Here's an explaination (via the first link) of what they do...
Stan
A heat recovery ventilator (HRV) can help make mechanical ventilation more cost effective by reclaiming energy from exhaust airflows. HRVs use heat exchangers to heat or cool incoming fresh air, recapturing 60 to 80 percent of the conditioned temperatures that would otherwise be lost. Models that exchange moisture between the two air streams are referred to as Energy Recovery Ventilators (ERVs). ERVs are especially recommended in climates where cooling loads place strong demands on HVAC systems. However, keep in mind that ERVs are not dehumidifiers. They transfer moisture from the humid air stream (incoming outdoor air in the summer) to the exhaust air stream. But, the dessicant wheels used in many ERVs become saturated fairly quickly and the moisture transfer mechanism becomes less effective with successive hot, humid periods. In some cases, ERVs may be suitable in climates with very cold winters. If indoor relative humidity tends to be too low, what available moisture there is in the indoor exhaust air stream is transferred to incoming outdoor air.
Although some window or wall mounted units are available, HRVs and ERVs are most often designed as ducted whole-house systems. The heat exchanger is the heart of an HRV, usually consisting of a cube-shaped transfer unit made from special conductive materials. Incoming and outgoing airflows pass through different sides of the cube (but are not mixed), allowing conditioned exhaust air to raise or lower the temperature of incoming fresh air. ERVs also allow the exchange of moisture to control humidity. This can be especially valuable in situations where problems may be created by extreme differences in interior and exterior moisture levels. For instance in cold, heating-dominated climates, better air flow and the introduction of humidity to the indoor environment can help control wintertime window condensation. In humid summer climates which are cooling dominated, it can be critical to dry out incoming air so that mildew or mold do not develop in ductwork.
After passing through the heat exchanger, the warmed or cooled fresh air goes through the HVAC air handler, or may be sent directly to various rooms. Stale air from return ducts pre-conditions the incoming flow before exiting. Systems in various sizes and configurations are available to automatically maintain 0.35 air changes per hour, the rate usually recommended to maintain good air quality. Many systems include filters to further control contaminants that would otherwise re-circulate through the home.
Conventional fan and vent assemblies for bathrooms and kitchens, often required by code, may allow significant energy losses. An HRV system can incorporate small, separately switched booster fans in these rooms to control moisture or heat generated by activities like showering or cooking. Odors and pollutants can quickly removed, but energy used to condition the air is recycled in the heat exchanger. Some codes or applications may still require stoves to be separately vented for removal of grease or gas fumes.
PATH Attributes
By transfering energy from exhaust air to incoming air, less energy has to be put into conditioningthe supply air, reducing consumption.
Because indoor air quality is generally lower than it is outdoors, ventilation is sometimes vital. This equipment help improve those conditions.
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Ease of Implementation
HRVs are readily available from several manufacturers, though not always stocked by distributors. ERVs are now also becoming commonly available. While providing better long-term energy performance HRVs, ERVs, and other mechanical ventilation systems add higher front-end costs to homeowners. Occupants must also understand the additional mechanical complexities of the systems, and be responsible for maintenance, particularly changing or cleaning filters as required. Gas stoves will still require a separate exterior vent system.
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Initial Cost
HRV systems generally cost between $700 and $2,000 installed, with variables like home size and the design of existing or planned HVAC systems being major factors. ERVs will cost more.
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Operational Cost
Costs of running the ventilation fans can be offset by savings on heating and cooling in areas where ventialtion is needed. Periodic replacement of filters may also be necessary.
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U.S.Code Acceptance
HRV and ERV systems should be covered by the HVAC sections in applicable local building codes. Many jurisdictions require conformance to model energy codes requiring higher insulation standards, increasing the likelihood that mechanical ventilation will be needed. Some codes also have minimum ventilation or air exchange requirements.
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Field Evaluations
Bruce Davis Construction: Washington Square, La Plata, Maryland
MADE Project: Bowie, Maryland
Warren Builders: Site 1. Albertville, Alabama
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Installation
Heat recovery ventilators can be installed by qualified HVAC contractors using conventional ductwork, tools and materials. The heat exchange units are typically installed in an attic, crawlspace, or storage/utility area, often adjacent to air handlers. Humidity recovery systems involve the use of condensers that may require defrost functions in areas subject to freezing conditions. Depending on the home layout and degree of incorporation to the HVAC system, significant additional ductwork may be required, plus fresh and exhaust air intakes, dampers, grilles and fan controls. Optional controls for bath fans include pollutant sensors with high speed overrides. As with any complex mechanical system, proper design, installation and testing of the system is needed to assure efficient performance.
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Warranty
Warranties vary by manufacturer and model. HRV/ERV cores are usually warranted for 1to 5 years and parts for 1 to 2 years. Some manufacturers offer lifetime warranties for their cores.
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Benefits/Costs
Heat recovery ventilation systems save heating and cooling energy in applications where mechanical ventilation is needed. Ventilation systems place control of air quality and air exchange in the hands of homeowners rather than relying on leaky structures or wind speed. Better air quality can have a positive effect on respiratory health and help control overall pollutant levels within the home environment. ERVs can help maintain humidity levels, also important for comfort and general health. Maintenance of proper humidity and air exchange levels can be important to controlling problems like window condensation in hot humid climate conditions.
http://www.toolbase.org/techinv/techDet ... logyID=122
http://www.heliosfans.co.uk/katalog/pdf ... uk_kwl.pdf
http://www.fantech.net/vhr1404.pdf
http://www.nu-airventilation.com/produc ... sheet.html
http://www.nutone.com/product-detail.asp?ProductID=2189
http://www.americanaldes.com/HRVRes.html
http://www.nypha.com/prodhrv.htm
http://www.vanee-ventilation.com/eng/produits.html
http://aprilaire.com/pdfs/3010.pdf
http://lennox.com/products/overview.asp?model=ERV1
http://renewaire.com/
http://customer.honeywell.com/Honeywell ... 218DD64%7d
http://broan.com/display/router.asp?doc ... goryID=512
Duct/Vent info...
http://www.ultivent.com/index.php?contest=housesmart01
