Barometric dampers are used to control the draft in ventilation systems used in a wide range of heating appliances including furnaces, boilers, fireplaces and stoves. In order to explain how they function, it is important to know how draft works. As air is heated it rises and hot air is less dense than cool air and as a result has less mass.
Inside ventilation systems, the hot air produced by the heating appliance e.g. furnace is less dense than the air outside in the external environment. This cool air causes the hot air to be forced up the ventilation or chimney, which is referred to as the draft. The speed at which the draft travels is dependant on many factors such as temperature, wind, and ventilation height. These conditions fluctuate and as a result so does the draft velocity. On chilly winter days the draft can be so strong that it is able to remove the flame completely away from the burner, or alternatively it could result in heat being pulled away, resulting in fuel wastage, this is referred to as the chimney effect.
When ventilation is attached to a furnace there is massive difference between the two in terms of drafts. The furnace requires a constant, stable temperature and environment, while the ventilation’s environment can change dramatically. In order for this change to be evened out so no dramatic changes result, a draft regulator or barometric damper is the solution to the problem. The real issue with high velocity drafts is that they can result in flue gases being removed to quickly and also negatively impact heat transfer from the boiler to the environment, which is of course the ultimate goal of any heating appliance. Today, the majority of appliance manufacturers provide guidelines on the draft conditions that should be aspired for. The barometric damper is typically installed in between the furnace and the main ventilation. This draft regulator will alter the draft velocity using weights, for instance if the draft is excessive it will open and allow air to flow from the boiler to the ventilation.
Draft regulators are usually used when the stack ventilation exceeds 25 feet. Too much draft in a furnace or heating appliance can result in very bad conditions. For example, as well as leading to increased heating bills since the draft can result in flame impingement, which can also lead to hazardous levels of carbon monoxide. This effect can also impact the furnace or appliance metal causing brittleness and reducing the lifespan of your appliance considerably. In simple terms, embrittlement is the loss of quality of the metal caused by the flame touching areas of the appliances metal construction not designed to withstand such high temperatures.
If you are looking for a barometric damper, note that there are two main types: single and double acting. A single acting damper will have a stop that just enables the damper to swing a single direction. While a double acting damper will enable the damper two swing in both directions. The single damper will shut if there is pressure inside the ventilation, whereas the double acting type will enable flue gases into the furnace area if the flue becomes obstructed. This may sound dangerous, however, bear in mind that if there is a gas spillage from the furnace into the room the majority of dampers are fitted with a sensor that will trigger the burner to shut off. In many states a spill switch is a code requirement.
Also bear in mind that different types of furnaces that utilise different types of fuel will have a certain kind of barometric damper for them. For example, single acting dampers are usually used for oil based appliances, while double acting are typically used for gas appliances.