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Home , Building envelope, Chimney

Best Practices for Woodburning Installation

Prevent smoking, drafts and odors from installed in new

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ntroduction not only in lost time but in the repu- lems are difficult and expensive to cor- The ideal wood-burning fireplace is tation of everyone concerned. rect after the fireplace is installed, so a pleasure to use. It doesn’t Over the years the fireplace indus- the installation design stage is critical I when lit or spill cold air and odors try has spent a lot of time and money to success. when not in use; it doesn’t backdraft investigating problems and working to Although installations that meet all when the is on, and it works improve fireplace performance. We now of these best practices are ideal and are well regardless of wind speed or direc- know how to prevent problems through most likely to give trouble-free perfor- tion. For obvious reasons, everyone effective installation design. This paper mance, sometimes designs or involved in putting fireplaces into provides a concise overview of the char- client objectives make compromises , including manufacturers, archi- acteristics of good design. But before necessary. Where one aspect of best tects, builders and installation contrac- getting to the details of best practices tors, want every fireplace to give for integrating fireplaces into today’s Fig. 1 The system illustrated pleasure and never frustrate the - houses, there is one essential fact you above would not operate reliably owner. But sometimes fireplaces don’t need to know: because it has many of the features that work well and the results are costly, The most common fireplace prob- to problems. 1

