Variable Air Volume Ventilation
Standard practice in California for many years has been use of constant volume systems, which leads to over-ventilation. As a default, many areas of a hospital are reheating all the time. Those areas can be operated as VAV, reducing the need for reheat. This can reduce ventilation.
Cost to implement 💰♦♦♦♦
Carbon reduction potential 🌲🌲🌲🌲♦
CAV to VAV conversions require redesigning the room ventilation schedule to ensure all space pressurization requirements are met as per applicable code. If terminal units with modulating dampers and VAV capability exist, implementing the measure will only require reprogramming the terminal box minimums, supply and return/exhaust offset, and a final air balance to ensure minimum air changes per hour (ACH) and pressurizations are met.
How does this decarbonize?
Per the ASHRAE Advanced Energy Design Guide for Large Hospitals, CAV reheat systems use the most reheat energy, and they are not recommended. VAV reheat systems reduce reheat energy, especially in unoccupied periods when air change per hour (ach) requirements generally do not apply, but reheat is still often the largest component in energy models of these systems. Depending on core space air change requirements and space load, this may also eliminate the need for reheat in core spaces with the CAV to VAV conversion VAV systems reduce ventilation levels based on each space's demand. Reduced ventilation means less reheat for that air and less overall demand for boiler heating. VAV systems, on average, are 30-40% more energy intensive than VAV systems.
Where has it been done before?
Kaiser Permanente as done that at a number of facilities including South Bay Medical Center, where a demonstration project was completed to convert a CAV system to VAV while monitoring IAQ throughout the facility to ensure air quality was maintained at acceptable levels for occupant safety and comfort. They have also done VAV conversions at Woodland Hills Medical Center.
What are the barriers to successful implementation?
As of the 2019 California Building Code, OSHPD code requires return boxes for VAV systems. Return boxes are a costly addition and will not help the payback for retrofit applications.
How to implement
Conversion to VAV can be as simple as changing the control programming on a terminal unit or as involved as replacing pneumatic or mechanical terminals with Direct Digital Controls (DDC) VAV terminals. The Kaiser South Bay site required reprogramming the terminal box minimum ventilation levels followed by an air balance.
This measure typically costs $1-5/sf with a payback of 1-5 years.
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