What Is An ERV (Energy Recovery) And How Does It Work?
Maintaining indoor air quality is vital to the health and well-being of a building’s occupants. An Energy Recovery Ventilator (ERV) allows fresh air into a building, while retaining pre-conditioned heating or cooling.
Let me better explain. Let’s assume you have a 4 Ton Unit (Click Here to understand and interpret HVAC Tonnage), and are required by code to introduce 25% fresh outside air (in this case 400 CFM) into your building. With a standard economizer you draw 400 CFM of outside air, then proportionately remove the same quantity of air using a Barometric Relief, or Power Exhaust.
If the outside temperature is 0ºF, and the indoor air temperature is 70ºF, your HVAC unit must heat the mixture of outside air & building’s return air to maintain a building temperature of 70ºF.
Coincidentally, 400 CFM of pre-conditioned 70ºF air was forced out your building to meet the fresh outdoor air code. What if you could recover that energy?
An ERV is designed to do just that, recover pre-conditioned air before it is removed from the building.
How Does An ERV Work?
Hypothetically speaking… Two air-streams travel in opposite directions; one at the temperature of 0ºF, and the other at 70°F. Most would assume each air stream would balance out to the average of 35ºF each.
The air-streams actually switch temperatures; the 70°F becomes 0ºF, and the 0ºF becomes 70°F. This is the core theory behind Energy Recovery AKA — Air Exchange.
An ERV operates on the basis of this air-to-air exchange theory. A spinning wheel, or stationary core device utilizes the counter-flow of air to remove the stale air from inside a building, while retaining air temperature.
However, an ERV wheel or core cannot produce a 100% theoretical energy recovery result. Therefore, they are rated to return a percentage of effectiveness (Certified By AHRI) depending upon your environmental conditions. These percentages will vary under different environmental conditions, and are usually defined as summer and winter conditions. Illustration #2 displays an ERV Wheel offering an 80% Energy Recovery result (14ºF ÷ 70°F = 0.8 — equivalent to 80%). This 80% recovery introduces a mild 56ºF to mix with the indoor air, instead of 0ºF.
Other ERV products exist. For the sake of simplicity this article will only address desiccant wheel and core theory.
Replaces An Economizer
An economizer will meet code by introducing, and mixing the return air from the building with the outside air, and in most cases contains a barometric relief to remove the pre-conditioned air. However, it performs the operation at a higher energy cost to the building owner. How? By removing conditioned air and forcing the HVAC Unit to recondition fresh outside air when Free Cooling isn’t available.
Free Cooling occurs during cooling mode when the outside temperature & humidity is ideal. The economizer will shut off the compressor, while allowing the HVAC supply fan to draw in cooler outside air at the approximate temperature that the HVAC unit would produce & supply mechanically into the building.
An ERV has an additional advantage over economizers by extending the free cooling range. A properly configured ERV will allow you to utilize free cooling using lower outdoor temperature set-points due to the advantage of temperature recovery.
ERV Unit ratings are based upon CFM, and offer two types of Energy Recovery; sensible and latent.
Sensible Recovery is a temperature recovery as described above.
Latent Recovery involves the environmental factor of humidity.
Downsizing Your HVAC Unit
An ERV is designed to transfer Sensible only, or Sensible & Latent combined.
Latent allows the removal of outside air humidity before it reaches your HVAC Unit. It will literally redirect a large percentage of the outdoor humidity by transferring it to the opposing airstream — out of the building, allowing the HVAC to operate with a reduced load. This reduction allows an engineer to downsize the tonnage of a HVAC Unit at the design stage. Why, because an ERV changes the environmental conditions.
Same applies with the Sensible factor: The HVAC unit is no longer contending with 0ºF on that cold day illustrated above, or high humidity and temperature in July. Instead the HVAC unit is confronted with more year round moderate conditions. Therefore, the HVAC unit can now be downsized because the overall load conditions have changed.
Many More Factors & Applications
Energy Recovery is very complex. Many other factors not noted in this article come in to play and for the sake of simplicity are not noted here. Facilities from Natatoriums, to Animal Shelters to Hospitals have very strict indoor air qualities. Each one necessitates a specific design requirement. Pairing the proper ERV design specification to the job is critical.
ASHRAE Standards 62.1, and 62.2 – Offers Standards for Ventilation and Indoor Air Quality.
ASHREA Standard 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings
The MicroMetl Blog will continue to delve further into many of these critical issues. Please feel free to contact our Customer Support Department with any, and all of your ERV inquires.