Controls: Very Basic Question

[Horton]

New to the field, is this a true statement? If I have a Constant Air Volume (CAV) system, then is it always true that the OA damper and RA damper are linked together to maintain constant air volume (i.e., as the OA damper opens, the RA damper closes)?

And, related to this question, if a packaged unit delivers air at a constant volume, can I assume the OA and RA dampers came already provided from the manufacturer and already set-up to behave this way?

[WYO10tech]

Yes most all osa damper and ra damper are linked together. In most small cav they share the same actuators. A packaged unit econmizer in most cases will be will require some assembly but the osa and ra damper should be set-up. However check with the install manual there are some settings on the econmizer that may need to be set.

[kostasgreece]

In my area it is very hard to identify a standard from unit manufacturers provisions..

Just inspect the unit's mixing box, and try to identify damper actuators (one-two or three), in case there are less that three (OA, RA mixing) check for an assembly that connects them...

Try to move manually the actuator(s) and check the rotation of the mixing box...

Simple as stealing a kids candy...!!!

[DaveCR]

That could easily be a trick question. A constant air volume unit can also be 100% outdoor air.

[sysint]

No.

[Cagey57]

No.

And Yes, it probably is a trick question !

[tlp261]

It's not that common on a CV unit but the OA could be controlled separately based on CO2 control.

[crab master]

If I have a Constant Air Volume (CAV) system, then is it always true that the OA damper and RA damper are linked together to maintain constant air volume (i.e., as the OA damper opens, the RA damper closes)?

No, not always. Many times it's done with direct linkage cause it's cheaper & easier. Think about controlling building pressure as well. You can get better control by controlling return and and OA separately. Open the OA damper enough to help keep the building pressure positive, but leave the RA damper in the full return air open position until you start economizing, then start backing off the RA damper after the economizer signal gets greater than the OA damper min. position. Granted typically w/many man hours one could adjust the linkage as such to do what I am suggesting, but it is very difficult.

And, related to this question, if a packaged unit delivers air at a constant volume, can I assume the OA and RA dampers came already provided from the manufacturer and already set-up to behave this way?

Most of the time yes, but not all the time. You can special order package units to behave almost however you desire. Also on larger units you normally will see separate actuators due to torque requirements.

http://en.wikipedia.org/wiki/Glossary_of_HVAC