fireplace specs

Old 10-18-02, 02:37 PM
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Question fireplace specs

I have a 30 year old fireplace that smokes. I have learned that the fireplace dimensions are critical to preventing a smoking fireplace. My flue is 8x8 and I belive the firebox is oversized for that size flue. How can I obtain firebox specs so I can figure out corrective action. Note, I have already addressed down draft problems
Old 10-19-02, 08:39 AM
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fireplace specs

What are the demenions of the firplace opening. How far is it from the bottom to the damper.
Old 10-19-02, 11:28 AM
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The Smoking Fireplace

Investigating causes and solutions
Backdrafting is one vexing problem sweeps are likely to encounter on a regular basis. Because of the difficulty in determining the cause, it can be one of the most aggravating, both for the sweep and for the homeowner. But bringing the customer into the process may lead you to the cause more quickly, will help you find a solution that fits the customer's needs and budget, and will certainly build goodwill between you and your customer.
Ask and observe
The first step in diagnosing any performance problem is simple: Ask a lot of questions. When a customer calls, ask them the following:
Is the problem constant or intermittent?
Is it getting worse over time?
Does it happen only during starting?
Does it happen only on windy days? Does it happen only during cold weather? Does it happen only during mild weather
Does one fireplace smoke when another is in use?
Does it happen only when a door is open?
It is possible that the homeowner won't know the answers, or that the information they provide may not be completely accurate. If possible (without risking personal safety or property damage) light a fire in the fireplace or appliance and observe the symptoms yourself.
Make a preliminary diagnosis
After gathering all available information from the homeowner and from your own observations, you'll probably be able to make some educated guesses about the cause of the backdrafting problem. It is very important to make note of each solution attempted. Never try more than one solution at a time and never make costly or permanent changes until you are certain your diagnosis is correct. Use the information you have gathered, your experience in similar situations and your knowledge of how chimneys work to determine the category of the problem even if you can't pinpoint the cause.
Check it out
Next, try out some temporary solutions. The simpler solutions, like opening a window or turning off a fan, can be tried by the homeowner before your visit. Others are more complicated, like temporarily extending the height of a chimney or reducing the size of the firebox opening. If the chimney smokes all the time, test for the following problems:
Damper. Don't forget to check the obvious. Make sure the homeowner has the damper open when there's a fire going. Check the damper yourself, it is possible that the damper is in backwards.

Fireplace opening that is too large for the flue/ flue that is too small for the fireplace opening. Hold a piece of sheet metal at the top of the fireplace opening and lower it until the smoking stops. To permanently reduce the size of the opening, install a metal strip across the top. For correct installation, slide the metal down until the smoking ceases, mark the point and measure up to the lintel. The width of the metal sheet should equal this measured distance.

Improper construction. The lintel should be approximately 7 inches below the damper. If the smoke shelf and the lintel are on the same plane, it is more likely that the fireplace will smoke. If the lintel doesn't project below the damper, smoke will flow into the room. Smoke chambers may also be too tall, too deep or too wide. Check NFPA 211 for recommended specifications. Large smoke chambers reduce flow due to the expansion and cooling of gases in the chamber itself: These chambers can also create more turbulence as the gases pass through the damper, resulting in even more cooling.

Insufficient chimney height. Most chimneys require at least 10 feet of flue. Many don't have that. In particularly short chimneys it may be necessary to have a larger flue to make up for the area. That could be aggravated by exposure on all four sides making the chimney likely to remain cooler. The way a fireplace works, flow is often more important than draft. Taller chimneys contain a taller column of warm, rising gases. The movement within the flue of this taller column of gases also increases the pressure difference (or draft) at the bottom of a chimney. The actual height of the chimney is the critical factor, not the volume of gases contained in it. Permanent Solution: Raise the chimney.

Improper termination height. Chimneys should terminate at least three feet above the point they pass through the roof and at least two feet higher than any portion of the building within 10 feet of the chimney. These are minimum standards and may not be adequate. Remember to look at turbulence from surrounding trees and buildings to determine that the chimney is venting into undisturbed atmosphere. Temporarily fit a clay tile to the top of the chimney to see if that alleviates the problem. If it works, you will need to raise the chimney. Don't leave the temporary tile up there-it could fall of and cause personal or property damage.

Weatherization of the house. When draft takes place, air is pulled out of the house. Smoke can't rise up the chimney any faster than air can be drawn into the house to replace it. If air isn't being drawn in quickly enough or in sufficient quantities, smoking can occur. Test by opening a window near the fireplace while the fire is lit. For a more permanent solution, install an outside air supply for the fireplace.

Anything that creates resistance to flow, including obstructions or blockages like leaves and pine needles or creosote buildup. Friction always exists between moving gases and the passageways through which they flow. It is a rule of thumb that the longer the total venting passageway, the greater the resistance will be. Since draft is necessary to overcome flow resistance and to initiate the flow of gases, systems with a large resistance to flow need greater draft to function properly.

If the problem has gradually worsened over time, a creosote blockage might be indicated. If there is a chimney cap, the mesh screen can easily become plugged with creosote. Creosote can also cause resistance to flow because the surface of creosote is rougher than the surface of a flue.

Solutions: Clean the chimney, remove any obstructions and take steps, like installing a chimney cap, to prevent their reoccurrence.

