Chimney Genie believes that a knowledgeable consumer is our best client. The information below is a summary of the parts and function of the typical chimney. All chimneys are designed to exhaust waste gases or smoke and they normally have all the the same basic parts. The section on drafts (below) explains how the chimney works based on the “stack effect”. Many times this basic information will he
lp you understand why your chimney may have an issue. The chimneys we service in Nassau, Suffolk, and all of Long Island have very similar and consistent types of problems and usage. This is simply because they all experience the same basic usage and weather conditions on Long Island. Parts and Function of Your Chimney
Starting at the top of the chimney, we have the cowl, which is meant to prevent birds and other animals from nesting in the chimney. They often feature a rain guard to prevent rain or snow from going down the chimney. A metal wire mesh is often used as a spark arrestor to minimize burning debris from rising out of the chimney and making it onto the roof. The chimney cowl can also function as a wind directional cap that rotates to align with the wind and prevent a backdraft of smoke and wind back down the chimney. Next down the chimney you will find the damper, which is a metal plate that can be positioned to close off the chimney when not in use and prevent outside air from entering the interior space, and can be opened to permit hot gases to exhaust when a fire is burning. A top damper or cap damper is a metal spring door placed at the top of the chimney with a long metal chain that allows one to open and close the damper from the fireplace. A throat damper is a metal plate at the base of the chimney, just above the firebox, that can be opened and closed by a lever, gear, or chain to seal off the fireplace from the chimney. How Your Chimney Works:
At its basic level, a chimney is a structure that provides for the ventilation of smoke or waste gases from a boiler, stove, furnace or fireplace to the outside. Many of the residential chimneys on Long Island will look the same. They are as near as possible to vertical, to ensure that the gases flow smoothly, and air can be drawn into the combustion area in what is known as the stack, or chimney effect. Since hot air rises, and so does the warm air in your home, we find that the warmer air moves to the top of room, building or structure. The height of a chimney influences its ability to transfer flue gases to the outside air. We find that trapped air creates a pressurized area and forces its way to the outside – through even very small openings such as recessed light fixtures and window frames. At the same time replacement air is trying to enter in the lower part of the building to make up for the escaping air. Ultimately, stack effect is the movement of air into and out of buildings, or chimneys, resulting from air buoyancy. This Buoyancy occurs due to a difference in indoor-to-outdoor air density caused by temperature and moisture differences. All this finally results in either a positive or negative buoyancy force that keeps the process going and the greater the temperature difference and the height of the structure, the greater the buoyancy force, and the resulting stack effect. The Chimney in Action:
As combustion gases inside the chimneys are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. The taller the stack, the more draught or draft is created. However, there can be cases of diminishing returns… for example if a stack is overly tall in relation to the heat being sent out of the stack, the flue gases may cool before reaching the top of the chimney. This condition can result in poor drafting, and in the case of wood burning appliances, the cooling of the gases before emission can cause creosote to condense near the top of the chimney. The creosote can restrict the exit of flue gases and may pose a fire hazard. Designing chimneys and stacks to provide the correct amount of natural draft involves a number of design factors, many of which require iterative trial-and-error methods. Chimney Genie has the experience to recognize these problems and make the necessary adjustments to your chimney. In addition, depending on the type of furnace you use, regular cleaning is important to minimize the creosote build up on the inside lining of the chimney surface. Each year there are many fires in homes due to poor maintenance. To learn more you can review at the chimney institute for Safety Website. Wind Loading, Internal Pressures, & Chimney Draft
Wind-loading is the effect on interior house pressures caused by the wind. When wind strikes a building, it creates high pressure on the side that it hits and low pressure on the downwind side. Any open windows or doors on the windward side will help to pressurize the house, increasing chimney draft. However, openings on the downwind side will depressurize the house and increase the likeliness of backdrafting from chimneys or vents. Backdrafting is a reversal of the airflow in which the smoke is coming into the house instead of going up the chimney. Interior mechanical devices and heaters such as clothes dryers, kitchen fans, bathroom fans, attic fans and central vacuums can also create depressurization by removing large volumes of air from the house. The result is often negative pressure in the area of a fireplace, woodstove, or other fuel-fired heating appliance making it increasingly difficult for natural draft chimneys to function as intended. When a fireplace chimney is full of hot air, it actually pulls air through the firebox. This pulling effect is called draft and it corresponds to the amount of pressure in a water hose – the only difference is that the air pressure is negative and the water pressure is positive (think of using a straw to drink with instead of to blow bubbles). Thus, a chimney is called a negative pressure system. Increasing the draft in your chimney is like opening the faucet wider on the hose. The simplest way to increase the draft in your chimney is to burn the fire hotter – hotter air is lighter, so it has more pull. Another way to get more draft is to increase the height of your chimney – except when the chimney is already so tall that frictional forces negate the effect of the extra height.
