|What is a demand-controlled ventilation system?
A demand-controlled ventilation system is an integral part of a building's ventilation design. It adjusts outside ventilation air based on the number of occupants and the ventilation demands that those occupants create.
What systems typically qualify?
Systems could include those based on CO2, occupancy sensing, turnstile counting, or methods where the building system tracks occupancy (through occupant activity) or ventilation demand.
What are the typical components for each type of system?
Many of the components that are used in controlling outside air may already be in place. These existing components could include an economizer or air makeup unit with modulating dampers. The additional components would be control sensors to communicate either directly with the economizer or with a central computer. Programming, while not a physical component, is a critical element of system operation.
What systems would have DCV?
Large assembly spaces such as gymnasiums, auditoriums, lecture halls, conference rooms, churches, and theaters are good candidates for DCV. These spaces are designed for large numbers of people with high outside air requirements. However, the spaces are frequently only partially occupied. It is expected that, in the future, most spaces with ventilation air capacities of at least 1,500 CFM and serving areas having an average occupant load factor of 20 or less will be designed with DCV features.
What critical components would be expected?
Each space being controlled should have some form of occupant sensing. One CO2 sensor can serve a single conference room. In some cases, multiple sensors would be needed. It would normally take multiple sensors for a large Variable Air Volume (VAV) system serving multiple spaces.
How are the sensors attached?
The sensors could be connected directly to the economizer or to the Direct Digital Control (DDC) system. Equally as important, the economizer or DDC system should be properly programmed to accept the sensor's input. Improper programming can negate any potential benefits. For example, if the system is set up to open up the outside air full-open at the first sign of people, it will over-ventilate unless the group of people is normally a very large group.
What drawings show the sensors?
The sensors should be indicated on the HVAC drawings. Notations should include sensor ID number and to which control device the sensor is attached. This will permit the drawing inspector to know which control sequence to review, as well as the installer to know where to run the signal wire.
What building codes apply?
The State of Oregon building codes
allow demand-controlled ventilation. A proposed code change would require demand-controlled ventilation for larger spaces with high occupant densities.
The State of Washingtonbuilding codes
allow demand-controlled ventilation via the use of "alternate methods" utilizing ASHRAE 62.
The State of Idaho adopted the International Building Code
(IBC) effective January 1, 2003. There is a clause in the IBC that permits reduced ventilation during reduced occupancy.
The State of Montana has adopted the Model Energy Code
for residences and ASHRAE 90.1 for commercial spaces. These ASHRAE standards allow for ventilation to be adjusted for variable occupancy. ASHRAE standard 90.1 allow for ventilation to be adjusted for variable occupancy.
How should the system be properly installed and tested?
There are different methods for confirming that the system is functioning properly. The sensor type, control hardware, damper hardware, and programming affect how the system should be checked. For example, a CO2-based system would require a known purge gas to be applied to the sensor. The system should detect the change and the dampers respond accordingly. The response to a change in occupancy sensing would depend upon the system design. An occupancy-based system that reported to an HVAC computer may modulate the outside air differently than a system where only the VAV terminal boxes were affected. The designers should provide a test plan to facilitate this inspection.
The programmed interaction between the economizer and DCV system should be checked carefully. When the system is in free cooling mode, it should override the DCV system. If indoor air quality becomes a problem, additional fresh air will be required.