By Greg Stopher
Part 2: Whole Home Ventilation Systems
Last month I covered basic home ventilation including the bathroom fan and air leakage. With homes being tighter and more insulated, indoor air quality is starting to suffer because of the lack of ventilation. This month I am going to cover the benefits and some of the drawbacks of a whole house ventilation system.
There are two types of mechanical whole house ventilation systems out there. One is the Energy Recovery Ventilation system and the other is the Heat Recovery Ventilation (HRV) system. In Southeast Alaska, Energy Recovery Ventilation is not and should not be used. Both systems use a heat transfer core but the Energy Recovery core transfers heat and some water vapor back into the supply air where a Heat Recovery core only transfers heat. So an Energy Recovery Ventilation system will only add to the humidity problems that exist in Southeast Alaska. A Heat Recovery Ventilation system or HRV will only transfer the heat from the exhaust air to the incoming air. This process of heat transfer acts as a natural dehumidifier for the house and is useful in our climate.
An HRV rating is based on how effective it is at exchanging heat from heated indoor air to unheated outdoor air. The rating is based on the rise in temperature of unheated outdoor air and is called the Apparent Sensible Effectiveness or ASE. The ASE of most HRV systems range from 70% to 90% ASE and will be labeled on the HRV. For Example: An HRV with an ASE of 80% when the outdoor temperature is 40 degrees and the indoor temperature is 68 degrees (pretty common temperature in Southeast Alaska). Take the difference between 68 and 40, which equals 28. Multiply 28 by .8 or 80% which equals 22.4. Add that to the outdoor air temperature of 40 degrees and you get 62.4 degrees, which is the temperature of the incoming supply air from the HRV, a big difference from 40 degrees.
Naturally, warm air holds more moisture than cold air so heating cold air will reduce its relative humidity. With an outdoor temperature of 40 degrees and an outdoor relative humidity of 80%, the incoming outdoor supply air is heated to 62.4 degrees in the heat exchanger core, as described above, the humidity of the air drops to just 35% relative humidity because HRV systems act as natural dehumidifiers.
As I discussed in last month’s article, code requires 50 Cubic Feet per Minute (CFM) of intermittent ventilation in bathrooms, such as a bathroom exhaust fan or 20 CFM continuous exhaust in bathrooms, such as an HRV. HRVs exhaust the moist air in bathrooms, kitchens, and other potentially hazardous areas and supply air to areas that require fresh air, such as bedrooms and living areas of the home. The exhaust and supply of an HRV should be balanced to not create a positive or negative pressure within the home. A slightly negative pressure is better than a slightly positive so that water vapor is not driven into the walls of the home. Although, if a garage is attached to a home, the garage pressure should always be negative of the home so the chemicals and carbon monoxide in the garage are not driven into the home. It can be a delicate balance between healthy and unhealthy air quality, but by only using natural air leakage and bathroom fans the balance is harder to achieve. With the use of an HRV, the balance is easier to control.
Bathrooms need a continuous exhaust of 20 CFM to exhaust moist air. A bedroom requires about 15 CFM per person supply air. I like to supply all bedrooms with about 15 to 20 CFM and a master bedroom with about 30 CFM of fresh supply air. Most people spend at least eight hours in their bedroom every 24 hours so why not have fresh air to breathe while you sleep? I know that I sleep a lot better knowing that I am breathing healthy air. Most standard sized homes with four people and a dog require about 120 CFM exhaust and 120 CFM supply air to produce a healthy and balanced system.
Often, indoor air quality is worse than outdoor. Studies have shown that intermittent ventilation cannot handle the pollutants that we bring into our homes. Cleaners, candles, furniture, fixtures, cabinets, pets, combustible appliances, and attached garages are just a few of the items that can bring unwanted hazards into the home. These pollutants just increase over time. However a home with a continuous ventilation system exhausts and dilutes pollutants. We can also control and filter the air we bring into our homes through the use of an HRV. There are too many potential air quality hazards to list but a good place to begin your research is the EPA website at www.epa.gov/iaq/ia-intro.html.
HRVs require about as much energy as it take to run a 100-watt light bulb each day and they also require twice per year maintenance. Filters and the heat core of a HRV should be checked and cleaned twice per year so that airflow remains unhindered and the machine works properly. HRVs require a condensate drain that should be checked for obstructions and the exhaust and supply ports on the exterior of the building should be checked regularly. HRVs work best if a home is tightly sealed and insulated properly. If installed properly and maintained regularly an HRV will supply years of healthy indoor air and efficiently.
A lot more work needs to be done on the education and research of home ventilation and indoor air quality. This series of articles only touches on the basics of home ventilation. New products and research keep the field interesting. As homes are built tighter and insulated more I am sure that home building and indoor air quality will continue to improve. All homes should be comfortable and safe to live in, as builders it is not only important to get the structure and aesthetics right, we also need to make sure that we also get the indoor air quality right.
Greg Stopher has over 16 years of experience in the construction field and earned a degree in Construction Technologies from the University of Alaska – Southeast. His company, Stopher Construction, LLC, is a general contracting company specializing in remodeling, custom finishes, additions and new home construction projects. He can be reached at 907-321-2350.