Use this, not that: Choosing ventilation systems

By Steve Konstantino

THE BUILDING SCIENCE COMMUNITY widely agrees that ventilation is needed in homes, particularly newer ones constructed with good air-sealing techniques and materials. The earlier approach to this was to use bathroom exhaust fans that would run at a constant low speed and allow outside (fresh) air to enter the home through holes and cracks in the building envelope. However, this method can bring air and moisture into the envelope, where it is not wanted, and provides no filtration of incoming air.

Today, the preferred method is to use a “balanced” ventilation system such as an ERV (Energy Recovery Ventilation) or an HRV (Heat Recovery Ventilation). They are called balanced ventilation systems because they bring in the same amount of air that they exhaust. These systems are composed of a control unit and an insulated box containing fans, filters and dampers.

There is also a general consensus that an ERV is the better choice for most climates, including cold climates like Maine’s. An ERV recovers some heat and moisture while performing its primary job of providing fresh air and exhausting indoor pollutants and CO2. HRVs recover only heat and can lead to over-drying a home in winter.

The purpose of balanced ventilation systems is to improve indoor air quality. They do not provide heating, cooling or summertime dehumidification. And because mechanical ventilation consumes energy, it is best to choose equipment that uses the least amount of energy to get the job done.

New homes need balanced ventilation. Along with the tips below, learn the differences between manufacturers and models, which can differ significantly.

An ERV is a balanced ventilation system, meaning that it brings in as much air as it exhausts. It recovers some heat and moisture while performing its primary job of providing fresh air and exhausting indoor pollutants and CO2.

What to consider when choosing an erv or hrv

When choosing an ERV or HRV, these are the factors that I consider, in order of importance:

It must have an electronically commutated motor (ECM). Fans consume electricity and they need to run almost all the time. So choose an ERV with an ECM fan. These motors use about half the power of ordinary ones and are quieter and easier to control to get a slower speed.
Solid core exchanger. The exchanger is where the heat and moisture are transferred from the outgoing air to the incoming air. Choose equipment that has a solid core that is easy to remove and clean. Some systems have a larger revolving wheel as an exchanger. These have an additional motor that uses some electricity, adding another component that may break or add unwanted noise.
Solid construction that’s easy to clean and maintain. Choose a model with a solid box, good insulation and quality collars and fittings. ERV systems need to have their filters replaced and their core cleaned a few times a year, so you want it to be easy to service.
MERV 13 or better air filter. Most systems have at least two filters, one for incoming air and one for outgoing air. These help keep the core clean and filter out dust from incoming air. Choose MERV 13-rated filters or higher. It’s important to note that many have a third filter for occasional recirculation; these often have a lower MERV rating, which is fine for that purpose.
Defrost strategy. Some systems need electric heat to keep the core from freezing and some use a recirculation strategy. They both have merits. The electric resistance method will use more electricity in below freezing temperatures. Know what you are choosing.
Good efficiency rating. This is related to the efficiency of the exchange core. The higher the number, the more heat and moisture are captured during the exchange. Pick a system that has a rating above 70% at 32ºF outdoor temperature.