CFM Corporation 2461 Single Venting, Backpuffing, Fuel, Creosote, Draft Testing, Negative Pressure

Single Venting

Dutchwest

Backpuffing

Your stove requires a dedicated flue. Do not connect the stove to a flue used by any other appliance. Chimney draft is a natural form of energy and follows the path of least resistance. If the stove is vented to a flue that also serves an open fireplace or another appliance, the draft will also pull air in through those avenues. The additional air flow will lower flue temperatures, reduce draft strength and promote creosote development; overall stove performance will suffer. The effect is similar to that of a vacuum cleaner with a hole in the hose. In some extreme instances, the other appliance can even impose a negative draft and result in a dangerous draft reversal.

Fuel

Even the best stove installation will not perform well with poor fuel. If available, always use hardwood that has been air-dried (“seasoned”) 12-18 months. Softwood burns more rapidly than hardwood and has a high pitch content condu- cive to creosote production. Decayed wood of any type has little heat value and should not be used.

All unseasoned (‘green’) wood has a high moisture content. Much of its heat value will be used to evaporate moisture before the wood can burn. This significantly reduces not only the amount of energy available to warm your home, but also the intensity of the fire and temperature of the exhaust gas. Incomplete combustion and cool flue temperatures promote creosote formation and weak draft.

You can judge the moisture content of wood by its appear- ance and weight or use a commercially available moisture meter for an exact measurement. Unseasoned wood will be a third heavier than dry wood. Also, look for cracks (“check- ing”) in the ends of the log that result from contraction as the wood dries. The longer and wider the cracks are, the dryer the wood is.

Purchase your fuel from a reputable dealer.

Creosote

Creosote is a by-product of low-temperature stove op- eration, weak draft, or both. It is a tar that results when unburned gases condense inside the flue system at tem- peratures below 290˚ F. Creosote is volatile and can gen- erate a chimney fire. All of the installation characteristics that adversely affect chimney draft also promote creosote condensation. Consequently, you can minimize creosote accumulation with an effective chimney design and the use of operational techniques that encourage good draft and complete combustion.

Inspect your chimney frequently and clean it whenever accu- mulation is exceeds 1/4”.

Backpuffing is a condition that results when the draft is too weak to pull flue gases out of the chimney system as fast as the fire generates them. Volatile gases build up within the firebox until reaching a density and temperature at which they ignite. With this ignition, you may hear a muffled pop- ping sound and see a bit of smoke forced out of the stove.

This condition is most likely to occur in the Spring or Fall when moderate outdoor temperatures and low intensity fires produce weak draft. If your stove back-puffs, open up the damper to let the smoke rise to the flue more quickly. Also, open the air inlets to induce a livelier fire and speed airflow through the stove. Avoid large loads of firewood at one time. You should always see lively, dancing flames in the firebox; a lazy, smoky fire is inefficient and will promote draft prob- lems.

Draft Testing

An easy way to determine whether your chimney draft is strong enough is to close the stove damper, wait a few minutes to let the airflow stabilize, and then test whether you can vary the strength of the fire by swinging the air control open and closed. Results here are not instant; you may need to wait a few minutes for a change in the air control setting to have an effect on the fire. If there is no change, the draft is not yet strong enough to let you close the damper. You will need to open it for a while longer and manage the fire with the air inlet until the draft strengthens. Keep a record of your operating habits and relate them to their effects on the stove’s function. You’ll be rewarded with safe and efficient performance.

Negative Pressure

Good draft also depends on a sufficient supply of air to the stove. The chimney can’t pull in more air than is available to it. Sluggish draft can be caused by a house that is tight enough to prevent the ready flow of air to the stove, or by competition between the stove and other appliances that vent indoor air to the outside; i.e., exhaust fans for range hoods, clothes dryers, bathroom fans, etc. If the chimney draws well when all such equipment is turned off (or sealed, in the case of fireplaces and/or other stoves), you need

to be attentive in timing the use of the other appliances. If you need to crack a nearby window or door to enable the chimney to pull well, you should install an outside-air intake to bring combustion air into the room.

Conclusion

Woodburning is more an art than a science. Art includes technique; and since installations, homes, and fuel vary, the stove operator must also vary technique, (mostly timing), to achieve satisfying results. Over time, you will become famil- iar with the features of your particular installation; you will be able to identify cause and effect in a variety of seasonal circumstances, and adapt your operating habits to changing conditions.

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