Issue
Recommended method for replacing an MR-VAV-AX controller. Where to get ASC Parameters for the new device.
Environment
- I/NET software all versions
- MR-VAV-AX controller
Cause
An MR-VAV-AX needs to be replaced on an existing VAV box.
Resolution
The ideal way to change an MR-VAV-AX or similar would be to use an airflow hood to calibrate when the device must be changed. However, if no airflow hood is available, see the options below:
- Obtain the ASC parameters of the existing MR before removing the device from service. Re-use the original settings.
- If these values cannot be recovered, then ask the customer for the original drawings or T&B reports on the device in an effort to properly establish correct duct diameter and total airflow. Rarely is this information readily available at the time of the change.
-
Without such info, an estimation will have to be made. A rule of thumb is:
- 250 CFM per diffuser and an eyeball estimate of the duct diameter
- The pickup factor is left at factory default (3) or compared to other similar devices and an average of other pickup factors is used.
- For setting CHF, base it on the above estimate of 250 CFM per diffuser and make sure that the box just reaches 100% damper at the maximum airflow it is physically capable of at the AHU's normal static
- CLF is an estimate based on the use of the space and the temperature needs of the occupants. CLF is about 10% of the CHF.
