Negative Building Pressure and Fireplace Draw
You don’t wonder if you have a draft problem when you start your fireplace and smoke bellows into your home, you know you have one. When you have a draft problem, the problems may not be just restricted to a problem with your fireplace but may be a negative pressure problem in your home affecting your fireplace and other gas fired appliances. If combustion gasses from these appliances cannot exit the building, they can accumulate in the building and result in serious health problems.
There can be more than one cause for poor draft at a fireplace. Once a chimney sweep has performed an inspection and ruled out other probable causes you may find you have a negative pressure problem in your building. Negative relative pressure between the inside of your home and the outside can have many causes. Causes of negative pressures can be related to poor design of the air conditioning system, damage to the air condition system allowing leaks outside the building envelope, and poor design of ventilation systems such as your kitchen range vent hood. The problem can be local to a room or may be a building wide concern. Detecting the cause for a negative pressure conditions requires inspection, careful consideration, and in some cases special testing using gas detection equipment and low pressure manometers to evaluate.
How Do I Know If I Have A Negative Pressure Problem
Although poor draw at the fireplace may be the first symptom there can be other less apparent indications. In a warm humid climate such as ours, uncontrolled air drawn through the envelope of the building is unconditioned and carries moisture often resulting in mold in strange places such as on the ceiling around air conditioning register grilles, at outlet and switch cover plates on outside walls, or around door and window casings. You may also have a carbon monoxide detector that goes off frequently indicating flue gasses accumulating in the building.
If you suspect a problem with your fireplace draw, the first thing you should do is contact a chimney sweep. Eliminate other potential causes for poor draft at your fireplace first. If a negative pressure problem is suspected, hire an engineer or air conditioning contractor who can inspect the building and take interior pressure readings to provide guidance in developing a solution. Corrections for negative pressure can include adjustments to air distribution in the building and improvements to ventilation air; however, design considerations should be discussed with an engineer to ensure the solution is proper and will positively affect the comfort of the building.
Edward Robinson, P.E. President of Professional Engineering Inspections, Inc. A Houston based building inspection firm Mr. Robinson has been providing inspection services since 1987.