ERVs

How an ERV Works

An ERV operates using a central component called a heat exchange core. This core is a complex matrix of plates or channels that keeps the incoming and outgoing air streams separate while allowing for the transfer of heat and water vapor.

1. Incoming Air: Fresh but often warmer, colder, or more humid outdoor air is drawn into the ERV.

2. Outgoing Air: Stale, contaminated indoor air is simultaneously exhausted from the building.

3. The Exchange: The two air streams pass through the heat exchange core.

   • In Winter: The warm, humid outgoing air transfers its heat and some of its moisture to the cold, dry incoming air. This pre-warms the fresh air, reducing the energy needed for the furnace or heat pump to bring it to a comfortable temperature. The transfer of moisture also helps to humidify the incoming air, which can be beneficial in dry winter climates.

   • In Summer: The cool, dry outgoing air transfers its "coolness" and some of its dryness to the hot, humid incoming air. This pre-cools and dehumidifies the fresh air, reducing the energy needed for the air conditioner to cool it down.

This process is a form of counter-flow heat exchange, where the two air streams flow in opposite directions to maximize the efficiency of the transfer.

Key Components of an ERV

• Heat Exchange Core: The central component responsible for transferring both sensible heat (the kind you feel with a thermometer) and latent heat (the energy in water vapor). This distinguishes an ERV from a Heat Recovery Ventilator (HRV), which only transfers sensible heat.

• Fans/Blowers: Two dedicated fans, one for the supply air (bringing fresh air in) and one for the exhaust air (pushing stale air out). These fans are designed for quiet, continuous operation.

• Filters: Both the incoming and outgoing air streams are typically filtered to protect the heat exchange core from dust and debris and to prevent pollutants from entering the building.

• Drain Pan: Because moisture is transferred, particularly during summer operation when condensation can occur, most ERVs are equipped with a drain pan to remove excess water.

• Controls: A control system allows for different modes of operation, such as continuous low-speed ventilation, boost mode for high-occupancy periods, and defrost cycles in cold climates to prevent ice buildup on the core.

ERV vs. HRV: A Crucial Distinction

While often confused, ERVs and HRVs (Heat Recovery Ventilators) serve different purposes and are best suited for different climates.

• HRV (Heat Recovery Ventilator): Transfers only sensible heat. They are ideal for cold climates with long heating seasons, as their primary goal is to recover heat from outgoing air without transferring excess moisture. This helps prevent condensation and frost buildup in the core in very cold temperatures.

• ERV (Energy Recovery Ventilator): Transfers both sensible and latent heat. This makes them superior in mixed or hot, humid climates where controlling humidity is as important as controlling temperature. By recovering moisture, an ERV can reduce the dehumidification load on the air conditioning system in summer and prevent over-drying of indoor air in winter.

Why Use an ERV?

1. Improved Indoor Air Quality (IAQ): Modern buildings are built to be airtight for energy efficiency. While this is great for insulation, it can lead to a buildup of indoor pollutants, such as volatile organic compounds (VOCs), carbon dioxide, and allergens. An ERV provides a continuous supply of fresh, filtered air, effectively diluting and removing these contaminants.

2. Energy Efficiency: By recovering energy from the exhaust air, an ERV can reduce a building's heating and cooling costs by up to 80% compared to opening windows for ventilation. This makes them a critical component for achieving high-performance building standards like Passive House.

3. Moisture Control: An ERV helps to balance humidity levels, preventing excessive dryness in winter and high humidity in summer. This not only improves comfort but also helps prevent mold growth and damage to building materials.

4. Meeting Codes and Standards: As building codes become more stringent regarding ventilation and energy efficiency, ERVs are increasingly becoming a requirement for new construction. They are an essential tool for architects, engineers, and builders to meet or exceed standards from organizations like ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers).