HRVs

How an HRV Works

An HRV operates on a simple but effective principle using a central heat exchange core. This core is a series of plates or channels that keep the two air streams separate while allowing heat to transfer from the warmer stream to the colder one.

1. Exhaust Air: Stale, warm air from inside the building (e.g., from kitchens, bathrooms, and laundry rooms) is drawn into the HRV.

2. Supply Air: At the same time, fresh, cold air from outside is drawn in.

3. The Exchange: The two air streams pass through the heat exchange core. The warm exhaust air transfers its sensible heat—the heat that can be measured with a thermometer—to the cold supply air. The two air streams never mix; the heat is transferred through the core's conductive material.

4. Distribution: The now pre-warmed fresh air is then distributed throughout the building, and the cool exhaust air is expelled outdoors.

This process is a form of counter-flow heat exchange, which maximizes efficiency by allowing the coldest air to meet the coolest part of the core, and the warmest air to meet the warmest part, ensuring optimal heat transfer.

Key Components of an HRV

• Heat Exchange Core: This is the most crucial part of an HRV. Made of materials like plastic or aluminum, it's designed to conduct heat efficiently between the two separated air streams.

• Fans: HRVs use two separate fans, one to bring in fresh air and one to exhaust stale air. These fans are engineered to be quiet and energy-efficient for continuous operation.

• Filters: Both incoming and outgoing air streams are filtered to protect the heat exchange core and prevent the entry of outdoor pollutants like pollen or dust. Regular filter maintenance is essential for performance.

• Drain Pan: Unlike an ERV, an HRV does not transfer moisture. As the warm, humid indoor air is cooled by the core in winter, condensation forms. A drain pan collects this water, and it's then routed to a floor drain or condensate pump.

• Controls: A control unit allows the user to operate the HRV at different speeds (e.g., low-speed continuous ventilation or high-speed "boost" mode for a party or after a shower). Many models also have a defrost cycle to prevent ice buildup on the core in very cold weather.

HRV vs. ERV: When to Use Which?

The key difference between an HRV and an ERV (Energy Recovery Ventilator) lies in their ability to transfer moisture.

• HRV (Heat Recovery Ventilator): Transfers only sensible heat. It's the ideal choice for cold climates with long, harsh winters. By not transferring moisture, an HRV helps to avoid excess humidity buildup in the home during winter, which can lead to condensation on windows and other surfaces. The exhaust of humid air also helps prevent ice from forming inside the heat exchange core.

• ERV (Energy Recovery Ventilator): Transfers both sensible and latent heat (the energy in water vapor). This makes an ERV superior in mixed or hot, humid climates. In summer, the ERV pre-dehumidifies the incoming fresh air, reducing the workload on the air conditioner. In winter, it prevents the incoming air from becoming too dry.

Why Use an HRV?

1. Healthier Indoor Environment: Modern, airtight construction, while energy-efficient, can trap indoor pollutants like carbon dioxide (CO2), volatile organic compounds (VOCs) from building materials, and cooking odors. An HRV provides a constant supply of fresh air, diluting and removing these contaminants to improve IAQ.

2. Energy Savings: By recovering the heat from the exhaust air, an HRV significantly reduces a building's heating costs. This is a critical factor for achieving high-performance building certifications like Passive House.

3. Moisture Control: In cold climates, HRVs prevent excessive indoor humidity, which can lead to mold growth, mildew, and damage to building components. By exhausting moisture-laden air, an HRV helps maintain a healthier and more durable indoor environment.