In cold winter, people seek thermal comfort from the fireplaces in their houses. Is it really a good idea in terms of energy efficiency for the entire house? When a fire is lit, the fireplace creates an updraft force that draws the warm air from the house to the outside through the chimney and the cold air from the outside into the house through small cracks in the building envelope. This is called the stack effect. So while you are getting heat from the burning of wood in the fireplace, you are also losing heat in the house at a faster rate through convection. The following simulation illustrates this (more info).