Demand controlled ventilation consists on adjusting airflow according to the number of occupants, with the goal of reducing fan energy consumption. Less airflow also requires less heating and cooling, leading to additional HVAC savings. However, energy efficiency can be improved further with a heat recovery system.
Heat recovery ventilation (HRV) consists on exchanging heat between the outdoor air (OA) supply and the exhaust air. This reduces the workload for both air conditioners and space heating equipment.
- During summer, when indoor air is cooler, an HRV system can precool the outdoor air. This reduces the air conditioning cost, since outdoor air arrives at a lower temperature.
- The opposite process is carried out during winter, when indoor air is warmer. The HRV system preheats the outdoor air in this case, reducing the space heating load.
- As the temperature difference between indoor and outdoor air increases, there is more heat to exchange. Thus, the potential savings from HRV are higher when the outdoor temperature reaches extreme values.
These two ventilation systems, DCV and HRV, can achieve synergy, improving the overall efficiency of HVAC systems. While DCV reduces the cfm of outdoor air supplied to indoor spaces, heat recovery reduces the energy needed to heat and cool that air.
Energy recovery ventilation or ERV is an enhanced version of heat recovery, which also exchanges humidity between the outdoor air supply and the exhaust air. Thus improving HVAC systems efficiency even further, since it reduces humidification and dehumidification loads as well.
Consider that the humidity exchange feature makes an ERV system more complex and expensive. If local weather conditions provide no opportunity to save with humidity exchange, an HRV system is enough.
How DCV and HRV Operate Together
Demand controlled ventilation and heat recovery both ventilation systems that offer energy savings. However, the savings from each technology depend on different factors:
A DCV system saves more energy when a building is frequently at partial occupancy. When the number of people present is low, the ventilation airflow can be reduced more. DCV saves less when a building is close to full occupancy most of the time, since the cfm cannot be reduced by much.
There is an important difference when DCV is designed with the Indoor Air Quality Procedure (IAQP) from ASHRAE, since it responds to air pollutants rather than the number of people. In this case, the outdoor air supply can be reduced more when indoor air is clean, regardless of people present. However, several building codes only accept the Ventilation Rate Procedure and not the IAQP. CO2 concentration measurement for ventilation control is also a common requirement.
An HRV system achieves higher savings when the outdoor air is very hot or very cold. With a large temperature difference with respect to indoor air, the potential to precool or preheat the outdoor air is higher. The potential savings from HRV are smaller with moderate weather, since the temperature difference between the air inside and outside buildings is less.
DCV and HRV can operate independently, even when both interact with the outdoor air supply. An HRV system can exchange heat and achieve savings even when the DCV system is not reducing airflow. The opposite also applies: DCV can achieve ventilation savings even when there is little heat exchange between indoor and outdoor air.
Combining DCV and ERV in Building Certification Projects
Green building certifications such as LEED and WELL award points for both energy efficiency and indoor air quality. However, these two goals are sometimes in opposition.
- An increased outdoor airflow is normally beneficial for indoor air quality. However, this does not apply when the outdoor pollution level is higher. However, it consumes more fan power and requires more heating and cooling.
- The opposite also applies: An outdoor airflow reduction saves energy, but air pollutant concentrations tend to increase. However, airflow can be reduced without losing air quality, when pollutants are monitored with a dedicated device like Foobot.
- Heat recovery can be a very powerful tool in this case. An increased airflow leads to a higher energy consumption, but HRV can help compensate.
The combination of DCV and HRV is a very effective way to improve energy efficiency. HRV enhances the energy savings of DCV when airflow is reduced. On the other hand, it helps compensate the extra ventilation cost when airflow is increased.
The LEED and WELL certifications use different performance categories and scoring systems. However, both systems establish minimum performance requirements for energy efficiency and indoor air quality, and they give points for surpassing that level.
- DCV and HRV are ventilation systems that can both contribute to energy efficiency.
- A DCV system can respond to air pollutants, if equipped with the right sensors.
- Humidity control can be improved with an ERV system (enhanced version of HRV).
Air humidity is not a pollutant, but it has a detrimental effect on indoor air quality when left unchecked. Excessive humidity favors the growth of mold, dust mites and bacteria. On the other hand, excessive dryness causes dust, viruses and other particles to stay airborne longer.
DCV is effective as an energy efficiency measure on its own, and can also contribute to air quality if it responds to pollutant measurements. However, even greater savings are possible when heat recovery is added to the ventilation design. While the DCV system ensures a suitable outdoor air supply, the HRV system reduces the cost of heating and cooling that air.