Positive Pressure Plane

Lack of draft in the fireplace makes sitting in front of the fire unpleasant since smoke cannot properly escape through the chimney. Chimney sweeps get several calls during the winter months asking, “How can I stop the backdraft in my fireplace?” Or, “Why is there a draft in my fireplace?” ”.

Attic doors are often large, poorly sealed holes in the Positive Pressure Plane that provide access to the attic and outside. Given that the attic is the highest point in a house and so has the least positive pressure, its doors are typically located there. They get the most air pressures of any house opening. Even still, these entrances aren’t the best in the house when it comes to weather sealing. Most homeowners don’t want to deal with the hassle of changing the attic access door on a regular basis, but adding weather stripping can assist. When utilized, attic and whole-house fans usually bring in the unpleasant smell of the fireplace. If you have a drafty chimney, you’ll need to heat extra air if it leaks into the attic.

A Vent from a Dryer
The flapper door on a well-installed dryer vent allows for minimum air loss. If, however, the dryer is not located in the basement but rather on a higher floor, the small gravity-powered door may likely gap and lose air even when the dryer is turned off. By venting hundreds of cubic feet of air outdoors whenever you run the dryer, you raise the level of the neutral pressure plane within your home. This creates extra stress on the house and reduces the efficiency of our backdrafting fires. Since air loss below the neutral pressure plane may reduce that plane in the residence, laundry facilities should be located in the basement (rather than on upper levels).

Draft from a Gas Fireplace Chimney
Indoor air is used by any equipment that burns it for combustion and then releases the exhaust outside. The problem is that when there is no heat source to push the vent to draft, exterior air will be drawn into the home through the chimney of any naturally vented appliances.

In other words, the chimney may be used as a straw to either blow or drink through. The same holds true for the requirement of makeup air in the lower floors of the residence. Because of the air loss above the neutral pressure plane, the house will suck air down the chimney flues in the basement to make up for it. This phenomenon is called a “downdraft” in the chimney. Both external factors (such as wind) and internal forces (such as positive/negative air pressure zones) can contribute to downdrafts in a chimney.

What occurs when an inside heating appliance like a stove or fireplace has to vent to the outside air via a chimney? It’s possible that the water heater or fireplace won’t produce enough warm, buoyant exhaust to counteract the backdraft when the chimney is downdraft. This might cause dangerous levels of carbon monoxide to build up in gas appliances.

Back Draft from a Fireplace in a Basement
The same is true for gas fireplaces that backdraft in basements. If a safe chimney design has already been established, then the downdraft condition in the fireplace is most likely the result of the Stack Effect (as discussed above). Assuming, of course, that its placement lies below the plane of neutral pressure.

There is no “check valve” in a chimney to restrict the flow of smoke and exhaust. We can only hope for a damper on the fireplace or stove (only with wood-burning fireplaces or wood-burning stoves). Most damper systems are flimsily constructed, using shoddy materials that lack even the most basic gasket or weather stripping. With the most typical material being cast iron, fireplace dampers are a staple in most homes.

If you suspect that your chimney or fireplace is having some issues, have an expert inspect it. Call Local Chimney Sweep Cleaning today. We also offer Chimney Cleaning Clayton and other affordable services.

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