Pneumatic Question

[whec720]

I'm working on a AH with a bad HOA switch. I'm replacing the switch tomorrow as I did not have one handy today. Anyway, my question is on the same control circuit to the SF. The control voltage to the SF mtr starter first goes to the HOA, then to the EMS S/S relay and then to a Honeywell EP switching relay. The relay has three ports 1, 2, and 3. When voltage is applied to the coil, ports 1 and 3 and energized. No voltage puts the switch in its normal position of de-energized at ports 2 and 3. 16.5 psi is needed through ports 1 add 3 to run the SF.
My question is, where is this air pressure going? At this point, I do not completely understand the control scheme to the SF. Any feedback is greatly appreciated.

[adrianfromoz]

Port 1 is the main air supply from the air station and Port 3 goes to the mains controls for the Air Handler. When de-energised the the mains air to the control circuit bleeds out through port 2 and shuts off the main air supply at port 1, thus returning the pneumatic control circuit to its "de-energised" state.

Regards
:)

[whec720]

Port 1 is the main air supply from the air station and Port 3 goes to the mains controls for the Air Handler. When de-energised the the mains air to the control circuit bleeds out through port 2 and shuts off the main air supply at port 1, thus returning the pneumatic control circuit to its "de-energised" state.

Regards
:)

Thanks adrian. I can see how it all comes to play now.:)

[adrianfromoz]

Glad to be of help!

Stay cool, not that you've got any choice in the wrong half of the world!

[e154940]

The part number for the E/P relay is RP418A....
The ports are 3-2 de-energized, 3-1 energized.
The main air is most likely connected to port 1, port 3 is then considered a switched main which is connected to the main port of the pneumatic controllers for the AHU and/or the main air to pilot positioners.

When the fan is Off, the E/P switches 3-2 allowing the switched main air to blead back through port 2 (which should not be connected to anything). This will allow all pneumatic devices to return to normal postions (i.e O/A & E/A dampers close and R/A dampers open).

Hope this helps.