Air-to Air Exchangers
by Building Inspector and Indoor Air Specialist, Dan Schilling
© Copyright 2002 Residential Inspections LLC, All Rights Reserved
Air-to-air exchangers, sometimes called heat recovery ventilators, may go by different brand names, but they all have the same primary function of introducing fresh air from the outdoors into the interiors of homes. The reason they are sometimes called heat recovery ventilators is because they temper the cold incoming air by extracting heat from the stale air being simultaneously exhausted. These devices are being heavily promoted and sold by some HVAC contractors and, in some states, they are an option to fulfill new construction code requirements designed to improve indoor air quality.
Provided that these devices are diligently maintained, and I repeat “diligently” maintained, they can be an advantageous appliance to have installed in a home or office building. On the other hand, air-to-air exchangers should never be considered a viable solution to the indoor air contamination problems causing such a wide array of health concerns. Furthermore, if not consistently maintained, these devices can become seriously detrimental, increasing the contamination of indoor air.
Allow me to share a list of what I regard as the advantages and disadvantages of these air-to-air exchangers/heat recovery ventilators.
Air-to-air exchangers bring in small amounts of oxygenated air into a home on a varying basis, depending on how the system is installed. This influx of oxygenated air is typically introduced directly into the interior of the air duct system and then distributed through the ducts. When considering that each human in a home breathes about 2,500 gallons of air every day, one begins to understand the benefit of replacing oxygenated air indoors. Replacing oxygenated air to the indoor environment can help us feel more energetic, mentally alert, and reduce symptoms associated with high levels of carbon dioxide (CO2). While carbon dioxide should not be confused with highly poisonous carbon monoxide (CO), high levels of carbon dioxide can also cause poor health affects, such as headaches, dizziness, drowsiness, shortness of breath and eye irritation.
Air-to-air exchangers assist in the removal of indoor moisture which could lead to mold and bacterial contamination of indoor air. The amount of moisture actually being eliminated from the indoor environment may be nominal in comparison to the amount of moisture created within a home, however, even small amounts of moisture removal are a step in the right direction. This is particularly true for newly constructed homes and homes that have otherwise been made more energy efficient.
Rancid indoor air is a serious problem in virtually all indoor environments. Adding a small percentage of oxygenated air to an indoor environment does not solve the indoor pollution problems we have today. It is very misleading to presume that these devices are an answer to indoor air quality. Contamination sources and elimination methods are very complex and often require a variety of measures to correct and improve indoor air quality. As I have said many times before, there is no silver bullet.
Air-to-air exchangers typically will not remove enough moisture from the indoor air to eliminate moisture related problems, hence, all other methods of indoor moisture control must still be considered. High levels of moisture are known to be the leading cause of biological contamination found in indoor air.
Similar to the evaporator media used in built-in humidifiers and the fins of the air conditioner evaporator coil (both hidden from view inside duct work), the media inside of a typical air-to-air exchanger also secretly collects dust particles. These particles can promote hidden mold growth and can potentially circulate the mold spores into the breathable air. As an air quality inspector, I never recommend any unnecessary media that can promote hidden bio-growth within the air circulation system.
Electrical ions, richly present in the air outdoors and vital to cleaning ultra-fine particles from the air, are not brought in through air-to-air exchangers. It is unlikely that the beneficial ions would make it past the outdoor intake vent, much less through the ducts of the system. Ions lose their electrical charge the moment they touch something, such as the interior of ducts or filter media. Therefore, air-to-air exchangers are an inadequate solution for the removal of particulate pollutants that cause many ill health affects.
Naturally “activated” oxygen, also called “ozone” (not to be confused with manmade pollution levels), is also present in the air outdoors and is necessary for eliminating foul odors and chemical gasses through a process called oxidation. Ozone is what leaves that fresh, clean smell in the air outdoors. Air-to-air exchangers may bring in small amounts of activated oxygen, but not nearly enough to maintain natural outdoor levels for proper indoor air purification. This is particularly true in new or remodeled homes where the rate of off-gassing chemicals from building materials is excessive. Due to the half-life of ozone, one would have to leave all house windows wide open 24 hours a day in order to maintain the outdoor purification levels of ozone. Highly impractical for most of us. Many are now turning to residential air purifiers as a practical way to replace the missing ozone. (Note: Air purifiers that produce ozone indoors should have complete adjustability of ozone output.)
