Exhaust duct static pressure transmitter

[chadtech]

I am needing to install a exhaust duct static pressure transmitter, it will be connected to control system and modulate bypass dampers to maintain 3.5" w.c static.

The exhaust is for lab exhaust fume hood, where should the transmitter reference point be? Lab space or outside air?

[TriSTar]

You want to maintain a dust static pressure most of the time. These systems are tricky depending on how they are laid out and the number of fans involved. Lab exhaust also generally requires a minimum stack velocity to move the exhaust air away from the roof and personnel. You probably have a constant volume fan, like a Strobic fan, which has a modulating damper which provides the make up air to the fan when the lab valves, typically Phoenix valves, go into an unoccupied mode. No matter, you still probably want to maintain the exhaust duct static pressure. Your response times will need to be very smooth to prevent hunting with the modulationg damper as the system will be very dynamic. The engineer should have specified this in the sequence of operation.

[chadtech]

Yes sir you are correct about the fan type setup and the phoenix valves. The question is where to place the reference pressure of the transmitter. Lab space or outdoor?

[TriSTar]

If I understand your concern correctly the reference point should not be to the outside. The control would be similar to a variable volume air handler where you maintain fan speed based upon duct static pressure. Here you want to modulate the damper position to maintain the exhaust duct static pressure set point. Unless there is a specific requirement to maintain building pressurization, the reference points should be between the duct and the normal indoor building pressure.