| Demand-Controlled Ventilation Installers |
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Basically, what is demand-controlled ventilation system designed to do?
Demand-controlled ventilation (DCV) system is designed to provide the amount of outside air the human occupants need, and no more. This accomplishes two things. First it saves energy by not heating or cooling unnecessary quantities of outside air. Secondly, it can provide assurance that sufficient outside air is being supplied for the number of occupants present.
How are DCV systems hooked up?
The heating, ventilation, and air conditioning (HVAC) system already contains ventilation hardware. The DCV system is typically a modification incorporating CO2 sensors into the existing or base system design. CO2 sensors commonly use 4-20 mA calibrated signals that are sent back to the HVAC computer or economizer. The computer or economizer must be able to accept these signals.
Occupancy sensors used for lighting control can be purchased with auxiliary contacts that will close when the space is occupied. Signals from the occupancy sensors will be sent to the HVAC computer for analysis. The computer will then adjust the outside air dampers accordingly. Turnstiles or ticket sale systems likewise will communicate with the HVAC computer to indicate the needed outside air.
How is the DCV system initially adjusted?
The CO2 sensor is normally factory calibrated. However, to assure proper performance, the sensor should be flooded with a reference test gas to check for accuracy. Another method is to use a second, hand-held unit as a reference. The sensor data should be reflected on the HVAC computer. At the same time, the HVAC system should be monitored to assure that the outside air dampers open to the expected position. Additional sensors are sometimes needed for critical spaces. If there are multiple CO2 sensors, they should all be tested. Current models of CO2 have built-in capability to do in-place calibration if needed by using a reference gas from a small cylinder.
Occupancy sensor sensitivity is normally adjusted at the sensor per manufacturer's instructions. It should sense occupants in the room, not those in adjacent spaces. The sensor may directly control the lights or HVAC in the room. In this case, an additional wire will be required to run to the HVAC system computer or ventilation controls.
If using occupancy sensors, a trip through each room of a particular HVAC system or zone will create an increase in the fresh air demand. Once all rooms have been sensed as "occupied," the outside air damper should be in its maximum position. The outside air damper will then close a pre-set amount as each lighting circuit timer turns to "unoccupied."
In both cases, the flow rate of outside air should be measured at maximum conditions and compared to designed values. These measurements should be made during balancing and commissioning. The designed value of outside air should not be reduced for "diversity" due to the fact that the dampers are closing when the rooms are unoccupied. This does not allow time to purge out stale air.
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DCV Guide:
DCV Systems and ASHRAE
Other links
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