There is a lot of talk about Covid-19 (and other airborne transmittable diseases) and leakage rates in energy recovery ventilation units. This is an important conversation but it requires perspective.
Energy Recovery Ventilation Unit (ERV)
Most energy recovery ventilation units are used as part of a Dedicated Outdoor Air System (DOAS) that is paired with a separate HVAC system such as VRF, fancoils, WSHPs and chilled beams. The DOAS system is a single pass system delivering ventilation air to the space and removing an equal amount of room air with no intended recirculation. Some form of energy recovery device in the DOAS ventilation unit is often used to greatly reduce the energy use and improve operating cost and carbon footprint of the building. There is usually some amount of air that can leak between the supply and return air paths that leads to unintended recirculation.
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Air to Air Energy Recovery
Most ventilation systems include an air to air energy recovery device to reduce the energy cost of conditioning outdoor air by as much as 80%.
Depending on location and application, the ventilation air can be one of the largest loads in the HVAC system. Energy recovery in the ventilation system is a major contributor in improving the Energy Use Intensity or EUI of the building. It is so important that air to air energy recovery use is spelled out clearly in both ASHRAE Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings and the Canadian Energy Code.
All air HVAC systems such as Variable Air Volume (VAV) are also very popular. These systems integrate the ventilation function into the heat and cooling function. They remove air from the occupied space and return it to an air handling unit where the air is filter, heated or cooled and then return it to the space. A small portion of the air is exhausted from the building and is replaced with outdoor air providing the necessary ventilation. A VAV system is connected to multiple zone in the building so the bulk of the air is not only recirculated, it is mixed from one zone to another as a part of normal operation.
Let’s compare a VRF system with DOAS to a VAV system for a typical office application.
VRF With DOAS
Variable air Volume (VAV) System
DOAS System with Demand Control Ventilation
VAV System
The image above shows a DOAS system with additional Demand Control Ventilation (DCV). DCV allows the system to vary the actual ventilation airflow rate based on indoor air quality. For this comparison, we will assume the DCV has been turned off and the system is running at design ventilation airflow rate (turning off DCV is recommended by ASHRAE and others during epidemic situations).
The image above shows a VAV system with two control zones (typically there are more than just two zones). The Air handling unit serves both as the conditioning unit for indoor air and the ventilation unit for introducing outdoor air to the building.
The Ventilation airflow rate for offices is based on ASHRAE Standard 62.1 – Ventilation for acceptable Indoor Air Quality and is around 0.11 cfmoutdoor air/ft². The DOAS system will deliver 0.11 cfmoutdoor air/ft² through the building and it will exhaust an equal amount.
The Ventilation airflow rate for offices is based on ASHRAE Standard 62.1 – Ventilation for acceptable Indoor Air Quality and is around 0.11 cfmoutdoor air/ft². The air handling system will deliver 0.11 cfmoutdoor air/ft² through the building and it will exhaust an equal amount.
In addition, the ventilation units will filter, heat and cool the outdoor air as necessary based on outdoor climate conditions. For most locations, some form of energy recovery will be used to greatly reduce (as much as 80% reduction) the energy required to heat and cool the outdoor. The ventilation air will not be recirculated and it will not be mixed from one part of the building to another.
The air handling unit will introduce outdoor air into the recirculated air in the mixing box. The combination of recirculated and outdoor air will be delivered through the building to meeting space heating and cooling requirements. Recirculated air from one part of the building will be delivered to other parts of the building.
The actual heating and cooling of the space will be accomplished by the VRF system. There will be a local terminal unit within the control zone the will recirculate air through the terminal unit filtering, heat and cooling the air as necessary to maintain room temperature. Air will not be recirculated from one part of the building to another as a normal part of the operation.
To meet typical heating and cooling loads the VAV system will be able to deliver around 1 cfm/ft². The air will be removed from the room and returned to the air handling unit (AHU) where it will be filtered, heated and cooled as necessary. In a VAV system, the air delivered back to the space around 55°F. The amount of air delivered is varied by a “VAV box” based on the cooling load typically around 0.3 to 1.0 cfm/ft². Approximately 11% is outdoor and the rest is recirculated.
Plate, rotor and heatpipe air to air energy recovery devices have some amount of leakage. This can range from 0.5 to 10% depending on the product and the application. Using 1% leakage of 0.11 cfmoutdoor air/ft² means the recirculated amount is around 0.0011 cfmrecirculated air/ft².
The mixing box in an air handling unit by design will mix 89% return air with 11% outdoor air so the recirculated portion is 89%.
There is nothing wrong with VAV or VRF HVAC systems, they are excellent systems when applied correctly. As well, there are many more considerations when selecting an HVAC system that is not covered in this article. The point is all HVAC systems recirculate air to balance the energy cost of operating the system with maintaining good indoor air quality.
The best advice for operating HVAC systems during epidemic periods is to operate the system at the highest ventilation rate possible without losing control of space thermal comfort. Poor space comfort will stress the occupants and may make them more susceptible to contracting the disease.
For a DOAS system, turn off DCV (if the system has it) and operate the maximum airflow rate possible. Use a pre-purge and post purge before and after operating hours to clear the building. The main HVAC system can be shut down during unoccupied hours.
Shutting down the energy recovery device is not recommended. With the fans running, the ventilation unit will have whatever leakage rate whether the device is operating or not. The energy cost of the building will increase (potentially significantly) and the HVAC system may not be able to maintain good thermal comfort without the assistance of the energy recovery device.
For all air systems such as VAV, increase the percent of outdoor air to the limit of the unit’s capability (the unit was not likely designed to be able to heat and cool large amounts of outdoor air). Use a pre-purge and post purge before and after hours to clear the building.
