The Hearth in the House as a System
The human factor
Unlike automated hearth and heating systems using oil, gas or electricity, woodburning systems demand regular input by the user. There is a wide variation in skill and knowledge among operators of woodburning devices. Survey research has shown that a skilled operator could avoid spillage from a fireplace or heater, even if the system had several adverse design characteristics. On the other hand, some users could induce spillage from systems of good design by using inappropriate operating techniques.
Open door spillage
The most common spillage incident occurs when the appliance loading door is opened for stoking, a form of spillage referred to as smoke roll-out. Roll-out can be induced in virtually any woodburning system by opening the loading door quickly, particularly before draft has been fully established or during a low fire. Failure to open a flue pipe key damper or an appliance bypass damper before opening the loading door are also common operating errors leading to smoke roll-out. Although smoke roll-out is common when the appliance door is opened for stoking, it is the least dangerous form of spillage because it occurs when the user is present to take corrective action by closing the door.
Smoke roll-out from an open fireplace is more serious since there are no doors to close in order to stop the spillage. The problem of smoke roll-out from an open masonry fireplace can sometimes be reduced by changing its internal shape (see Combustion spillage from open fireplaces).
Closed door spillage
Closed door spillage is of greater concern because it can occur when there is no one nearby to take remedial action, such as over night when people are sleeping. Spillage or backdrafting induced by severe house depressurization when a large exhaust system is turned on can usually be corrected by turning off the exhaust system. Many houses have exhaust devices that can depressurize the space enough to cause backdrafting of natural draft combustion systems, but it appears that most homeowners figure out the problem on their own and manage their fans so that backdrafting is avoided. This result was not predicted by housing technologists who thought such houses would full of smoke much of the time.
Spillage that is not associated with room depressurization but occurs with the appliance door(s) closed is associated with a combination of inadequate temperature difference (low draft), appliance design, and adverse pressures caused by stack effect or wind effects. Although users cannot control these adverse pressures, they can have some control over temperature difference. The following actual cases of closed door spillage illustrate the mechanisms of user input.
The owner of a new side-draft wood stove with an internal bypass damper was unfamiliar with the operation of the combustion air control which included a bimetallic coil to moderate the firing rate. The stove had a rear exit flue collar and a very short flue pipe assembly directly entering the breech of an inside masonry chimney having a nominal 8" x 12" flue tile. The user complained of a slight but chronic smell of wood smoke in the house. Diagnosis revealed that, although the user's firewood was fine and his kindling and stoking techniques were good, he was setting the air control far too close to the closed position in the attempt to achieve an overnight burn. The result was that the fire starved for air and combustion collapsed to a smolder. The chimney was not receiving enough heat to produce reasonable draft, the appliance was acting as its own short stack, and a small amount of smoke leaked steadily from a gasketed section at the top of the stove. The remedial measures were simple: the user was informed about the bimetallic mechanism and its proper use, and the combustion air control was adjusted to make its range of movement more consistent with what the user should expect in terms of firing rate. The spillage was eliminated and the smell of smoke did not return.
In the second example, a 30 year old cooking range was used for both cooking and supplementary heating in a rural home. It had a key damper in the flue pipe leading to an outside masonry chimney with a nominal 8" x 12" flue. The users were long-time woodburners who had experienced brief episodes of spillage of the type that is common with older cook stoves. However, one night the system suffered a full backdraft that filled the house with smoke as the family slept. Although there were no injuries and no damage, the family was frightened by the experience. The users had largely diagnosed the problem by the time professionals became involved. They reported that it had been a particularly cold night, that larger than normal pieces of firewood had been used and that the flue pipe key damper had been closed more than normal in order to achieve an overnight burn. The actual mechanism of the failure was probably as follows: combustion collapsed to a smolder due to the combination of large wood pieces and the low draft caused by the key damper setting. The chimney was not receiving much heat and was being further cooled by cold weather. Smoke began to spill from the leaky cooking surface of the stove, diverting some of the combustion heat to the house rather than to the chimney, and a full backdraft followed shortly thereafter. As a result of this episode, the householders resolved to be more careful in the use of the flue pipe key damper and to use other means for over night heating until they could afford to have a new chimney installed inside the house envelope.
Both of these examples of closed door spillage have four key factors in common. First, in both cases the fire was in an extreme smolder mode, producing a lot of smoke and very little heat. Second, both systems had a damper between the combustion chamber and chimney which was used to route the exhaust through relatively restrictive internal passages. Third, the internal passages routed the exhaust downward before exiting the rear of the appliance. And fourth, user input was a significant factor in the mechanism of spillage in both cases.
In neither case were the effects of adverse wind directly implicated in the spillage, although it is easy to imagine how it could come into play in similar cases.
While the internal characteristics of the two appliances were implicated in these episodes of closed-door spillage, such designs should not necessarily be avoided. A venting system with good (driving) characteristics, combined with an informed user can easily overcome the somewhat adverse characteristics of such appliances.
Influence of operator skill
Since temperature difference is the most important ingredient in successful chimney venting, the practices of the user in building and maintaining wood fires is a critical factor. Users who permit wood combustion to collapse into a smolder are far more likely to experience both open and closed door spillage than those who use appropriate techniques.
Since temperature difference is the most important ingredient in successful chimney venting, the practices of the user in building and maintaining wood fires is a critical factor. Users who permit wood combustion to collapse into a smolder are far more likely to experience both open and closed door spillage than those who use appropriate techniques. It has also been shown that users of woodburning appliances that have glass doors with "air-wash" systems learn proper techniques more readily because they can see the effects of their input. Users who are informed that the door glass should stay clear and that if it doesn't, something is wrong, are far more likely to avoid smoldering than users of appliances with solid doors. This visual feedback mechanism is an unexpected but significant benefit of glass air wash systems.
Another beneficial coincidence is that the appliance features and operating procedures that are needed to reduce outdoor air emissions are precisely the same as those that will minimize indoor air pollution resulting from combustion spillage. These include internal characteristics that promote stable combustion, properly seasoned and sized firewood, firing practices that produce quick ignition when a fire is started, and air control settings that will sustain flaming combustion or catalytic combustion, (depending on appliance design) until the coal bed phase.
Tolerance of wood smoke in the house
The amount of wood smoke permitted to spill into a house is governed to some degree by the tolerance of the householders to this form of indoor air pollution. It is apparent that some householders accept the smell of wood smoke as a by-product of using a stove or fireplace, even associating "that nice woodsy smell" with the pleasures of woodburning. Others are intolerant to the slightest evidence of spillage. Part of the solution to the problem of combustion spillage from woodburning systems is to inform users that the smell of wood smoke in the house is not normal and should not be tolerated.
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