The size of the venting passageways and the amount of resistance offered by obstacles or bends in the system can also affect the flow capacity of the system. Changes in shape or direction of the venting system add extra resistance to gas flow. Chimneys should be constructed as close to vertical as possible.

Wind-induced downdraft problems. In this case, smoke is forced down the chimney because of unusually high pressure at the top of the flue. If smoking occurs only when the wind blows or if smoking is inconsistent and marked by puffing, wind-induced downdrafts are often at fault. This type of downdraft problem is usually caused by the air flow relationship between the house and the chimney termination. Often, the top of the chimney is exposed to air turbulence caused by wind rushing over or around some nearby object or barrier, like a taller building. Air movement hits the top of the chimney in a downward direction and forces smoke down the chimney. This problem is very common when the chimney is located downwind of a barrier (the side situated away from the wind), and it normally occurs when the wind blows from a particular direction at a particular speed.

Similar problems can occur when a chimney is located on the windward side of a barrier (the side from which the wind is blowing). Wind blowing against a barrier can create a high- pressure zone on the windward sides as air piles up while waiting to get around the barrier. When the chimney top is situated in this high pressure zone and the pressure in the zone is greater than the pressure in the house, draft in the chimney is overcome, and smoke flows downward.

The surest solution to chimney performance problems caused by wind-induced downdrafts is to extend the chimney above the turbulence or the high pressure zone. Sometimes a chimney cap will do the trick and sometimes simply cutting back nearby tree branches will work.

There should be no interference from tree branches within 15 feet of the top of the chimney.

A chimney which is located in a zone of turbulence can experience wind-induced downdraft problems.

Temperature Difference. The principle of buoyancy states that warmer, less dense gases rise in the chimney and are replaced by cool, heavier gases. The rising gases create a partial vacuum at the bottom of the chimney and in any appliance connected to the chimney. If there are openings into the venting system-stove air inlets or a fireplace opening-room air starts to flow into the system to relieve the pressure difference. The greater the temperature difference between the gases inside the flue and the air outside the chimney, the greater the draft. The more pressure difference, the more forcefully air is drawn into the system.

If the chimney is too large or situated on an outside cold wall, the flue gases can cool down to the point where they lose their buoyancy. See earlier mention of this problem.

If the fireplace works well when it is very cold outside but smokes when the outside temperature is close to the temperature inside, the easiest solution would be to refrain from operating the fireplace unless there is a difference of 20 degrees between the indoor and outdoor temperatures.

Flow reversals. Resistance to flow is greater than natural draft. This is often the most common performance problem and generally the most difficult to diagnose. Although flow reversals are caused by low or negative pressure at the base of the flue creating a pull or drawing effect down through the flue, inadequate flow capacity contributes to the problem. If the fire gets very low then the reversal might have to do with the velocity of the smoke in the system. Try starting a roaring fire to see if that solves the problem. The more rapidly smoke moves through, the less likely it is to reverse flow.

Check to see if there is an exhaust fan operating somewhere in the house. Attic exhaust fans, bathroom fans, kitchen range hood fans, indoor barbecue exhaust fans, and indoor air purifiers and air-duct booster fans that vent to the outside can cause flow reversals.

If there are leaks or cracks in the upper section of the house, or if an upstairs window is open, warm air could be escaping upstairs faster than it is being replaced downstairs. The resulting lower pressure around the fireplace can cause the smoke flow to reverse. The best solution would be to seal cracks upstairs and to install an outside air supply for the fireplace.

Flow reversals force smoke down a chimney and into the room.
Finding a permanent solution
Once you have identified the cause of the problem with reasonable certainty and have confirmed your diagnosis through testing, you can act on your diagnosis, making sure it is logical and consistent with all observed symptoms.

Ethical chimney sweeps always prescribe the simplest and most cost-effective solutions to chimney performance problems. Since homeowners often assume that chimneys work automatically, they may not appreciate the importance of the problems you have detected.

Therefore, rather than suggesting the most expensive solution, offer the homeowner a range of options and explain the results and advantages of each.

Sometimes chimney performance problems can be controlled by changing the way the homeowner operates the appliance or fireplace. Often, the homeowner is willing to make these changes before committing to a permanent physical change in the system.

Eventually, however, the consumer may find this approach bothersome and hire a sweep to make the necessary physical alterations to the heating/venting system. Regardless of the final solution, consumers always appreciate sweeps who give them the opportunity to try less expensive solutions.

Sound familiar? Most of the information in this article is drawn from Successful Chimney Sweeping, so if you haven't cracked the books for a while remember that the manual can be a valuable reference tool for your business. Additional information was drawn from The Homeowner's Guide To Chimneys, Fireplaces and Woodstoves by Jim Brewer and Ashley Eldridge.

The Smoking Fireplace. National Chimney Sweep Guild. (1996) Retrieved 19 October 2002.
Old 10-21-02, 02:14 PM
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Here is someadditional info: Originally I reported the flue size as 8x8. That is incorrect. The flue size is 8.5x13. Fire box dimensions are as follows: front Width 35.5; height 32.5 at center(arched) depth of box at base is 22.75; backwidth is 28.75; top of arch to smoke shelf is 6.5
Old 10-21-02, 05:47 PM
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Flue size for opening is barely enough.....should have been about 12x12 or better.....

Opening height I would have made 28-30", never built an arched opening, so I don't know how that affects draw....but I suspect in combination with the flue size, is your problem.

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