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why stoves overfire from too much draft
Read about why non-cat stoves are subject to over firing:
This article first appeared in 2006 as a two-part feature in WETT Ink, the newsletter of Wood Energy Technology Transfer Inc.
The Florida Bungalow Syndrome
Dealing with Uncontrollable Non-cats
By John Gulland
The owners of the new EPA certified non-catalytic stove say they cant control the fire and you are called in to diagnose the problem. It is a beautiful installation with the flue pipe running straight up to the base of a factory-built chimney that is routed up through the second story and the steeply-pitched roof. The owners say that even when they turn the air control off, the stove continues to fire too hard and they cant get it to burn for more than four hours. You check for leaks, but the door and glass gaskets and ash pan seals are fine. So why is the stove over-firing?
The story begins over twenty years ago in the western United States. In the mid-1980s, the U.S. Environmental Protection Agency was under pressure from the courts to meet its obligations under the clean air act and it turned its sights on wood stoves. The Agency began to develop an emissions test procedure based on one created by the State of Oregon a couple of years earlier. The laboratory test involved a total system height of 4.6 metres (15 feet) from the floor the stove sits on to the top of the chimney.
If all wood stove users had very short chimneys and only burned wood in warm summer weather, then stoves would behave in the real world much like they do in the lab. But few People who Heat with Wood live in bungalows in Florida.
What happens when we take that EPA certified non-cat stove and connect it to a tall, straight venting system like the one you were called in to diagnose?
This raises an interesting question. If draft level is so important to stove behaviour which everyone acknowledges then why is it assumed that a stove that is optimized for low draft conditions (with a fixed minimum air control leak) will work properly under high draft conditions? It would appear, judging from the absence of discussion and debate on the subject, that few if any People who worked on the development of EPAs emissions regulation were aware of the relationship between temperature difference, system height and draft, and its consequences for stove operation.
The first step in refining a standard after several years of use is to recognize that there are aspects that need correcting and the second step is to reach a consensus on changes that will solve the problem. In this case, however, a simple revision will not be possible.rt chimney and very high outdoor temperature.
Now, lets review each of the four possible solutions.
It has been suggested that a restrictor plate with an opening of perhaps four or five inches be installed under the chimney cap to reduce draft. Such a restrictor would work by reducing the flow capacity of the chimney, slowing the rate of flue gas flow and providing more residence time for the flue gases to give up their heat to the chimney structure, leading to a lower average flue gas temperature and lower draft. The restrictor might resolve appliance over-firing, but could create the more serious problem of smoke roll-out when the loading door is opened for refuelling. That is because when you open the stove door to load it, the flue gas flow rate doubles or more as room air rushes in and up the chimney. It is under these conditions that you need the flow capacity of a six inch chimney to fully vent the exhaust, especially with EPA non-cats which have restrictive firebox exits and need a lot of draft to prevent smoke roll-out. The restrictor plate solution is probably not a good one because it tampers with the chimney structure, which is a certified device and isnt really the cause of the problem in the first place. Not to mention that it creates the likelihood of smoke spillage indoors because it reduces chimney venting capacity.
The second option in our list is to install a key Damper in the flue pipe. This is by far the most popular solution because it is cheap, easy to install and it works. Unlike the restrictor plate method, the key Damper is only used when high draft conditions exist, so it can be more selective in its effect. However, even when a key Damper is open, it represents a significant restriction in the flue pipe, especially after it collects a coating of deposits. In a six inch flue pipe, a key Damper could effectively block 15 to 20 percent of the flow area. In very tall systems, high draft might be able to overcome this resistance, but in marginal situations, such as that of a 25 foot system operating in mild fall weather, for example, a key Damper could contribute to smoke roll-out when the door is opened for loading. Furthermore, in a study done for Canada Mortgage and Housing Corporation some years ago, it was shown that appliances with key dampers or by-pass dampers are more likely to be implicated in open door smoke spillage, if only because users either forget to open the Damper first, or because the Damper doesnt fully open. Flue pipe key dampers are a throwback to earlier and less appealing forms of wood heating and in an ideal world they would be relegated to the history books.
Restricting chimney draft downstream of the appliance can lead to smoke spillage.
The third potential solution is to install a barometric draft control in the flue pipe. Although a barometric control could work, it would also vastly increase the amount of air consumed by the system. The higher the draft, the more the control swings open and the more room air would flow into the venting system. These effects would reach their maximum during the coldest weather when draft is highest. That would represent a considerable energy efficiency penalty and could create depressurization problems in a well-sealed new house. Also, the barometric control could be the easiest path for smoke spillage into the room in a house with negative pressure problems.
The oil and gas industries have worked to eliminate dilution devices like barometric controls from their equipment because of the energy penalty and operational problems. The wood heating industry should probably do the same. In fact, the CSA B365 technical committee has already moved in that direction. A new clause proposed for the next edition of the code reads: A Barometric Damper shall not be installed in the flue vent of an appliance unless the appliance has been certified for use with a barometric Damper."
A barometric draft control admits air to the flue pipe assembly to bleed off draft pressure.
The final option is to modify the appliance air control so it can admit less air. Since there is so much variation in stove air inlets and control design, it is not possible to specify exactly how this should be done. In the case of some stoves with fixed secondary air inlets, aluminium tape has been used to block part of the opening. In stoves with a single control for both primary and secondary air, the air lever stop may be changed to allow the Damper to close more.
This air control was adjusted by bending the fixed stop.
The first step in considering this method is to understand exactly where the air enters the stove and how its flow is controlled. This option tends to be controversial, even though it is a precise response to the cause of the problem, which is an excessively leaky stove. Concern has been expressed that this option involves tampering with the pollution control features of the appliance. But it could equally be argued that any blockage of or opening into the venting system, as implied by the three other options, represents similar tampering because the use of such devices is not permitted during emissions testing. Provided it is done carefully and in full knowledge of the consequences, this method does not create other problems, such as an increased likelihood of smoke spillage.
None of the optional solutions is without drawbacks. The result is that whatever option the dealer or installer chooses, he or she could be open to criticism. Here are a few recommendations that might help to resolve this problem over a period of time.
The EPA and the committee responsible for the CSA B415 standard (which has the same problems) should acknowledge that when stoves are optimized to operate under test conditions, they do not perform as intended when connected to tall chimneys and in cold climates.
When designing new stoves or reworking existing models, Manufacturers should develop air controls with a means of adjustment to deal with this problem. Such adjustments should not be readily accessible to the stove user.
Stove Manufacturers and regulators responsible for wood stove emissions should consider it acceptable for a WETT certified technician (and the U.S. equivalent) to make air control adjustments when they encounter short burn times with stoves connected to tall, straight venting systems.
This is a complex issue that has not been widely discussed. As a result, I have expressed my own views here with the intention of igniting a dialogue on the subject. I look forward to hearing any comments you might have.;why_stoves_overfire_from_too_much_draft
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