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MR-VAV-AX Replacement CFM Settings

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:

  1. Obtain the ASC parameters of the existing MR before removing the device from service. Re-use the original settings.
  2. 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.
  3. 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.
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