How Chimneys Work
The main function of a chimney is to carry combustion products such as gases and smoke from the appliances towards the outer atmosphere of your house. At the same time, it draws air for combustion in the appliance. The movement of combustion air and exhaust is known as "draft".
In short, it is the difference in the amount of pressure between gas and air in the chimney and the external air that develops the movement. Lighter and warmer gases in the flue move upwards.
You will need a high column of warm air in the chimney and cool air outside. Warm air will rise and draw out the exhaust from the appliance. As the air exits the chimney, fresh air is drawn in the appliance for combustion.
The draft of a chimney is affected by pressure conditions in the building or the house because basically draft is a measure of pressure. There are various factors that affect the draft.
There should sufficient amount of air movement in the house to make way for the air to exit through the chimney. In case the house is compactly insulated the amount of air drawn up the flue will surpass the amount of air entering the house and gradually the building or house will become depressurized. If the pressure in the house or building is lower than outside, there will be a tendency for the air to make way inside the building or house from all possible openings.
The movement of air in the house should not obstruct with the chimney. If the upstairs windows of a house are open, warm air inside will exit from the windows. As a result, the entire building or house becomes just like a huge chimney. Just as the air flows out from the upstairs window, fresh air enters from downstairs to replace it. This process is known as "The Stack Effect" because the house or building acts like a chimney or stack.
The open windows upstairs are only one root cause. A badly-insulated roof, a drafty attic, a tall stairwell or anything which allows a sufficient amount of hot air to exit the house can create "The Stack Effect" problem.
There should not be extreme competition with the other appliances in the house or building such as the air-exchange systems or exhaust fans. If another factor is causing the air to exit, the chimney may not be strong enough to overcome it and the exhaust might have to be drawn in the house from the chimney.
A chimney should be designed to adjust the type and volume of exhaust that is to be emitted by the appliances it serves. It involves accurate sizing, proper construction, satisfactory height and use of suitable building materials.
In short, it is the difference in the amount of pressure between gas and air in the chimney and the external air that develops the movement. Lighter and warmer gases in the flue move upwards.
You will need a high column of warm air in the chimney and cool air outside. Warm air will rise and draw out the exhaust from the appliance. As the air exits the chimney, fresh air is drawn in the appliance for combustion.
Factors affecting the draft
The draft of a chimney is affected by pressure conditions in the building or the house because basically draft is a measure of pressure. There are various factors that affect the draft.
1. Adequate amount of air
There should sufficient amount of air movement in the house to make way for the air to exit through the chimney. In case the house is compactly insulated the amount of air drawn up the flue will surpass the amount of air entering the house and gradually the building or house will become depressurized. If the pressure in the house or building is lower than outside, there will be a tendency for the air to make way inside the building or house from all possible openings.
2. Movement of air in the house
The movement of air in the house should not obstruct with the chimney. If the upstairs windows of a house are open, warm air inside will exit from the windows. As a result, the entire building or house becomes just like a huge chimney. Just as the air flows out from the upstairs window, fresh air enters from downstairs to replace it. This process is known as "The Stack Effect" because the house or building acts like a chimney or stack.
The open windows upstairs are only one root cause. A badly-insulated roof, a drafty attic, a tall stairwell or anything which allows a sufficient amount of hot air to exit the house can create "The Stack Effect" problem.
3. Competition for available sale
There should not be extreme competition with the other appliances in the house or building such as the air-exchange systems or exhaust fans. If another factor is causing the air to exit, the chimney may not be strong enough to overcome it and the exhaust might have to be drawn in the house from the chimney.
4. Appropriate chimney design
A chimney should be designed to adjust the type and volume of exhaust that is to be emitted by the appliances it serves. It involves accurate sizing, proper construction, satisfactory height and use of suitable building materials.
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