With the emphasis on energy conservation and efficiency, new home construction can create a problem of indoor air pollution. Vapor barriers, thermal windows, weather-stripping and caulk have reduced or stopped fresh air from infiltrating and replacing stale air. Entering and exiting the house through doors isnt always enough air changes. Cooking, aerosol sprays, cleaning agents, paints, and in some cases excess humidity if the house is sealed too tightly can create an undesirable environment. Keeping windows or doors open does not conserve energy. A device known as an air-to-air exchanger is used to recover heating or cooling and improve air quality.
There are many different designs, depending on the manufacturer, but the principle is the same. Fresh air is drawn in from a port open to the outside of the building, and passed through a chamber, also know as the exchanger, that is surrounded by indoor air. Highly conductive metal or other materials removes the energy (heat) from the warmer air and gives it to the cooler air. The fresh air is then ducted into the house, and the indoor air is ducted to a port and expelled outside. Up to 80% of the energy can be exchanged. During the energy exchange, moisture (humidity) can condense into water. A drain pan inside the cabinet will allow the water to be collected for removal. If the unit is installed in the basement, a condensate pump might be used to eject the water outside.
Most air exchangers are controlled simply be on and off switches, but in applications for removal of humidity, a humidistat can be used to turn the machine on and off to achieve the desired level.
Routine maintenance involves replacing or cleaning the filters if so equipped, and keeping the outside air intake free of debris. After years of service the exchanger will become crusted with grease and dirt that collects during condensation. Removing this accumulation will probably take solvents or degreasers and require opening up the cabinet and compartments.
One method of adding fresh air to the space is to duct outside air directly into the return air plenum of the heating or air conditioning system. Usually required by code for commercial applications, it is an inexpensive way of improving air quality, but not the most efficient, since it means heating or cooling outside air, which could be very cold in winter or extremely humid on a midsummer day.