Q. The indoor air gets stale and too dry or too humid during winter and summer when we heat or air-condition. Is there any efficient way to get fresh air indoors without opening windows and wasting energy?
A. The air inside an energy efficient house can get stale both summer and winter. Not only is it unpleasant and uncomfortable (too dry or too humid), but it can be unhealthy. Many of today's products and household cleaners emit dangerous chemicals, especially when they are new.
Actually, just opening several windows for cross-ventilation for a short period of time on a breezy day during winter is not very inefficient. Much of the air inside your house can get exchanged fairly quickly without a huge heat loss from the house structure. The heat content of air is relatively low.
This method is not as effective during summer in humid climates. The humidity from the incoming fresh air permeates the walls and items in your house. In order to remove it, the air conditioner must run longer. If the air conditioner does not have a variable-speed blower with humidity control, your house gets chilled before the humidity is removed.
For continuous fresh air inside your house in the most efficient manner, install a complete heat recovery ventilation system (HRV). During winter, heat from the stale outgoing warm air is transferred to the incoming cold fresh air. During summer, the stale outgoing cold air precools the incoming hot outdoor fresh air. Up to 75 percent of the energy can be saved.
A HRV is a simple system with a heat exchanger inside a cabinet and two blowers, one incoming and one outgoing. It has its own duct system drawing the stale indoor air usually from bathrooms and the kitchen. The incoming fresh air ducts often lead to the living room or hallway. The two air flows pass each other in the heat exchanger, but stay separate.
In many climates, indoor humidity levels are also a concern. For example, during summer, bringing in precooled humid air may not greatly improve comfort and may exacerbate allergies. Excessively dry air during winter can be uncomfortable for the skin and can cause other problems.
An energy recovery ventilation system (ERV) is a variation of a standard HRV. The design of the heat exchanger and its materials are different from a HRV. In addition to transferring heat, the heat exchanger in an ERV also transfers moisture. During the summer, the incoming fresh air is partially predehumidified by the outgoing cool dry stale air. During winter, the indoor humidity is recaptured.
There are various types of automatic controls that determine how long and how fast the blowers operate. An indoor air humidity sensor is common. You can also manually override the sensors and run it when you choose.