ASHRAE recommends increasing outdoor airflow as much as possible to help prevent COVID-19 in buildings. This entails a significant energy cost, as more outside air needs to be conditioned. However, an energy recovery ventilator (ERV) or airside economizer can help offset the additional cost.
Before installing an ERV system or airside economizer, building owners should obtain an assessment from a professional HVAC engineering firm. The effectiveness of these technologies varies depending on various conditions, including local climate. Two identical airside economizers can have varying results when used in different locations, and the same applies to energy recovery ventilators.
Improve your ventilation system and indoor air quality and reduce the risk of COVID-19 transmission.
Coronavirus prevention takes priority over energy efficiency until the outbreak is contained. However, there are still opportunities to save energy without compromising the health of building occupants. For example, demand-controlled ventilation (DCV) systems reduce outdoor airflow in response to occupancy, and ASHRAE recommends disabling them to allow more outdoor air. However, an airside economizer does not interfere with ventilation of outside air and the system may even allow greater ventilation under some weather conditions.
How an airside economizer can increase outside air ventilation
A building needs heating or air conditioning depending on the time of year. Some applications require permanent heating or cooling, depending on the time of year. For example, a large data center may need 24/7/365 cooling to manage all the heat released by IT equipment.
Under some weather conditions, outside air may be of suitable temperature and humidity to be used instead of air conditioning. An airside economizer can detect this and the outside air supply increases. Because outside air provides free cooling, air conditioning systems can reduce their output, saving energy. Under the most favorable conditions, the air-side economizer can take on the total cooling load of the air conditioning systems.
During the coronavirus emergency, an airside saver has a double benefit. It contributes to increasing the outdoor air flow recommended by ASHRAE, while saving on air conditioning. The energy savings and external ventilation of an airside economizer can be further increased by setting the thermostat to a higher level during the summer. For unoccupied buildings, ASHRAE recommends a cooling set point of 80°F and a maximum relative humidity of 60%.
Using energy recovery ventilation with increased outdoor airflow
There are two types of ERV systems: a heat recovery fan (HRV) exchanges only heat between both air streams, while an enthalpy recovery fan (ERV) exchanges heat and moisture. These systems can save a lot of energy, especially when outdoor airflow increases. However, if mixing is not controlled between the two air streams, ERV can have a negative effect on indoor air quality.
The AHRI 1060 Standard used a metric called Exhaust Air Transfer Ratio (EATR) to describe air mixing in ERV systems. The EATR should ideally be kept below 5%, but well-designed systems can go as low as 1-3%. Air mixing in an ERV can be minimized by maintaining a higher pressure on the air supply side.
It is not possible to eliminate air mixing, but the ERV can allow for a greater supply of outside air with less recirculation. The impact of mixing in an ERV system on air quality is small compared to the negative effect of indoor air recirculation. An ERV system also improves humidity control, and the building can stay within the recommended relative humidity range of 40-60% more easily. This range of humidity minimizes biological risks such as viruses, bacteria and fungal growth.
Conclusion
When buildings increase their outdoor airflow, as recommended by ASHRAE to prevent COVID-19, airside economizers and energy recovery ventilators can help conserve energy. Using an airside economizer is simple, as long as the local climate is suitable. However, an ERV should only be deployed if air mixing can be minimized.
Regardless of how the ventilation system is configured, ASHRAE recommends high-efficiency air filters combined with UVGI (Ultraviolet Germicidal Irradiation). There are many air purification methods, but ASHRAE prefers the combination of outdoor air augmentation, MERV 13 or better filters, and UV disinfection.
