Protecting Property, HVAC Systems from Black Soot Deposition
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Protecting Property, HVAC Systems From Black Soot Deposition Part two of a two-part series addresses what can be done to avoid IAQ problems created by candle soot deposition and how to fix them once they are discovered BY DAVID KRAUSE AND TIM HEBERT Once these particles are charged they can clump together with other soot particles, forming larger particles. These Most homeowners do not realize larger particles can adsorb more organic vapors and gases, thus growing in size the adverse effect candles can have on and becoming attracted to surfaces that indoor air quality. Soot deposition can have slight electrical charges. present hvac systems with several problems, including decreased energy All three of these events will cause efficiency, condensate drain pan particles to deposit onto a surface faster overflows and soot blowout from the than it normally would. A good indicator supply diffusers. In addition, candle soot of turbulence in the ductwork is will deposit itself on carepting, drapes excessive noise that often is created and other household furnishings and when local air speed is greater than 2,300 items. feet per minute. Obvious constrictions and abrupt airflow direction changes Before treating these conditions one especially at the main supply plenum must determine if a soot problem exists. should be avoided. Accurate contamination assessments can be performed via a thorough inspection Finally, the use of certain electronic of the air handler unit and sections of the return and supply ductwork. air cleaners can cause particles to become charged and to deposit onto Deposition of candle soot inside hvac surfaces with an opposite charge. Often systems results from soot entering these air cleaners will show signs of through a return vent and being severe soot deposition due to the charges transported throughout the system during they possess on their surfaces. normal operation. Deposits are heavily concentrated in areas of impact or The last group of contributing factors airflow alteration, such as turning vanes. may be most unlikely, but can explain Coils are prime areas of deposition. why two homes with seemingly similar soot concentrations and charging Several simultaneous events must mechanisms have different soot take place for deposition to occur, so deposition rates. If the materials within a removing one factorAny combustion of home have different attractions for the carbon-containing fuel can produce soot electrostatically charged soot particles, but incomplete combustion is the most deposition may differ substantially. A serious source. Other sources of soot home without carpet, plastic appliances formation in homes include fireplaces, and with wood coverings may not furnaces, diesel-powered automobiles experience soot deposition while an parked in garages and unvented gas identical home built with typical appliances. materials used today may experience severe soot deposition. can prevent property damage. Contributing factors are divided into three major groups. The first group is the soot source. Many sources have been proposed, but historically the most prevalent source is from combustion. As particles are moved throughout a home they can become electrostatically charged. This can occur several ways, including high-speed transport through fiberglass-lined duct, turbulent transport in the ductwork and passage through an electrostatic air cleaner. These and possibly other charging mechanisms can charge soot particles. Porous duct liner is a common place for candle soot to deposit itself in residential HVAC systems. If soot is accumulating inside a home, Avoiding Soot Creation then carbon monoxide and other products of combustion may also be Even when soot is not visible, a present in sufficient concentrations to candle, especially those that are cause acute respiratory injury or even aromatic, and with long, thick wicks, can death. In these cases the most hazardous generate enough soot to cause property combustion pollutant is the carbon damage in a short time. Advise your monoxide and not the soot. residential customers to avoid using heavy soot-producing candles. Similarly, a cracked heat exchanger emitting flue gasses into a supply air A second factor in soot production is stream can present conditions similar to the airflow dynamics of a house. This candle soot deposition. This condition includes the home's ventilation rate, poses a much greater danger in terms of particle charging mechanisms and carbon monoxide poisoning and should airflow disruption. Many new and be one of the first items checked when renovated homes exchange minimal evaluating a soot deposition problem. amounts of air with the outdoors, which are commonly known as "tight houses." One key to distinguishing between Energy-conservation measures are a candle soot deposition and a cracked heat major factor in such situations. exchanger is the prevalence of soot deposition beginning at the return of the By reducing the ventilation rate to system up to and including the heat save energy, builders have allowed exchanger area and downstream of the pollutants generated in a home to stay supply. inside for longer periods of time. The longer a soot particle stays in a home, the It also is important to determine when greater the chance it has of depositing the onset of soot deposition began. itself onto a surface. Candle soot deposition occurs over an extended period of time, but typically is By avoiding the construction of not noticed until household items fiberglass-lined ductwork with abrupt suddenly look dingy. On the other hand, airflow changes and constrictions that evidence of a cracked heat exchanger can cause air velocity to increase, hvac be pinpointed to a specific point in time. installers may be able to avoid the deposition of soot particles. By avoiding these conditions and factors, property damage from soot Another category of contributing deposition can be prevented or reduced. factors involves materials onto which If builders and hvac installers avoid charged soot particles deposit installing ductwork with high air velocity themselves. Investigations of this process and excessive turbulence, some cases of reveal that soot deposits onto injection- soot deposition may not develop. In molded plastic surfaces such as garbage addition, ensuring adequate ventilation to cans, ice trays, kitchen appliances and outside air can prevent accumulation of synthetic carpets. These items possess an soot for extended periods of time. electrostatic charge under many common environmental conditions. While very weak, this charge is sufficient to attract soot particles. All combustion appliances, another source of soot, should be vented to the outdoors as a general rule. Combustion appliances designed to vent into a home should be adjusted so complete fuel combustion occurs. If a combustion appliance is suspected, then a true emergency exists and immediate action is required. Cleanup Time While it is good practice to clean Once it is determined that candle soot evaporator coils while in place, the deposition exists, successfully removing problem with candle soot deposition is it it from most hvac component surfaces is reacts when it contacts most coil dependent on several factors. These cleaners. This results in the formation of factors are the overall condition of the hard, crystallized material that requires material prior to soot deposition, the sufficient force to remove from the coil amount of deposition and the cleaning fins. accessibility of the various components. As a result it will be necessary to Sheet metal surfaces typically are evacuate and remove the coil for proper more easily cleaned than porous cleaning. For best results, use a high- foaming alkaline coil cleaner surfaces, such as duct liners. However, duct liner should be considered accompanied with a minimum 1,500 psi salvageable if the deposition is only on pressure washer. In our experience, this the top of the insulation and the method has been the most effective. insulation will recover from a thorough HEPA vacuuming. Following a cleaning Cleaning interior surfaces of any of these materials, some type of sheet metal ductwork should be resurfacing coating should be applied to accomplished by using either a HEPA fiber-lock the materials and increase the vacuum and/or aggressively brushing all life expectancy of this portion of the air these surfaces while under negative handling system. pressure. The use of compressed air or "air-brushing" alone has been ineffective All nonporous metal surfaces that are at removing normal household dust and easily accessible should be cleaned by does not work when dealing with soot HEPA vacuuming and by hand wiping deposition. with a mild biodegradable cleaning solution. Typically, most flex duct installations that have been subject to candle soot The blower assembly should be deposition lend themselves to a proper removed, disassembled, thoroughly wet- cleaning with mechanically driven cleaned and reinstalled following the brushes and negative air pressure. cleaning of the air handling unit cabinet. However, on occasion a technician must determine salvageability of these In the case of a gas furnace, hand- components and the cost of cleaning cleaning of the exterior walls of the heat versus the cost of replacement. exchanger can be challenging but can be accomplished with due diligence. If the Decontamination of a heavily system is a heat pump, remove the heat contaminated hvac system can be strips, then thoroughly clean and dry challenging.