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Fig. 2 This system will function reliably because it has the characteristics that promote successful venting. practice cannot be met, fireplace per- odors and cold air when not in use and formance might not suffer noticeably. 1. Install fireplaces and smoke when a fire is lit. A fireplace However, installations with several char- chimneys inside the depends on the buoyancy of its hot acteristics known to cause problems are envelope. exhaust to draw in air and much more likely to create user dis- create flow up the chimney. But when satisfaction because of smoking and 2. Penetrate the building the chimney runs up the outside of the backdrafting. Therefore, while com- envelope at or near its house, its ability to resist the negative plying with all these best practices is highest level. pressure in the house due to a good target to aim for in all instal- is weakened (see discussion of stack lations, some compromise will not nec- 3. Avoid large, uncompensated effect below). essarily result in poor performance. exhausts. Draft, which is the pressure differ- These best practices can be expected ence needed to vent fireplaces success- to produce good performance in most 4. Avoid very short chimney fully, is influenced by the temperature fireplaces. However, the installation systems. of the exhaust and the height of the instructions provided by the fireplace chimney. That is, the hotter the exhaust manufacturer take precedence, so they 5. Use straight chimney systems. gases, the stronger the draft. And the should be followed if they differ from taller the chimney (at a given temper- the recommendations made here. 6. Provide glass . ature), the stronger the draft. In well- Venting failure is usually caused by designed systems there tends to be a the location of the fireplace and its low level of draft present during the chimney in the house and the way Locate Fireplaces and heating season even when the fireplace other equipment affects fireplace oper- Chimneys Inside Houses is not in use. This “standby draft” is ation. For this reason the best time to A common installation practice involves produced because the tempera- prevent smoke spillage from fireplaces locating the fireplace and its chimney ture air in the chimney is warmer than is during the planning stage as the fire- in a chase projecting outside the build- the outdoor air, so it rises. Without place system is integrated with the ing envelope to conserve interior1 standby draft, chimney flow can reverse, house design. Following, and in Fig- space. Despite its popularity, the prac- spilling foul odors and cold air from ures 1 and 2, is a summary of the ele- tice of locating fireplaces in outside an unused fireplace. ments of best practice that are discussed chases is one of the main causes of Under standby conditions the air in in this paper. complaints about fireplaces that spill a fireplace and chimney installed in an Chimney offsets can be impli- 5 cated in smoke Penetrate the Top of the should be carefully sealed to the build- spillage. Building Envelope ing . Keeping a chimney at room tempera- To avoid these problems, a chim- ture during standby conditions deals ney should penetrate the highest part with one of the main 2influences on of the building envelope. For most com- mon house designs, this means that the fireplace and chimney should be located on an interior rather than a perimeter wall. outside chase tends to cool grad- To produce enough Fireplaces and chimneys that ually to below room tempera- are located on perimeter ture, even if the chase is sealed standby draft, a chim- (or outside perimeter walls in and insulated. As the chimney ney must not only be chases) are more vulnerable to temperature falls, draft declines the adverse effects of wind along with it. The loss of standby as warm as the because their chimneys tend to draft is worsened by the fact that house, but it must be low relative to lines. The negative pressure inside the house general strategy for preventing can work against upward flow also be as tall as the cold backdrafts under standby in the chimney. The three most highest part of the conditions is to locate fireplaces common causes of negative pres- and their chimneys centrally in sure in houses are stack effect, building envelope in rather than against exhaust fan effects and air cir- order to overcome the perimeter walls. The same culation system imbalances. One approach is effective in reduc- or more of these sources are negative pressure due ing the potentially adverse effects found in all houses to varying of wind by placing the top of degrees. to stack effect. the chimney above areas of wind turbulence. Stack Effect Even in cases where venting Just as warm air in a chimney problems are assumed to be produces draft, the air in the wind-related, the replacement of house is warm relative to the chimney draft. The other factor is height the standard chimney cap with outdoor air so it tends to rise, produc- and it is just as important. To produce one of specialized design should only ing a pressure difference called stack enough standby draft, a chimney must be done with the approval of the fire- effect. Stack effect creates negative pres- not only be as warm as the house, but place manufacturer. The standard chim- sure in the lower levels of the house, it must also be as tall as the highest ney caps supplied by manufacturers of where fireplaces are normally installed. part of the building envelope in order factory-built fireplaces and chimneys The negative pressure due to stack effect to overcome the negative pressure due are usually of good design, so replac- is enough to force a cold backdraft in to stack effect. ing standard caps supplied with these a fireplace chimney that is cooler than A chimney installed in a single-story fireplaces is not necessary. In fact, the . If one attempts to section of a two-story house can suf- supplied cap should never be replaced light a fire in a backdrafting fireplace, fer cold backdrafting at standby (Fig. or left off for aesthetic or other reasons smoke will flow into the room instead 1). This is because the chimney may of up the chimney. The negative pres- be shorter than the warm part of the Location Recommended Marginal Not sure due to stack effect experienced by building and in addition, cooling of the Location Location Recommended the fireplace is strengthened if the house chimney’s exposed section reduces its has excessive leakage at high levels average temperature. A chimney (see below). The cure for this form of installed low on the eaves of a house venting failure is straightforward: Install with a vaulted can backdraft for chimneys through the warm space the same reasons. enclosed by the building envelope. The problems resulting from a chim- A chimney installed inside the house ney being installed in an outside chase will produce more standby draft than or penetrating the building envelope at the house produces in stack effect. This or below its top are made far worse if is because the air inside the chimney the majority of leaks in the building is at room temperature, but the chim- envelope are concentrated at high lev- ney is taller than the warm space of els. High level leakage can be created the house. The negative pressure due by recessed light fixtures, ceiling fan to stack effect in the house is never mounts, poorly fitted hatches, fold- strong enough to overcome standby draft up and skylights. High level leak- in an interior chimney (subject to the age sites should be minimized if pos- Chimney should penetrate the envelope effect discussed below). sible and, where unavoidable, they at or near its highest point. because the result could be venting fail- being aware that it is operating. special attention to the other aspects ure due to adverse winds. Decorative Pressure imbalances in houses can of system design. Manufacturer’s instruc- shrouds for chimney tops should never also be created by design problems with tions for factory-built fireplaces should be used unless listed by the fireplace forced-air heating and cooling systems. be reviewed carefully for criteria asso- manufacturer. Leaking ducts and air handlers located ciated with the use of offsets.

