Interlock Between Fume Hood Exhaust Fan and Outside Air Unit

Shujinko

Senior Member
I have a client that has a very old lab type building. He is wanting to put new outside air units in the building and interlock them with some existing fume hood exhaust fans. The equipment is old so there is no type of control system. Basically when the lab hood exhaust fan turns on the outside air unit needs to turn on via an interlocking mechanism. What would be the best way to acheive this (contactor, relay, other)? Thanks for the help. :):):)
 

mgookin

Senior Member
Location
Fort Myers, FL
Sounds like you're adding makeup air to an exhaust system?

Why not just power it off the same switch that turns on the exhaust? Make it a dpst.
 

GoldDigger

Moderator
Staff member
The OP's situation may be one outside air unit per room but multiple hood fans.
Although having N switches in parallel directly feeding one fan may be code compliant, it does not seem wise to me.
And running all of the additional switch legs could be a pain.
If the different hoods are fed from more than one breaker the wiring just gets more confusing.

Tapatalk!
 

Shujinko

Senior Member
Yes, the owner wants to add outside air fans. There are different scenarios. Some scenarios we will have multiple hoods interlocked with one outside air unit. Other scenarios we have one hood for one outside air unit. I'm thinking a contactor might be the best solution, with the hood circuit as the coil circuit.

On another note hoods and outside air units will be different voltages.
 

mgookin

Senior Member
Location
Fort Myers, FL
Knowing now what's been revealed, I'd put relays at the makeup air fan(s). That way someone working on the system will see how it's configured. Every exhaust fan can have a traveler going to a makeup fan relay.
 

Shujinko

Senior Member
I'm still a little unclear on the scenario where there is 1 make up air unit (MAU) associated with several (in this case 3) lab hood exhaust fans (EF). The lab hood EFs are all 120V/1 phase and the MAU is 208V, 3 phase. Each EF is on its own circuit and obviously the MAU is on it's own circuit. Is there a contactor that has multiple coil circuits or would I have to do this with a relays?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
It will probably entail multiple contactors. If you get so specific as to completely explain what fans you have and which come on together, someone might draw it out for you.
 

kwired

Electron manager
Location
NE Nebraska
I'm still a little unclear on the scenario where there is 1 make up air unit (MAU) associated with several (in this case 3) lab hood exhaust fans (EF). The lab hood EFs are all 120V/1 phase and the MAU is 208V, 3 phase. Each EF is on its own circuit and obviously the MAU is on it's own circuit. Is there a contactor that has multiple coil circuits or would I have to do this with a relays?
A so called contactor with multiple coil circuits would likely be separate contacts all mounted on one circuit board anyway. Your easiest and least cost solution is likely to use independent relays instead of looking for something that is all combined in a single unit. You likey would use light duty compact relays for the controls portion and a drive a single heavier contactor to switch the motor load(s).
 

Shujinko

Senior Member
Wow George, thanks for the going through all the trouble of drawing up a sketch. I only have one question, on the line for the MAU, the one with the 3 contacts for each EF.....what type of device would that be? A single relay with 3 contacts or 3 different relays?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Wow George, thanks for the going through all the trouble of drawing up a sketch. I only have one question, on the line for the MAU, the one with the 3 contacts for each EF.....what type of device would that be? A single relay with 3 contacts or 3 different relays?
EF-1 Contactor is a DPST or better. The motor of EF-1, being 120V, could be supplied in parallel with the coil of Contactor EF-1, or additional poles could be provided to drive the motor of EF-1.

MAU Contactor is a 3PST. It just closes the contacts to energize the motor of MAU.

The three contacts you see driving MAU are second poles of the EF contactors.
 

Shujinko

Senior Member
Thanks George I think your diagrams really helped me clarify what needs to be done in the scenario at hand. I will look for 3PST contactors online to see what's available. I really appreciate the help.
 
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