Residential Mechanical Ventilation – The Minnesota Solution

The state of MN has been at the forefront of adopting residential building codes that incorporate energy efficiency designs for residential mechanical ventilation. These codes have additional provisions which address some of the serious side effects such designs can have on the home’s inhabitants and structural integrity.

Back in the early 1980’s, these energy efficient building designs introduced highly insulated air tight homes. These designs significantly reduce a home’s energy consumption but created adverse consequences that could not go ignored. Extensive research conducted over the past 30+ years was the driving force behind the current 2015 Minnesota Energy Code. Considered controversial by the builders and realtors, primarily due to the code’s cost impact on a home’s sale price, this code is also viewed as a showcase for all the advocates of the advancement of building science.

The 2015 MN Mechanical Ventilation Code

In the past, many residential ventilation systems were either unbalanced or exhaust only; typically, exhausting more air than was brought into a home. This approach was generally considered “easier” and” less costly.” The state of MN however, decided that the benefits of a balanced ventilation system far outweighed any cost concerns.

The 2015 MN Residential Energy Code requires a total ventilation rate based on the total square footage and the number of bedrooms there are in a home. A minimum of 50% of the total ventilation rate must be met by a continuous and balanced whole-house mechanical ventilation system. Kitchen and bath fans can be a part of the overall ventilation system, but an HRV/ERV system alone can meet both the total and continuous requirements of the current code. Such multi-purpose systems use minimal amounts of energy, are energy efficient (many models are Energy Star rated) and improve the indoor air quality of a home.

New homes already adhere to the 2015 MN Residential Energy Code. Builders are now required to install these ventilation systems during construction, but homeowners can retrofit older homes with an HRV/ERV system also. Though it is not required to add a balanced system to homes built before February 14, 2015, it is a relatively simple modification that connects to a home’s furnace ductwork to simultaneously exhaust indoor air and bring in outdoor air.

The Drawbacks of Unbalanced Mechanical Ventilation and Negative Pressure in Homes

An unbalanced mechanical ventilation system involves fans exhausting and/or supplying air in unequal amounts into a home. When the supply air exceeds exhaust air, it creates positive pressure in a home. When exhaust air exceeds supply air, it creates negative pressure. Homes operating under positive and negative pressures can affect the safe operation of fossil fuel appliances and expose a home’s inhabitants to serious health risks over time; toxic gasses and airborne contaminants such as radon, mold, allergens, bacteria and viruses. A balanced ventilation system in homes negates pressure differential consequences and helps minimizes the impact of such health risks by effectively diluting their concentrations in an efficient manner.

Negative pressure can cause back drafting, a likely reason why your fireplace doesn’t draft very well. Large exhaust devices such as kitchen range hoods (generally over 300 cfm) pull significant amounts of air out of a house, your fireplace chimney (when open) will be trying to bring air back into the structure to reach a point of pressure equilibrium. To remedy such situations, a make-up air device can be installed which is capable of mechanically bringing back into a home the same amount of air that’s being exhausted by the range hood. Make-up air requirements are also a part of the 2015 MN Energy Code.

The Air in Our Homes

The movement and types of air within a home is not a simple matter. The 2015 MN Residential Energy Code is very specific in its requirements. It outlines not only the need for ventilation in homes but also addresses combustion air (air needed for fossil fuel appliances), make-up air (air that deals with sources of significant negative pressure) and provisions to protect homeowners from radon (a soil gas commonly found in MN that contains carcinogenic particles). Most homeowners are unfamiliar with building codes so finding a competent builder and HVAC contractor is highly recommended.

It’s good to see that most new single family homes in MN today are implementing HRV/ERV balanced ventilation systems to meet the total ventilation requirements of the MN Energy Code. Homeowners of older homes need not despair, for retrofitting an existing home with a balanced HRV/ERV ventilation system is not overly complicated or expensive.