Adding fresh-air ventilation through your HVAC system
How can you get that "fresh air" feeling in your home if bringing in outside air means bringing in the cold or humidity?
In order to improve IAQ, the EPA recommends that homes receive “0.35 air changes per hour but not less than 15 cubic feet of air per minute (cfm) per person.” These air changes can occur through natural ventilation such as open doors or windows, infiltration such as cracks and joints around doors and windows, or mechanical means such as fresh air intake of HVAC units. This third option is what we’re aiming at in this post: consider this option like the “fresh air” vs. “recirc” buttons on your car’s climate control.
“Dilution Ventilation” is critical for improving IAQ because it adds fresh air to the soup we are breathing when staying indoors. Doing it through the HVAC system is best because on those days when it’s too muggy or too cold to open the windows, your IAQ should not suffer.
The simplest way of bringing in fresh air through the HVAC involves a motorized damper and controller. When the damper is open, it delivers fresh air from the outside to the “return” side of the furnace so that the incoming air can be properly filtered and conditioned before being distributed with the recirculated air. Here are several makes of simple damper/control systems: AirCycler and Honeywell. These do not exhaust air outside, however, so they result in a net positive pressure in your home as compared to outside.
When the fresh air coming in is very cold, warm or humid compared to the air inside your home, it’s most efficient to use the air that’s already in your home to change the temperature and humidity of the fresh air. An Energy Recovery Ventilator (ERV) is a heat exchange system to: 1) exhaust stale air from your home and 2) bring in and exchange energy with fresh air from outside. By doing a volume and energy exchange, you are getting net neutral air pressure in your home and less lost energy by using the stale air to heat or cool and dehumidify the fresh air. Here are several options to:
Exchange heat and humidity in equal volumes with each other. It only requires energy for the fan and controls.
Bring in fresh air and heat or cool it with a heat pump. This requires a little more energy but for small spaces, can even be the sole heat/cooling source.
Bring in fresh air and dehumidify it with a dehumidifier. This is best for hotter, more humid climates where cooling and dehumidification is needed more often than heating.
If you don’t know how or when to “let the fresh air in”, let your HVAC system do it for you!