Balanced fresh air ventilation design, installation, & troubleshooting in buildings:
Tthis building ventilation design article explains the use of balanced fresh air ventilation systems, heat recovery ventilators, and energy recovery ventilators to improve indoor air quality in homes.
Lots of people consider dust, pollen, mold, animal allergens, pet hair as the main culprits in indoor air quality issues, allergies, and asthma aggravators. But many indoor contaminants are simply too small to see, or are not particles at all but rather gases or chemicals. Balanced ventilation systems combine fresh air input with stale or contaminated air exhaust to improve indoor air quality while saving on building energy cost.
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As reported in Best Practices Guide to Residential Construction:
Balanced ventilation uses both a supply and exhaust fan to provide fresh air while keeping house pressures neutral. Linking a multiport supply system (described above) with a bathroom exhaust fan on the same switch is a form of balanced ventilation.
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Well suited to extreme climates, balanced ventilation provides optimal distribution of incoming fresh air and tempers it for comfort.
HRFs reclaim heat from the exhaust air while ERFs, recommended for hot humid climates, dehumidify and cool incoming air. For good performance, systems must be properly installed, balanced, and maintained.
Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above.
Most balanced ventilation systems, however, use a heat-exchanger to transfer heat and, with energy recovery ventilators (ERVs), humidity between the two air streams. These systems, sometimes called air-to air heat exchangers, are the most expensive option for whole-house ventilation; but, if installed properly and well-maintained, provide optimal comfort and ventilation. Depending on the type of heat exchanger, balanced ventilators are referred to as either heat-recovery ventilators (HRVs) or ERVs.
Balanced Ventilation System Controls
One of our clients became so frustrated with problems balancing fresh air in her home that she simply cut out a window pane. This article discusses a better approach.
HRVs and ERVs are typically run continuously, but they also may be set to run 8 to 12 hours per day when people are at home.
In addition, most have a high-speed mode that can be manually activated for spot ventilation of kitchens and bathroom.
Some balanced ventilation systems also use dehumidistats to automatically turn on or increase ventilation when the air reaches a preset humidity level.
Balanced Ventilation System Ducting
While a dedicated ductwork system is the best approach for HRVs and ERVs, to save money they are often piggybacked onto the home’s HVAC ductwork. In one approach, the HRV or ERV draws exhaust air from the return ductwork and feeds fresh air into the furnace’s return plenum. In a slightly better arrangement, the HVAC ductwork is used only for supply, while the exhaust side picks up stale air in bathrooms, laundry, and kitchen. Neither approach distributes fresh air as well as a dedicated duct system.
Also, since HRV/ERV fans operate at 100 to 200 cfm while air-handler fans are often sized at over 600 cfm, trying to integrate the controls, balance airflows, and provide the correct amount of ventilation air is challenging and rarely works well.
The most common approach is to run the ventilation system only when the thermostat calls for heating or air-conditioning, providing too little ventilation. Heat-recovery efficiencies are also compromised, typically due to unbalanced airflows.
Balanced Ventilation System Maintenance
One drawback of HRVs and ERVs is that they require more maintenance than other ventilation systems. Numerous studies have found that many of these systems significantly under perform in the field due to both installation errors and poor maintenance.
In addition to cleaning or changing intake, outtake, and internal filters, the homeowner or service person needs to clean the core once a year or more to prevent mold and bacteria growth. It is important to follow closely the manufacturer’s recommendations.
Unless the homeowner enjoys the responsibility of HVAC maintenance, the work is best handled by a professional service company.
Balanced Ventilation System Cost Effectiveness
For HRVs and ERVs to work properly and achieve the rated efficiencies, they must be installed correctly and balanced well, and the house must be very tight.
Even so, the added cost over a basic ventilation system will be recouped only in the most extreme United States climates with the highest energy costs. However, in very cold or hot climates where mechanical ventilation is needed during most of the year, the added cost may be justified by the comfort of tempered, filtered ventilation air, the effectiveness of the distribution, and the lack of pressure-related problems.
Used primarily in cold climates, HRVs have two air streams that pass over one another in a plastic or aluminum heat exchanger.At left we show a photograph of a heat recovery ventilator.
Used primarily in cold climates, heat-recovery ventilators (HRVs) pass two air streams by one another within a plastic or aluminum heat exchanger, recapturing 60 to 75% of the heat from the outgoing air stream. This unit, installed in an attic, is hung from chains to reduce noise and vibrations.
Recovery of heat from the exhaust air typically ranges from 60 to 75%, if properly installed and balanced.
During the summer, if air-conditioning is used, the heat transfer reverses, cooling the incoming hot air. Systems generally have exhaust ports in rooms that generate moisture or pollutants, including bathrooms, laundry, and kitchen and supply ports in bedrooms, living rooms, and other main living spaces.
Because they have both supply and return ducting, HRVs provide the best distribution, exhausting air from bathrooms and other wet areas and providing fresh air to primary living space. The kitchen typically has its own range hood, so grease does not get into the HRV system.
Original Best Construction Practices Guide photo source: courtesy of David Hanson, memphremagog Heat Exchangers, appearing in the text cited above.
Defrost cycle for heat recovery ventilators
In cold climate applications, a defrost cycle is required. It usually switches on at about 20°F to keep frost from building up in the core as condensation from the exhaust stream begins to freeze. Systems either recirculate indoor air or preheat incoming air to prevent freeze-ups.
More Reading about heat recovery ventilators:
Energy-recovery ventilators are primarily used in air-conditioned homes in hot, humid climates. They are generally recommended for climates where the cooling load exceeds the heating load and where sustained freezing temperatures are rare. Sustained temperatures below 10°F can damage the permeable core material used in many ERVs.
ERVs either use a dessicant-coated plastic wheel or a special “enthalpic” core material to move moisture (latent heat), as well as sensible heat, between the two air streams. In summer, incoming air is cooled and dehumidified. Since dehumidification is the biggest component of air-conditioning costs in humid climates, it is important to find a unit with a high TRE (total recovery efficiency) rating, indicating that it can transfer large amounts of moisture. To achieve the rated efficiencies, the units must be run at the recommended airflows.
In cold weather, an ERV will tend to humidify the incoming air, since the moisture transfer is always toward the less humid air stream. This is rarely a problem, however, since the cold incoming air holds so little moisture to begin with that the net effect of the air exchange is to remove humidity from the house.
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