Can a New Duct System Improve Indoor Air Quality?
In most homes, the duct system moves more air than any other system or force present in the home. Nationwide, about 85% of homes use central forced air systems – furnaces/AC and ducts – to heat and cool the living space. Most forced air HVAC systems move between 1,200 and 2,000 Cubic Feet per Minute (CFM). While forced air ducts are usually not the main source of indoor air quality problems (though they can be), ducts are a major transmission mechanism for contaminants to enter the living space and be distributed throughout it.
Forced air heating and cooling systems utilize a Forced Air Unit (FAU), typically installed in the crawl space, attic, basement, or garage of the home. The FAU utilizes a fan to drive air movement through the system and a gas burner, electric strip, or refrigerant loop to heat or cool air passing through the system. The FAU is connected to one or more return ducts, which pull temperate air from inside the home; once the air is heated or cooled, it is pushed back into the living space through a network of supply ducts.
A Filter On Its Own Can’t Improve Indoor Air Quality
Building codes require forced air systems to be filtered in order to prevent objects from being drawn into the equipment. Unfortunately, the code is neither stringent nor specific enough regarding placement and quality of this filter for the code to be meaningful in preventing the duct system from transmitting indoor air quality contaminants. In the drawing above, the filter is represented by a (#). Most forced air systems utilize a filter located just behind the return grille, accessible from inside the house for easy replacement. These filters are typically one inch thick, coarsely woven, and cost about $5 or less. They will protect the FAU fan from large objects, but won’t filter many contaminants.
Returns Ducts Don’t Just Leak, They Suck
More important than the particle efficiency of most standard filters is the placement. Most commonly, these filters are placed just behind the return grille for easy access. Unfortunately, this actually does nothing to prevent the intake of some of the most harmful contaminants. You see, forced air ducts leak. The typical duct system leaks out about 30% of the air moved by the FAU. The supply ducts are connected to the outlet of the FAU, so when the supply ducts are pressurized by air movement in the system, air leaks out, which wastes energy. The return duct is connected to the intake of the FAU, and when the FAU runs, it places the return duct under a vacuum. When return ducts leak, they pull air and particles in from the attic, basement, crawl, space, garage, or wherever else the return duct is run through. Since air is filtered before it enters the return duct, any air that is pulled in through return duct leakage eventually enters the home unfiltered. Among the particles that could be pulled in through your return duct leaks are:
- Mold and mildew
- Rodent feces
- Lead dust
- Carbon monoxide
- Fumes from solvents, paints, stored fuel, etc
Asbestos & Carbon Monoxide Are Big Concerns
Asbestos and carbon monoxide, in particular, should not be overlooked. Many duct systems constructed before 1978 were sealed and insulated with asbestos, which causes respiratory irritation and is considered a factor in the onset of mesothelioma and lung cancer when its particles are inhaled. So if the intact asbestos tape is what sealed the return duct in your system as that tape deteriorates over decades, those deteriorated particles are the first thing to be sucked into the system.
Not only can carbon monoxide enter the return duct from car exhaust if the return duct or FAU are located in the garage, the FAU and other gas-powered equipment in the home are common sources of carbon monoxide sources. A gas furnace is the most common type of FAU, and return leaks in proximity to the furnace can cause exhaust fumes, which contain carbon monoxide, to be pulled into the air stream. Return leaks can also cause gas water heaters in proximity to backdraft exhaust fumes into the air stream of the forced air system.
Fortunately, the harmful air quality properties inherent to most forced air systems can be remedied by proper installation of a new duct system. The two key points of improved indoor air quality with regard to forced air duct systems are leakage prevention and filtration.
New Ducts Can Help Improve Indoor Air Quality
Installing a tight duct system is critical to preventing unwanted particles from entering the forced air system’s air stream and improving indoor air quality. A new duct system should leak less than 10% of the amount of air they move, and less than 6% is ideal. It is becoming common for highly skilled install crews to install ducts that leak 2% or less of the air that moves through them! Unfortunately, the reality is that most new duct systems leak an unknown quantity, because most duct systems are not pressurized with a Duct Blaster to measure the amount of air they leak. Duct Blaster leakage testing was pioneered by the building performance sector of the construction industry, and has become the universal method for verifying the quality of a duct system installation. Though many cities and states have passed building codes requiring new duct systems to be airtight, enforcement has not caught up in most places. Therefore, it is up to installers to police themselves. A qualified installer will test the system with a duct blaster to ensure it meets airtightness standards, and make improvements as needed until the system is acceptably tight.
Adding Filtration Improves It More
It is practically impossible to install a totally airtight duct system, and many pollutants such as pet dander and hair spray originate from within the living space of the home. For these reasons, among others, it is still necessary to have an effective air filtration system. In the above drawing, the filter (#) is located at the point where the return duct connects to the intake of the furnace. By filtering the return air stream at the last possible point before the air moves through the fan and changes from being under a vacuum to being under pressure, the filter is able to capture particles originating from inside the living space, as well as those that originate from the crawl, attic, etc., and are introduced into the return stream. This provides the most thorough filtration and yields both pollutant-free supply air while protecting the FAU fan, air conditioner evaporator coil, and other mechanical components from dust which can shorten their service life.
Particle filtration is rated according to the Minimum Efficiency Reporting Value (MERV) scale. Most $5 filters from the hardware store filter at about MERV 4, which captures dust mites and carpet fibers, but not cement dust, hair spray, lead dust, or microbes expelled when sneezing. To capture all of these particles, a MERV 13 filter would be needed. A quality duct replacement should include a filter mounted at the fan intake of the FAU, with a MERV 11 or higher rating and an airtight cover and shroud.
Hire a Quality Contractor
If someone in your household suffers from allergies or other respiratory problems while in the home, replacing your home’s duct system could potentially have more impact than any other measure in improving the health of your loved one. A home performance contractor, or possible a very reputable HVAC contractor, will be best suited to carry out this project. When choosing a company to work with to carry out a duct replacement, a homeowner should verify that the company will:
- Perform EPA compliant asbestos remediation as needed
- Perform Combustion Appliance Safety (CAZ) testing before and after the project to remedy any existing problems and identify any potential problems with carbon monoxide, such as a backdraft of the furnace or water heater, cracked heat exchanger, or other source of carbon monoxide.
- Verify and guarantee the airtightness of the new duct system using a duct blaster test.
- Install a MERV 11 or better filter with an airtight cover at the furnace intake
If all these guidelines are followed, a new duct system can make all the difference in the quality of life for people living with indoor air quality problems.
Copyright September 2014 by Advanced Home Energy
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