Air-to-air exchangers require regular maintenance, which is almost never performed by homeowners, and likely to be overlooked by contractors during service calls. In fact, at the time of this article, I have yet to see one of these air-to-air exchangers properly maintained while conducting building inspections. Internal motors do not get oiled, allowing them to dry up, heat up and seize. The interior filters are not cleaned and, perhaps worst of all, the air intakes from the exterior are quite frequently clogged with dirt. Often they are so clogged that an ant couldn't get through, much less a supply of fresh air. If outdoor air cannot enter, there is no benefit whatsoever.
When the air intake vents get clogged, these systems which were intended to improve indoor air quality can become a detrimental health hazard. When clogged, air-to-air exchangers can make the indoor air significantly worse than it would be without the system installed at all. This point should be further emphasized by saying that clogging is a matter of when, not if. The reason for this has to do with indoor air pressures: When the intake vent gets clogged and the system can no longer bring in air from outdoors, it will still continue to exhaust air from the indoors. This creates “negative air pressure” within the home, which in turn can cause serious air quality issues due to a process called “back-drafting”. In essence, back-drafting causes critically important exhaust vents in the home to function poorly, or operate in reverse. Consequences of negative indoor air pressure and back-drafting are: A) prevents kitchen and bathroom exhaust fans from operating properly, which can over tax and burn out their motors faster, B) causes excessive indoor moisture levels which in turn causes health problems related to molds and dust mite populations, C) causes exhaust vents from furnaces, water heaters, gas ranges and fireplaces to function poorly, or in reverse, thus spilling poisonous carbon monoxide gasses and other combustion pollutants into breathable air, D) increases the infiltration rate of radioactive radon gas through the basement area of a home which increases the risk of lung cancer for the occupants of the home.
If the interior filter, designed to protect the heat transfer portion of the system, becomes clogged with house dust, the system can no longer exhaust the stale indoor air properly. If the system still has the ability to pull outdoor air into the home without the ability to exhaust an equal amount of air, the indoor air pressure can become “positive”. This positive indoor air pressure can cause “exfiltration” which pushes moisture-laden indoor air into the walls of the house, through electrical outlets, switches, light fixtures, box sills, and floor boards. Consequences of moisture exfiltration are: A) moisture can accumulate in wall cavities where it can begin rotting the structural frame of a home, B) the moisture rusts metal plate fasteners of floor and truss members which can lead to collapse, C) the moisture can cause the walls and floors to change shape, D) moisture attracts wood-destroying insects into wall cavities, E) cause paint to peel on exterior walls, F) moisture can wet and damage insulation causing energy loss.
Note: As you can see, positive indoor air pressure can adversely affect the structure of your home while negative air pressure adversely affects the health and safety of the occupants. Ideally, indoor air pressure should be as close to neutral as possible.
The installed price of an air-to-air exchanger can be $1,500.00 to $3,000.00 each.
In summary, if you bought a home that did not have an air-to-air exchanger, I would recommend that you do not purchase one. For the benefit of a little oxygen, you could open your windows on occasion, save the maintenance hassles and use the money for more important air quality improvements. If you bought a home that already had an air-to-air exchanger, keep it scrupulously maintained or disconnect it.
Three Better Solutions
In addition to proper cleaning and elimination of obvious sources of indoor air contaminants, I recommend that homeowners use a combination of the three proven methods of air quality control listed below. These methods will significantly improve indoor air quality at a fraction of the cost of an air-to-air exchanger:
Efficient Furnace Filters
Good furnace filtration, such as the high-efficiency pleated type or good quality electrostatic filters will provide A) a significant reduction of particles in the breathable air, B) keep the entire house cleaner, C) help protect mechanical equipment, D) help prevent mold and bacterial growth in air conditioner coils and, E) help prevent AC condensate drip pans and drain lines from becoming clogged and damaging the furnace.
Electronic Air Purifiers
Air purifiers (not to be confused with HEPA room filters or electronic furnace filters) replace the natural levels of electrical ions and ozone that are missing from indoor air. These portable systems eliminate house odors, including those from pets, smoke, mold and building materials, as well as, help cleanse the air of airborne particles. They require no installation, and just one machine will purify the air in an average size house.
Make-up Air Vents
The installation of a make-up air vent will bring small amounts of fresh oxygenated air into the home while helping to maintain neutral air pressures indoors. They require virtually no maintenance and use no electricity.
You can read about these three options in other articles here at indoorair.com.
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