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These are some simple scientific fundamentals to explain how and why a chimney works. This page should make it easier to understand how we can diagnose smoking or other problems with your chimney.
Your House as a System
Even though you can't see it, the air in your house is in constant motion. In general, airflow tries to flow out of your house in the upper parts and replacement air tries to flow into your house in the lower parts of your house. Thinking of your house as a system makes it easy to understand the reasons for that airflow. Many changing factors, including: stack effect, wind loading, interior mechanical systems, and fuel-burning appliances such as furnaces, fireplaces, wood stoves and water heaters influence a home's airflow.
Homes built or renovated in the past 25 years are more airtight than older homes. This makes it much more difficult for replacement air to enter the home. As the saying goes, “hot air rises”, and so does the warm air in your home.
When the warm air rises to the upper areas of your home, it's called the stack effect. That trapped air forces its way out - even through 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.
Somewhere in your house, amid all this airflow, is the Neutral Pressure Plane (NPP). Above this theoretical plane, the air pressure is slightly greater than the outdoor air pressure and it tries to force its way out of the house. Below the plane, it is slightly negative and the house is trying to draw air in from the outside. The location of the NPP changes in response to changing conditions. The factors that affect airflow in the house also influence the level of the Neutral Pressure Plane.
Anytime a fireplace or fuel-fired heating appliance (except direct vent) is below the Neutral Pressure Plane, air will tend to flow into the house through the chimney or vent. A common example of this is found in homes with two fireplaces, one below the other. As the upper level fireplace uses air for combustion and chimney flow, it depressurizes that level slightly causing air to flow upwards from the lower level. Since the lower level fireplace is below the NPP, it draws air into the basement through the chimney. Unfortunately, since those two flues generally exit the chimney close to each other, the replacement air can contain some smoke from the fireplace above and it can pick up unpleasant chimney odors as it passes down the chimney flue.
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 (leeward) 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 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 difficult for natural draft chimneys to function as intended. Another common mechanical system that removes air from the house is a forced-air furnace. Many such systems are out-of-balance due to leaks in the ducts. Leaky supply ducts force air into
the attic or crawlspace. Leaky return ducts draw air from the basement or other areas they pass through.
Furnaces, water-heaters, fireplaces and woodstoves are examples of fuel-burning appliances that need large volumes of air for combustion. Unless they are specifically equipped to draw air in from outside the house, such as direct vent appliances, operating them can reduce the inside air pressure. There are a variety of mechanical devices on the market that help provide the necessary replacement air to balance the air pressure needs of your house system.
Draft and Flow
Although most people don't realize it, the air moving up your chimney works under the same set of physical principles as water flowing in a hose or pipe. 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, chimneys are negative pressure systems.
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. Given the same amount of pressure, a larger pipe can carry a greater volume of water than a smaller one. The same is true for chimneys - with the same amount of draft (pressure) a larger flue will exhaust more smoke from your fireplace than a smaller one. Similar to how a water hose can be kinked or plugged, the airflow in your chimney can have a restriction that slows down the smoke flowing up the chimney.
Poor flow in a chimney can result from: excessive creosote deposits, closed or plugged dampers, improper construction, structural damage or even a dirty chimney cap. In fact, having a plugged-up chimney cap at the end of your chimney is like having a closed nozzle at the end of a hose - preventing airflow through the chimney. Your Chimney Sweep can check your chimney and recommend any corrective action needed for proper draft and flow.
Wood and Gas CANNOT be vented out of the same flue or liner. Wood burning liners cannot be used with gas appliances and vice versa. If you want to convert from one fuel to another, the liners MUST also be changed to conform to code.
• Contains combustion products
• Directs the combustion products to the outside atmosphere to keep them from
entering your home and causing poisoning or death
• Protects the chimney walls from heat and deterioration
• Addresses incorrect chimney construction
• Corrects safety issues by nature (Cracked flue tiles, shifting, loose or missing mortar joints)
• If a chimney is unlined
• Prevent a transfer of extreme heat to combustible materials to prevent fires or
• Prevents the flue gases to attack masonry structure and shorten it’s life
• Prevents carbon monoxide or smoke to leak into the home
• Prevents possible liner leaks to the combustibles
• If there are missing or misaligned flue tiles
• If there are missing mortar joints (Dry Stacked)
• Chimney fires
• Water damage
• System deterioration caused by acid and flue gases
When you shut a damper you want to keep the warm air in your home from escaping up the flue. The throat damper in most masonry chimneys does a poor job of this with its metal on metal seal. Lock-Top dampers seal tight with a silicone rubber gasket that is 90% more effective than a new throat damper.
Traditional dampers are located in the “throat” of a fireplace. They are made of cast-iron and are housed in a low-tech frame work. These cast-iron dampers do not seal well – they have a leaky, rough, metal-to-metal seal.
Recent lab studies have shown that even a new throat damper can leak up to $200 of a heating and cooling a year. Imagine how much energy an old, rusted out damper leaks!
What you want is the Lock-Top damper:
The Lock-Top Seals heat in like a thermos lid. The Lyemance shuts the door on energy loss. The Lock Top II offers the screen around the damper as an added bonus. The High efficiency dampers seal tightly to save you money.
• High efficiency dampers have a rubber gasket seals tightly like a storm door for your fireplace, and they keep heating and cooling dollars where they should be… In your wallet
• These Dampers mount on top of your chimney to keep out animals and rain
• These Dampers easily operate from inside your firebox and they seal tight every time you close it, eliminating any draft
Most masonry materials are porous and will absorb large amounts of water. Common brick is like a sponge, absorbing water and wicking moisture to the chimney interior. Defective mortar joints or the use of improper mortar or brick can greatly increase the tendency to absorb and convey water to the interior of the masonry chimney.
Several products have been developed specifically for use as waterproofing agents on masonry chimneys. These formulas are 100% vapor permeable, which means that they allow the chimney to breathe. Therefore, water that has penetrated and the vapors produced when the chimney dries out or the water vapors produced during use are allowed to escape, while the waterproofing agent prevents water from entering from the outside. These products usually have a five- to ten-year warranty. Paint or clear sealers should never be used as a waterproofing agent because they will trap water vapors and moisture inside the chimney causing further deterioration.
Waterproofing is a preventive measure. When damage or deterioration (gaps, voids, cracks, missing mortar, etc.) already exists in a masonry structure, the chimney should be repaired before the waterproofing agent is applied. The chimney exterior may also need to be cleaned before the waterproofing material is applied.