Understanding Sensible and Latent Heat in ERV Systems

To get a clear picture of how an ERV system operates, it is important to understand the difference between sensible and latent heat. Sensible heat is involved in temperature changes, while latent heat is involved in phase changes such as water evaporation. An ERV system can be designed to exchange only sensible heat, or both.

• An ERV system that exchanges only sensible heat preheats or pre-cools the air intake depending on the season, but without humidity exchange.
• An ERV system that exchanges both latent and sensible heat can use the exhaust air to humidify or dehumidify the supply air.

For example, an ERV system that exchanges both sensible and latent heat can remove humidity from outdoor air before it reaches air conditioning equipment. In other words, humidity from the air supply is transferred to the exhaust airstream.

• Without ERV, the air conditioning system must handle the full cooling load and dehumidification load.
• However, the ERV system can reduce both loads simultaneously.

In new constructions, using ERV from the start may allow HVAC engineers to downsize the tonnage of air conditioning systems. This reduces not only their energy consumption, but also their upfront cost. ERV operation is reversed during winter, but the benefit is the similar: the unit reduces the total heating output required from furnaces or boilers.

ERV systems lead to more moderate load conditions on HVAC equipment during the entire year. In particular, they mitigate the highest cooling loads on hot summer days, and the highest heating loads on the coldest days of winter.

ERV Systems and Code Compliance

Like many other additions for HVAC installations, energy recovery ventilation is subject to building code requirements. Also note that not all weather and building conditions are suitable for ERV, and the investment may not be justified under some circumstances.

• In the case of New York City, the Energy Conservation Code covers ERV systems in section C403.2.7.
• ERV systems must also meet the ASHRAE 62.1 and 62.2 requirements for ventilation and indoor air quality.

HVAC engineers can specify the most suitable ERV system according to your building needs and local weather conditions. They are also familiarized with code requirements and can help you get a quick project approval.