Balanced Ventilation Systems
Balanced ventilation systems exhaust stale air and provide fresh air through a ducted distribution system. Of all the ventilation schemes, they do the best job of controlling pollutants in the home.
Balanced systems move equal amounts of air into and out of the home. Most balanced systems use heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs). HRVs exchange only heat. ERVs exchange both heat and moisture from the exhaust air stream.
Balanced systems, when operating properly, reduce many of the safety problems and moisture-induced building damage that is possible with unbalanced ventilation. They are not trouble-free, however, and there are many homes with “balanced” ventilation systems that experience pressure imbalances and poor air quality due to poor design, installation, or maintenance.
These complicated systems can improve the safety and comfort of home, but a these ventilators need a high standard of care to assure that they operate effectively. Testing and commissioning is vital during both the initial installation and periodic service calls.
Heat and Energy Recovery Ventilators
The difference between an HRV and an ERV is that HRVs transfer heat only, while ERVs transfer both sensible and latent heat (moisture) between airstreams.
Balanced ventilation systems can provide superior indoor-air quality (IAQ) and comfort but they require exemplary installation and maintenance. Installers must test and commission the ventilation system during both the initial installation and periodic service calls. At least once a year, homeowners must clean the two filters inside the balanced ventilator along with grills and termination fittings in the ducts.
Currently, a lack of trained installers and educated consumers causes problems with existing balanced ventilation systems. The most common problem is insufficient airflow because of faulty ducts or dirty filters. Other problems include these.
- Occupants turn the ventilator off.
- Operation time is too long or too short.
Heat-recovery ventilators (HRVs)
The HRV core is usually a flat-plate aluminum or polyethylene air-to-air heat exchanger in which the supply and exhaust airstreams pass one another with minimal mixing.
Heat travels through the core, by conduction and convection, from the warmer to the cooler airstream. In heating climates, this means that heat contained in the exhaust air warms the incoming supply air. In cooling climates, the heat of the incoming fresh air transfers to the outgoing exhaust.
In cold weather, outgoing moisture condenses when it passes the cold incoming air in the HRV core. This condensate drains out of the unit. HRVs may require defrost mechanisms in cold climates to thaw frozen condensate in the heat exchanger. The most economical defrost method is periodic recirculation of warm indoor air, though some HRVs with aluminum cores use electric heat strips.
Energy-recovery ventilators (ERVs)
ERVs reclaim moisture along with heating or cooling energy, depending on the season. Energy recovery ventilation exchanges both sensible and latent heat between the outgoing airstream and incoming airstream.
Energy recovery ventilation removes between 20% and 50% of the water vapor from the more humid airstream and transfers that water vapor to the less humid airstream. In comparison to an HRV, ERVs add less outdoor humidity to the supply in warm humid weather and subtract less indoor humidity from exhaust in cold dry weather.
The ERV’s exchange of water vapor has no significant dehumidifying effect on the building. Instead this water-vapor exchange merely reduces the amount of water vapor that the ventilation air adds to the building. However in very humid weather, the moisture-transfer medium may become saturated and fail to remove much water vapor from incoming air.
ERVs also reduce the amount of water vapor leaving the building. During winter, the ERV reduces excessive dryness in dry climates, but increases excessive humidity in damp climates.
Experts disagree about whether the ERV has significant advantages compared to the HRV. In persistently humid regions, neither HRVs nor ERVs reduce indoor humidity. Dehumidification would require a whole-house dehumidifier or else a ventilating dehumidifier that both ventilates and dehumidifies using the refrigeration process.
For more information on ventilation, see Residential Energy, Chapter 11.