Maintaining healthy indoor air quality is essential for building comfort, safety, and energy efficiency. An Energy Recovery Ventilator (ERV) brings fresh air into a building while capturing a significant portion of the heating or cooling energy that would normally be lost.
Here’s a simple breakdown of ERV fundamentals.
Why Energy Recovery Matters
Commercial buildings are required to bring in a specific amount of fresh outdoor air. For example:
- A 4-ton rooftop unit may require 25% fresh air, equal to about 400 CFM.
- That same 400 CFM of indoor air must be exhausted to maintain balanced pressure.
On a freezing day, that means pulling in 0°F air and throwing away 70°F air that you already paid to heat. Without recovery, the HVAC system has to work harder to maintain indoor temperature—raising energy costs.
An ERV solves this by capturing energy from the outgoing airstream before it leaves the building.
How an ERV Works
An ERV contains either:
- A rotating energy wheel
- A fixed, stationary core
Both designs move two airstreams in opposite directions—fresh outdoor air and stale indoor air—through a heat-exchange section.
Although completely separate, the airstreams pass close enough that heat and moisture transfer between them. The result:
- Outdoor air is preheated in winter
- Outdoor air is precooled and dried in summer
- Indoor air leaves the building while giving up its energy
This process is known as air-to-air heat exchange.
Example: Winter Operation
- Indoor air: 70°F
- Outdoor air: 0°F
- ERV efficiency: ~80%
The ERV returns outdoor air at ~56°F instead of 0°F.
That means your HVAC unit only needs to raise the remaining 14°F to reach the desired temperature—dramatically reducing load.
Types of Energy Transfer
ERVs provide two forms of recovery:
Sensible Recovery
Recovers temperature (heat or cool).
Latent Recovery
Transfers moisture—reducing humidity load on the HVAC system.
Latent recovery is especially valuable in humid climates, helping maintain comfort and reducing the need for oversized equipment.
ERV vs. Economizer
Both ERVs and economizers introduce fresh air, but they operate differently:
|
Feature |
Economizer |
ERV |
|
Introduces fresh air |
✅ |
✅ |
|
Recovers energy |
❌ |
✅ |
|
Extends free cooling range |
Limited |
Enhanced |
|
Handles humidity |
❌ |
✅ (with latent capability) |
|
Reduces HVAC size |
No |
Yes (in many designs) |
Economizers are great for “free cooling” during mild weather, but when conditions are extreme, they bring in unconditioned air that raises operating costs. ERVs solve this by moderating the air before it reaches the HVAC unit.
Can an ERV Reduce HVAC Tonnage?
Yes—when properly designed.
Because an ERV moderates incoming air temperature and humidity, the rooftop unit:
- Sees less extreme conditions
- Requires less capacity to maintain comfort
- Can often be downsized or engineered with lower load requirements
This benefit depends on climate, building type, and ventilation rate.
Applications and Considerations
ERVs are used in spaces where ventilation load is significant or indoor air quality is critical, including:
- Schools
- Offices
- Fitness centers
- Hospitals and healthcare
- Natatoriums (pools)
- Animal care facilities
- High occupancy spaces
Proper selection must account for:
- CFM requirements
- Static pressure
- Efficiency percentage (AHRI-certified)
- Installation environment
- Humidity and climate zone
- Allowed exhaust-to-supply leakage (EATR)
Designing or replacing an ERV requires careful evaluation—not simply matching model numbers.
Standards to Know
ERV design and performance are aligned with industry standards:
- ASHRAE 62.1 and 62.2 – Ventilation & Indoor Air Quality
- ASHRAE 90.1 – Energy Standard for Buildings
These define minimum ventilation rates and performance expectations for commercial systems.
Final Thoughts
Energy recovery may look simple on the surface, but proper selection, integration, and performance evaluation involves many variables. When replacing aging ERVs, it’s important to re-evaluate the application and performance goals—not just replace “like for like.”
If you have questions about ERV principles, design considerations, or integration with rooftop systems, MicroMetl is here as a resource. Reach out to our technical team or share your question—we may feature it in a future educational post.
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