Piping low port of a duct Static Press Probe for VFD control?

[ddcman]

Typically, the high port of a duct static pressure transmitter is piped 2/3 distance down the longest supply duct when used for control of an air handling unit variable frequency drive. It is common practice to simply leave the low port vented to atmosphere. This causes problems if the transmitter is mounted at the control panel in a mechanical room if the room is used as a return air plenum. Does anyone have any recommendations for the best place to pipe the low port in this application?

 

What you're trying to do in most cases with the static pressure transmitter is verify the difference in duct pressure and "representative room" pressure. Using the mech room as the low isn't a good idea for the low port because the Delta-P measured by the p-sensor will be too high because the mech room is too negative. If piping length of the low doesn't get too long, try piping the low to a good "representative room" that is one of prime focus for conditioning and air balance.