Energy Recovery Ventilators

The Energy Recovery Ventilator (ERV) and Heat Recovery Ventilator (HRV) are fresh air exchange appliances that take fresh air into a dwelling, clean and evenly circulate the fresh air and expel stale, polluted air while controlling humidity and minimizing heat loss/gain.

The current trend toward sealing (tightening) houses to reduce air and moisture infiltration makes them more energy-efficient and less costly to own. However making them very tight, but with poor ventilation, can create many indoor air quality problems including mold, persistent and objectionable odors, and what is commonly known as "sick building syndrome."

While opening windows for ventilation is an option, this also defeats the purpose of building a tight home in the first place. It is advisable that such houses also have a controlled way of moving fresh air into the house and removing stale/contaminated air from it. Such a system must be able to maintain air circulation while minimizing energy loss. One of the better ways of accomplishing this is with Enthalpy Recovery Ventilators (ERVs aka. Energy Recovery Ventilators).

Transferring a little water vapor across the core is often desirable because in extremely cold weather [consistently below 20oF (-7oC)], the water vapor from the exhaust air of an HRV can freeze the core and the exhaust vent to the outdoors. If this should happen, the ventilation rate is severely reduced. HRVs are also known for drying the house air to uncomfortable levels in climates where winter can last months.

Since an ERV (using a Mitsubishi Lossnay Core) transfers some of the moisture from the exhaust air to the usually less humid incoming winter air, the humidity of the house air stays more constant. This also keeps the heat exchanger core warmer, and so helps minimize problems with freezing. Some systems also have an electric heating element for defrosting the exterior vent to minimize freezing in that location.

In the summer, ERVs may help to control humidity in the house by transferring some of the water vapor in the incoming air to the theoretically drier air that's leaving the house. If you run an air conditioner in the summer, an ERV generally offers better humidity control than an HRV.

Researchers have found that some of the water vapor can condense on the cool ductwork interiors and thus promote mold growth. This is especially a problem for buildings with over-sized cooling equipment or during partial cooling loads (i.e., when it's warm and humid outdoors, but still not hot enough for the equipment to turn on for more than a few minutes at a time.)

All properly designed residential ventilating systems should be able to at least move about one third of an air change per hour (ACH) or 15 cubic feet per minute (CFM) per person. For example, if your house has 2500 square feet (232 square meters) of floor space, and you have 8 foot (2.44 meter) ceilings, then the ventilation system would move approximately 189 CFM.

However, designers should not use the 1/3rd ACH guidelines as a "hard-and-fast-rule." There will be times in the house when more ventilation is desired (i.e., at a party with many people present) and times when little ventilation is wanted (i.e., when no one is home). To assist the occupants in this effort many manufacturers offer a variety of control strategies. A few of the more useful features are: water vapor and carbon monoxide sensors; multiple fan speeds; and programmable timers. Most manufacturers also offer high efficiency air filters to trap pollen and other undesirable pollutants from the outdoors before they enter the system. Some also offer pre-heater or pre-cooler coils to make the incoming air more comfortable during extreme weather conditions.

There are some small wall- or window-mounted models, but the majority are central models with their own duct system. To save on installation costs, many installations share existing ductwork.

An energy recovery ventilation system should be designed as simply as possible for economy of installation and energy efficiency. Duct runs should be as short and straight as possible. The correct size duct is necessary to minimize pressure drops in the system and thus improve performance. Insulate ducts located in unheated spaces to at least R-9, and seal all joints with duct mastic (never use ordinary duct tape on ducts.)

There are several ways of routing the ducts around the house. In general you want to have a supply and return duct for each bedroom and one for each common area. Do not locate returns within 10 feet (3.05 meters) of an oven or cooktop, as vaporized grease could clog the heat exchanger core and contaminate the ducts. Locate the ERV itself in an easily accessible area and as centrally as possible to all of the rooms. A sewer drain or condensate pump may be necessary for removal of water that the core extracts from the air during cold weather. Locate the outside air intake as far from driveways, clothes dryer and furnace vents as possible. Keep all supply and exhaust vents at least 10 feet (3.05 meters) from each other.

In general, simplicity is key to a cost-effective installation. Sophisticated and complex systems are not only more expensive to buy but they are generally more maintenance intensive and often consume more electric power. For most houses, attempting to recover all of the energy in the exhaust air will probably not be worth the additional cost. These types of ventilation systems are still not that common, and many heating, ventilation and air conditioning contractors may not have very much technical expertise and experience with them.

Source sited from:

http://www.eere.energy.gov/consumerinfo/refbriefs/ea5.html