Fireplace Design

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The majority of homes built in the United States this century are built with some sort of wood burning fireplace. As we become more aware of efficient use of energy, it is necessary to examine current fireplace design practices to determine how we can get the most out of our fireplaces. This article will summarize ways of increasing the heating potential of masonry fireplaces. The three fundamental questions addressed will be:

  • Where does the air that feeds the fire come from? inside or outside?
  • How can we reduce the amount of heat lost up the chimney? (stack loss)
  • How can we maximize the amount of heat that actually enters the room?

Provide a make-up air supply

The air that fuels a fire has to come from somewhere. With modern airtight construction, the home is a controlled environment where air enters at prescribed locations. This air is heated and distributed throughout the home. The fireplace draws air into the firebox to be consumed in the fire, and then expels the air up the flue and out of the house.In a cold environment, the air drawn into the firebox comes from the house and has already been heated to a comfortable temperature by the central heating system. As the air is expelled up the flue, it has to be replaced by more air. Where does this air come from? It must infiltrate the house from the outside, be heated up by the central heating, and then fuel the fire. The chart below illustrates the energy drain incurred from this process. The net energy shown on the vertical axis is the absolute level of energy available to for use in the house after the combustion air lost up the clay flue tile is deducted. A conventional fireplace with no glass doors or outside make-up air provides no heat gain below 35ºF, in fact it is an energy drain. The energy gain for a conventional fireplace with outside air for combustion and glass doors is constant across all temperatures and results in net heat gain. A source of outside air must be provided. In houses with a forced air heating system, the make-up air can be provided through a fresh air duct connected to the cold air return.