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Did you know your home's indoor air may be more polluted than Los Angeles on a smoggy day? Cigarette smoke, dust, soot, bacteria, molds, carbon monoxide, formaldehyde, radon, pesticides, and vapors from scores of household cleansers and building materials are just some of the contaminants that may be tainting your home's air. "Sufficient evidence exists to conclude that indoor air pollution represents a major portion of the public's exposure to air pollution," states a report issued by the Environmental Protection Agency (EPA).
Indoors, air pollutants get trapped and build up, particularly in today's air-tight, super-insulated houses. When pollutants accumulate, some concentrations can exceed 100 times outdoor levels.
High indoor humidity can be a serious problem, too. Humid air encourages mold, mildew, and bacterial growth. In the winter, condensation collects on windows and can damage paint, insulation, roofs, and exterior walls. And in the summer, high humidity feels uncomfortable and strains air conditioning.
Aside from removing the source of the problem, the best way to keep a house from building up toxins and humidity is to provide plenty of ventilation. But how can you ventilate a house without letting costly heated or cooled air out the window? Meet the heat-recovery ventilator.
ENERGY-SAVING VENTILATION
A heat-recovery ventilator (HRV), sometimes called an air-to-air heat exchanger, differs from conventional vents and fans. With standard ventilation, air circulates through static, open vents or is expelled by fans, such as those used in bathrooms, kitchens and attics. When room air escapes or is expelled, the energy that was used to heat or cool it is wasted.
An HRV can save 75% or more of that wasted energy. As it pushes out stale air, it pulls in fresh air, and--with little or no mixing of the two air streams--it transfers the heat or chill from the outgoing air to the incoming supply. The fresh air arrives pre-heated or pre-cooled and, with some units, pre-humidified or dehumidified.
Some HRVs mount like a room air conditioner in a window or wall opening; these are meant to handle individual rooms that have ventilation problems: bathrooms, laundry rooms, artist studios and darkrooms, for example. Larger, whole-house HRVs provide fresh air for all rooms. They often require routing ductwork to several places: to rooms where you want to exhaust stale air, particularly bathrooms, laundry rooms and kitchens; to the outdoors; and often to the central heating and air-conditioning system's return air supply. Though whole-house systems are installed primarily in new houses, they can be retrofitted into some houses that have good access for ductwork, particularly those with unfinished basements. To simplify installation, some systems can take advantage of existing ductwork--they're piped into the return-air system.
HOW IT WORKS
Inside an HRV, one fan draws outdoor air in; another fan blows stale room air outside. Both fans blow the air through a central heat-exchange core that extracts the heat from one air stream and transfers it to the other. In some units, the incoming air is filtered on its way through.
Different manufacturers employ varying technologies to perform this heat-transfer magic. Some send opposing air streams through alternating layers of aluminum plates. Carrier's Comfort Ventilator(tm) utilizes a polypropylene core to transfer heat and separate the incoming and outgoing air streams. Another type of exchanger transfers heat with refrigerant-filled pipes. None of these plate or pipe-type heat exchangers transfer any humidity (water vapor) from one air stream to the other.
Enthalpy-type exchangers have permeable exchangers that let some water vapor through. Honeywell's Perfect Window and the AirXchange models have a desiccant-coated wheel that rotates between the two air streams. Though all HRVs can lower household humidity in the winter by exhausting indoor air, the dessicant-coated wheel exchanger can control incoming humidity in the summer. For this reason, Honeywell refers to the unit as an "Energy-recovery ventilator," in reference to its ability to cut cooling costs. Ron Saldin, Honeywell's Senior Product Manager says, "It takes energy to humidify air. An ERV can reduce the load on air conditioning by rejecting moisture that would normally be coming into the home. "
The NewAire, from Altech Energy, has a fixed-plate type of heat exchanger that uses a Mitsubishi-patented, resin-inpregnated paper core. Like the Honeywell model, this allows water vapor--and the latent heat it contains--to move from one air stream to the other. In addition to reclaiming some heat, this exchanger pre-humidifies or dehumidifies incoming air to a comfortable range of 35% to 45% in most climates.
Condensation collects in most HRVs when warm, moist air contacts the exchanger's surfaces in very cold weather; this can freeze, blocking air flow. Liquid condensation is collected in a condensate pan or carried away by plumbing. Because the Honeywell and NewAire models transfer water vapor, they don't produce condensation.
Some units have electrical anti-frost heating elements that automatically turn on at about 10 degrees F. In addition to preventing freeze-up, these slightly warm incoming air. Others have a system of dampers that periodically recirculate room air through the unit when temperatures drop below 20 degrees F.
An HRV is operated by a wall-mounted control. Most offer more than one mode. For example, with the Honeywell model, you can set the HRV to exchange air continuously, to turn on only when humidity exceeds a certain level, or simply to circulate indoor air until humidity gets too high, then to exchange indoor and outdoor air.
THE PRICE OF EFFICIENCY
The cost of a ducted, whole-house HRV depends on the specific model, the amount of ductwork and accessory material needed, and the difficulty of installation. The units alone range from a low of $400 to about $1500; most run from $500 to $900. The only way to estimate total, installed cost is to get bids from air-conditioning contractors.
Mounted, room-sized models run from $350 to $450. Find out from your utility company whether or not they offer rebates for installing HRVs; some do. The amount of energy needed to run an HRV varies widely from one model to another and depends on the capacity of the unit.
THE RIGHT SIZE
A variety of sizes are available. Carrier, for example, makes 18 different models ranging in capacity from 150 to 1270 cubic feet per minute (cfm). The right size to buy depends on the number of occupants and the cubic capacity of the rooms or house you wish to ventilate. For complete ventilation, the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) recommends replacing indoor air with at least 15 cfm per occupant. Sizes and lengths of ductwork runs can affect efficiency, so it's important to work with a contractor when planning a system.
IS AN HRV RIGHT FOR YOU?
An HRV is most effective in a very "tight" (well-insulated) house where energy prices are high. A tight house collects more humidity and pollution than one with a lot of air infiltration. If a house has excess infiltration through leaky windows and a poorly-insulated shell, air flow bypasses the heat exchanger, negating its work.
Though they are considered to be most effective in very cold climates, HRVs make sense where summers are hot, too. The actual economics indicate that HRVs may offer better energy savings in hot, air conditioning climates. Electicity needed for air conditioning is much more expensive than any other fuel, so every BTU you can recapture may have three times the value of the savings of heating fuels like natural gas. (A BTU measures the quantity of heat required to raise temperature of one pound of water one degree F.) Of course, the higher the cost of energy, the more an HRV makes sense.
HRVs are practical for houses that have very high winter humidity and enthalpy-type HRVs can assist air conditioning in climates where summer humidity is high. An HRV may be a practical solution where there is a radon or formaldehyde pollution problem--though this should be determined by an expert.
In mild climates, exhaust-only ventilation may be a better solution, than an HRV. Where outdoor winter temperatures are fairly high, there isn't sufficient recoverable heat to repay the investment. And for cooling, the difference in temperature between indoor and outdoor air often isn't high enough to justify the cost of installation, operation, and maintenance.
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