# Motor Operated Dampers Control Relay



## snadeau (Sep 16, 2013)

I am trying to control one motor operated damper from two separate exhaust fans on two separate circuits. These dampers need to operate so that they actuate when either or both fans are running. How can I do this. The dampers are 120 volt.


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## Wirenuting (Sep 12, 2010)

snadeau said:


> I am trying to control one motor operated damper from two separate exhaust fans on two separate circuits. These dampers need to operate so that they actuate when either or both fans are running. How can I do this. The dampers are 120 volt.


Using an aux contact on the exhaust starters pass them to a relay that allows the damper motor to operate. 
Most of the times the damper opens before the exhaust fans kick on. It makes it easier on the damper modulator. Use a reed switch or limit arm to kick the exhaust fan on.


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## Semi-Ret Electrician (Nov 10, 2011)

snadeau said:


> I am trying to control one motor operated damper from two separate exhaust fans on two separate circuits. These dampers need to operate so that they actuate when either or both fans are running. How can I do this. The dampers are 120 volt.


Is this a real world job or a classroom project?

Real world would be to have two fans with a gravity damper on each outlet.

Relays, diodes etc would not be "listed" so would require field labeling by an NTRL recognized outfit, which might run your costs up.


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## EBFD6 (Aug 17, 2008)

Semi-Ret Electrician said:


> Relays, diodes etc would not be "listed" so would require field labeling by an NTRL recognized outfit, which might run your costs up.


What are relay's listed for if not for control of devices in the field?

I agree that the best way would be to use aux contacts on the exhaust fan starters. If the exhaust fans don't have starters, 2 RIB relays would do the job.


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## oldtimer (Jun 10, 2010)

EBFD6 said:


> What are relay's listed for if not for control of devices in the field?
> 
> I agree that the best way would be to use aux contacts on the exhaust fan starters. If the exhaust fans don't have starters, 2 RIB relays would do the job.


 






Call your FRIENDLY HVAC DEALER .

I am sure he will help you out .

:laughing:


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## Semi-Ret Electrician (Nov 10, 2011)

EBFD6 said:


> What are relay's listed for if not for control of devices in the field?
> 
> I agree that the best way would be to use aux contacts on the exhaust fan starters. If the exhaust fans don't have starters, 2 RIB relays would do the job.


I was wondering about the separate circuit comment, problem about back feed to the other circuit, argument with the inspector etc.

I wonder if anyone makes an isolated "OR" 120V relay or using diodes.

But mostly about the lack of a UL label which drive our AHJ nuts and liability.


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## Awg-Dawg (Jan 23, 2007)

EBFD6 said:


> If the exhaust fans don't have starters, 2 RIB relays would do the job.


 I like this idea.


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## varmit (Apr 19, 2009)

If this is an application where it would be OK for the fans to run for a few seconds until the damper opens: Parallel a N.O. auxiliary contact on each starter to open the damper when either or both fans start. If the fans have no starter, as a 120 volt motor, then add a relay to each fan in parallel with the motor.

If this is a situation where the fan(s) can not be allowed to run without the damper open: The "start" control would energize the damper first. Use a contact on either the damper actuator or a limit switch on the damper to energize a relay to start the fan(s).


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## ponyboy (Nov 18, 2012)

Timing relays come in handy in these instances to prevent pressure issues


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## Jlarson (Jun 28, 2009)

EBFD6 said:


> 2 RIB relays would do the job.


Everyone should have a few RIB's on hand.


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## JRaef (Mar 23, 2009)

I don't get why this is so complicated. Aux contact of each starter to a relay coil. Relay contact goes to damper motor. No cross circuit, no diodes, simple OR circuit. Just make sure the relay contacts are rated for the damper motor HP size.

Opening a damper BEFORE the fan motor gets to speed would INCREASE the load on the motor trying to accelerate by the way. Load = flow in a centrifugal fan. Damper closed = no flow = no load.


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## BBQ (Nov 16, 2010)

JRaef said:


> I don't get why this is so complicated. Aux contact of each starter to a relay coil.


You are assuming there are mag motor starters involved. 

I work with tons of roof mounted exhaust fans that do not use magnetic motor starters. Just simple wall switches.


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## JRaef (Mar 23, 2009)

BBQ said:


> You are assuming there are mag motor starters involved.
> 
> I work with tons of roof mounted exhaust fans that do not use magnetic motor starters. Just simple wall switches.


OK, I've not done much HVAC stuff so I bow to the superior experience here. But would they be big enough to have motorized dampers associated with them? From what I've seen, small stuff like that usually just has gravity dampers.


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## beardie (Sep 12, 2013)

What about protection for the relays and damper motor if you're running directly off the motor wiring


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