Avoid Large, Provide Glass Doors Uncompensated Open fireplaces are particularly Exhausts vulnerable to smoke spillage for When a large exhaust, such as Spillage resistance two reasons. First, the main a downdraft kitchen barbecue restriction to flow – the chim- fan, is turned on in a modern is incr eased signifi- ney – is downstream of the fire, tightly constructed house, the cantly if the main flow and there is little6 resistance to pressure inside 3can drop below spillage flow into the room. Sec- atmospheric pressure outside. restriction is on the ond, most new homes do not This negative pressure competes room side of the fire provide enough natural leakage with the upward flow in a chim- to supply the large amount of ney produced by draft and can in the for m of glass air that an open fireplace con- result in the spillage of smoke doors sumes. Spillage resistance is into the house. Once they are increased significantly if the aware of it, most homeowners main flow restriction is on the can manage this potential prob- room side of the fire in the form lem, but it can be alarming of glass doors, which also reduce when first experienced. outside the building envelope in the amount of air that the fireplace needs One way to prevent this is to avoid or crawl spaces are common causes of for satisfactory operation. the use of large exhaust fans in houses negative pressure problems. Unbalanced Many homeowners enjoy the crackle with wood-burning equipment. Another supply and return airflows can also of an open fire and some fireplaces depressurize the space where a fireplace in some houses can operate success- is located, leading to spillage of smoke fully this way. But changing condi- Why ducted outdoor and cold air. tions, like adverse winds, the operation combustion air is not in of an exhaust fan, unbalanced airflows this list of best practices. Avoid Very Short from one room to another, or even A ducted supply of combustion Chimney Systems people walking in front of the fire- air from outdoors has been At a given temperature difference, a place, can induce spillage from the promoted – and even made taller chimney will produce more draft open hearth. See-through or multi- mandatory in some building than a shorter chimney. While factory- sided open fireplaces are particularly codes – as a solution to spillage built fireplaces are normally approved vulnerable to these pressure differ- problems from fireplaces. for minimum system heights4 in the 14- ences and airflows. Although there is anecdotal to 15-foot range, taller systems than If smoke spillage occurs, the quick- evidence of effectiveness in this are preferable for good performance, est and most effective way to stop the some situations, the available particularly for fireplaces with gener- spillage is to close the glass doors. Glass research shows that outdoor ous hearth openings relative to chim- doors can also help to reduce air leak- air supplies do not reliably ney diameter. The larger the fireplace age up the chimney and hearth odors prevent smoke spillage. Where opening, the more air must pass through from spilling into the room when the required by regulation, these it to prevent spillage. For fireplaces fireplace is not in use. For these rea- outdoor air supplies must be with large openings or with more than sons all fireplaces should be provided installed, but they should not be one open side, such as see-through or with glass doors, even if the purchaser relied upon as a primary mea- corner fireplaces, a system height of expects to use the fireplace as an open sure to ensure good fireplace at least 20 feet is preferable. Note that hearth most of the time. performance. the installation instructions for listed factory-built fireplaces provide chim- For more information about the Best ney height limitations and these should Practices recommended in this docu- way is to compensate for the volume be followed precisely. ment, including technical references and of air exhausted with a roughly equal alternate design strategies, please visit amount of make-up air brought in from Use Straight Chimney Systems the HPBA Web site at www.hpba.org outdoors. A make-up air fan can be elec- Chimney offsets should be avoided where trically interlocked with the exhaust fan possible. Each elbow or offset in a vent- This article is a joint effort of Hearth switch so that house depressurization is ing system creates turbulence in the & Home Magazine, and the Technical prevented. Make-up air systems of this gas stream and therefore presents resis- Committee of the Hearth, and type can function without the home- tance to flow. Offsets in5 chimneys are Barbecue Association. Text created by owner having to take action or even not uncommon but their use demands John Gulland.