The function of a draft diverter
A draft diverter, or draft bonnet, plays a vital role in the operation of a combustion chamber, such as a fireplace or furnace. The draft diverter, which consists of a baffle plate positioned directly above the primary flue, regulates the direction and amount of air flowing in and out of the burner.
Though simple in construct, draft diverters ensure the proper function of a combustion chamber in more ways than one.
Combustion cannot proceed properly if a burner has a substantial lack or excess of oxygen. An incorrect air to fuel ratio can result in low combustion efficiency and even constitute a safety hazard in the form of carbon monoxide build-up and flame instability. A draft diverter ensures optimum burner performance by producing a nearly constant draft and a stable air supply regardless of external conditions.
Flue Gas Control
While a constant air supply does contribute much to minimising dangerous exhaust gases, a less than ideal burning condition nevertheless leads to incomplete combustion. The draft diverter addresses this issue by preventing the backdraught of exhaust gases into the room. It gathers nearly all combustion products from the primary flue, diverting them to the secondary flue, which then vents them safely outside. During the combustion gases' passage through the diverter, they mix with dilutant air, which along with the baffle plate reduces their temperature before release.
Aside from regulating the temperature of flue gases, the baffle plate also serves as protection against downdrafts. In the absence of a draft diverter, air blowing down the chimney in adverse conditions disturbs the combustion in the burner, or if the furnace is not in operation, extinguishes the pilot light. The baffle plate inside the diverter deflects this downdraft, leading the gusts of air safely away from the primary flue. This ensures that the combustion chamber remains undisturbed while the flue gases still get vented out properly.
Finally, a draft diverter contributes to the easy maintenance of a furnace by minimising soot build-up. This is particularly important during updrafts. Without a diverter, updrafts in the secondary flue would pull out excess air from the primary flue and the combustion chamber, increasing soot and smoke production. The diverter keeps updrafts from sucking out oxygen from the furnace, maintaining the necessary air-fuel ratio for optimum combustion. This reduces surface fouling within the system and minimises pollution from exhaust gases.