Chimneys
Purpose
The chimney has two tasks:
- to bring the required amount of air i.e. oxygen into the firebox by creating a draft
- to release the combustion gases into the atmosphere
Chimney physics
Operating principle
When you light a fire, the air inside the chimney heats up and becomes thinner than the outside air, and thus begins to rise. While the warm air rises, the cold air from the premises flows into the firebox and ignites the fire, creating more heat. Pressure differences also occur in this cycle.
Air movement
Although you cannot see it, the air in your house is constantly moving. It tries to get out by moving upwards and thus allows the outside air to enter the lower zone. The trapped warm air in the upper parts of the house creates a pressure which draws it out through the smallest openings. In order to compensate for the lack, at the same time, air enters the lower parts.
Chimney draft
When a chimney is filled with warm air, it ”pulls'' air through the firebox. This effect is called “draft” and is similar to the pressure in water pipes, except that the only difference is that the air pressure is negative and the water pressure is positive. That is why we can call chimney a "negative pressure system".
The most common reasons for poor chimney draft are the following:
- Large amount of combustion deposits on the walls
- Structural damages
- The existence of some foreign object in the chimney
- Influence of wind
- Dirty chimney cap
- Improper construction that disrupts chimney physics
Neutral pressure plane
Due to the air flow, somewhere in your house, the so-called "neutral pressure plane"is created. Above this imaginary plane, the air pressure is slightly positive in relation to the outside air pressure, and therefore the air in the upper zone is trying to get out of the house. Below the plane the air pressure is slightly negative, so the house is trying to "suck in air" inside. The location of the neutral pressure plane is variable due to the conditions change because all the factors which affect the airflow in the house also affect the position of the neutral pressure plane. Whenever the fireboxes devices are below that plane the air will try to enter the house through the chimney or vent.
The best example of that are houses with two fireplaces, one below the other. While the fireplace on the upper floor uses combustion air and flow of the chimney, it thereby reduces the pressure of that level and causes the flow of air from the lower floor upwards. Since the fireplace from the lower floor is below the neutral pressure plane, it pushes air through the chimney to the lower floor. If those two smoke extraction ducts are close to each other, the replacement air may contain smoke from the upper firebox and may pick up unwanted odors.
Influence of wind
When the wind blows into a building it creates high pressure on that side but low pressure on the side down the wind. An open window or door on the gusty side of the wind will increase the pressure of the house and increase the draft in the chimney. Also, opening the openings on the side down the wind will reduce the pressure and increase the possibility of "reverse drafts" of chimneys or vents. The reverse draft draws air into the house through the chimney instead of extracting it from the house. Home appliances, such as tumble dryers, kitchen hoods (aspirators), bathroom fans and other devices that extract more air from the house, reduce the pressure. This can cause negative pressure in the area of the firebox, which complicates the desired draft of the chimney and its normal operation.
If you think your chimney has poor flow or some other problem,
give us a call!
Our professional team will inspect your chimney and suggest corrective action in order to achieve its maximum functionality.
