johnson controls

[etheric999]

Working on a carrier 60 ton unit,has something called power exhaust by johnson controls,has four of them,could anyone give me some info about these things, and how they are wired,thanks.

[plain spoken]

Sounds like a RTU. When the unit takes outside air into the building the building may become positivly pressured. The power exhaust has a sensor located in the building that tells the unit when the building pressure becomes positive. At that time the Johnson controllers start the power exhaust fans to relieve this positive pressure and bring it back to a neutral pressure. There are multiple fans that are brought on in stages by the controllers. Each one of these Johnson controllers controls a fan. The further you are from set point the more fans you have come on. The controllers get their control signal from a static pressure controller that is located inside the unit door that opens to the filter area, down at the bottom.

[lwarren]

Originally posted by plain spoken
Sounds like a RTU. When the unit takes outside air into the building the building may become positivly pressured. The power exhaust has a sensor located in the building that tells the unit when the building pressure becomes positive. At that time the Johnson controllers start the power exhaust fans to relieve this positive pressure and bring it back to a neutral pressure. There are multiple fans that are brought on in stages by the controllers. Each one of these Johnson controllers controls a fan. The further you are from set point the more fans you have come on. The controllers get their control signal from a static pressure controller that is located inside the unit door that opens to the filter area, down at the bottom.

I will just add that most of these units can be configured to work either off of building pressure as stated or by a changeable setpoint in the unit based on percent open of the outside air damper.

[plain spoken]

Need model # to be sure, but the modulating power exhaust (which is the one that uses the multiple Johnson controllers) will only work with building pressure.

[twcpipes]

An additional point to ponder:

Ideally you want to control the actual positive pressure at the front doors as referenced to outside atmosphere.
When the sensor is in the return ducting, say by the unit, and the return is ducted this presents a potential problem.
A varying return static can exist. Especially if the supply air system is variable air volume. As dampers close the return pressure builds slightly at the unit especially during morning warm-up. This gives a false reading of actual building static to the controller at siad front doors. It also tends to make the controller hunt. If the high side sensor were put by the front door in the ceiling, the controller would always see actual building positive pressure. If the sensor uses a tube, the tube can be extended to the entry door. Keep in mind you are working with .03"wg to .05"wg which is a very small amount of pressure to deal with.
If you should happen to find this problem as mentioned above, that's one you "fix it".
Good luck,
tom