Fireplaces and Chimneys are primary key components in everyone’s house. Chimneys carry smoke, heat and harmful flue gases out of the house. Hence, it makes sense to be aware of the scientific fundamentals that will give an overview of why and how a chimney works. However, a certified Chimney Sweep knows it all and can easily diagnose odor or smoking problems of your chimney.
Although it cannot be seen, the air inside our homes is in continuous motion. The airflow usually moves out of the house through the upperparts and the replacement air flows into the lower parts of the house. If you consider the house as a system it will become easier to understand the reasons behind the airflow. There are numerous factors, such as wind loading, stack effect, interior mechanical systems along with fuel-burning appliances like fireplaces, water heaters, furnaces and wood stoves influencing the airflow of a house. It is observed, those homes that are renovated or built within the last 25 years are one step ahead in being airtight in comparison to older homes. This creates more difficulty for the replacement air to move-in to the house.
Stack Effect: When warm air upsurges towards the upper part of the house, it is known as the stack effect. This trapped air paves its way through any small openings that it can get through, such as window frames and recessed light fittings. Simultaneously the replacement air comes down to occupy the space created by the escaping air.
Neutral Pressure Plane: Somewhere within the house amongst all these airflows lies the Neutral Pressure Plane (NPP). Right above this neutral pressure plane, the air pressure is higher than the outside air pressure trying to exit the home. Whereas, under the plane the pressure is negative and there the home is drawing air from outside. The NPP changes its location depending upon the changing conditions and the factors affecting the airflow in the house also impacts the level of the NPP.
Whenever a fuel-fired heating application or a fireplace is placed lower than the Neutral Pressure plane, air will try to get inside the house through the vent or chimney. This is most common in homes with dual fireplaces, one on top of the other. The fireplace that lies on the top uses air for ignition and chimney flow depressurizing the level that causes the air to flow upwards from the bottom level. As the bottom fireplace is secondary to the NPP, the air flows through the chimney and reaches the basement. Since both the flues exodus the chimney in a close margin, there can be some fireplace smoke present in the replacement air which may create bad chimney odors while passing down the chimney flue.
Wind-loading: The effect of the pressure created inside the home by the wind is termed as wind-loading. Wind always generates high pressure when it strikes the wall of a house and low pressure on the side of the downwind. If there are doors or windows open on the side of the windward, it will increase the chimney draft, which in turn will pressurize the home. On the contrary, an opening to the side of the downwind will increase the back-drafting from vents or chimneys that will depressurize the house. Back drafting means the reverse movement of air that gets inside the house instead of going out of the chimney.
Interior Mechanical Devices: Interior mechanical devices like kitchen fans, attic fans, clothes dryers, bathroom fans, and central vacuums create depressurization by releasing huge volumes of air from the home. This often results in negative pressure in the fireplace area or in the woodstove increasing the difficulty for natural draft chimneys to perform as intended.
In short, to make sure that your chimney is performing as expected, remember to call a professional chimney sweep annually for inspection to prevent chimney fires and carbon monoxide intrusions. They are also capable of identifying potential issues that may require your attention before they become hazardous. It is your primary responsibility to check that the workflow of the chimney does not get disrupted and the furnace is well-maintained so that it can operate smoothly and generates comfort and warmth to your home-sweet-home.