: The new MA energy code will no longer allow for open fireplaces for just this reason, all new home fire places will have front doors to control heat loss.