Your fireplace chimney design can have a big impact on how well your fireplace works. It’s important not only to build a beautiful centerpiece for your main living area, but to ensure that it is as safe as possible and functions well.

First, it’s important to understand how your fireplace chimney fits into your home’s airflow system. Air flows through your home like a river and you’ll want to ensure that indoor and outdoor air are exchanged often. This can be achieved by ensuring that controlled air intake points supply the right amount of air for occupants, as well as other oxygen consuming appliances, including furnaces, stoves, and fireplaces. To make sure this happens, your contractor will be able to specify the appropriate intake fans, vents and access points that will meet the requirements for air exchange. If there is not a good and constant air exchange, you may run the risk of carbon monoxide buildup as well as poorly burning fireplaces.

Both temperature and air pressure can affect the airflow in your home. Wind blowing on one side of your home creates a wind load effect that raises the air pressure on that side of the home. The air pressure on the leeward or opposite side of the home is slightly negative pressure. Thus if your air intake point is on the windward side, then the chimney can work efficiently drawing smoke and hot air up and out.  However, if your intake is on the leeward side, the resulting slightly negative pressure will create a back draft and draw smoke back into the house.  Thus it is important to have large exhaust outlets such as chimneys or dryer vents balanced with equal controlled intake points to keep air pressure neutral or slightly positive.

This careful consideration to air flow in a home is particularly important with the modern, well-insulated home. Much of the outer shell is built tightly to conserve energy and heat, so it seems counterintuitive to create opportunities for air to draw into the house, but combusting elements need the oxygen and airflow to function properly.

Keeping the proper airflow in mind, here are some tips to ensure that your beautiful fireplace burns brightly.

  • Keep the chimney and fireplace in an interior location, not on an outside wall. Colder outside air will cool the rising hot air from the fire and cause the fireplace to burn sluggishly and smokily. With a colder chimney temperature, you may find that it’s hard to get fires started and to heat up the chimney column.  By keeping your fireplace chimney off of outside walls, you’ll also prevent unsafe creosote buildup, which can accumulate much faster on an exterior wall chimney.
  • Make sure your air intake matches the size of your chimney. Your chimney is a 12 inch by 12 inch column of air rising up away from the fireplace. It doesn’t take long for that flow to remove a lot of air from the home’s interior—air that needs to be replaced.
  • Construction can affect how well you chimney functions over time.  If your chimney is built around masonry blocks, make sure that the interior of the chimney is as smooth as possible. This will minimize creosote buildup and keep your chimney functioning properly.
  • Make sure your chimney is tall enough, but not too tall. A properly functioning chimney should be a minimum of 10 to 12 feet tall. If the chimney extends beyond the roof, the column should extend at least three feet and be two feet taller than any thing within ten feet of it, including other structures or trees.
  • Size the flue to handle opening of the fireplace. A simple ratio is that the fireplace opening should be no more than 10 times the opening of the flue. So if you’re planning a hearth grand enough to roast a pig, just make sure your flue is also as large.
  • Consider fireplace inserts or woodstoves to keep the heat inside. A beautiful fireplace doesn’t have to be just visually attractive, it can provide a more efficient heat source for the interior with the help of a fireplace insert that redirects the hot air back into the room. Many styles are available and they offer the look of a traditional fireplace without the loss of energy. One company, Heatilator, has a terrific website for inserts that are ready made for installation.
  • Is there a return air grill to your furnace in the same room? If you have forced warm air heating, this return air grill may be competing with your fireplace for the air in the room and creating a slightly negative air pressure, which hampers the chimney from functioning properly.