Commercial Hood EX fan and Make up air issue

[Strombea]

We install alot of hood contactor and captive air systems and on the last one I used a MWBC to feed the 2 fans on the roof- 1 exhaust 120v fan and 1 swamp cooler blower only 120v fan make up air. The captive air box has the 2 contacts set up for hot and neutral. i only used hot T1 terminals as I was sharing neutral.

problem: when we fired up the swamp cooler it ran fine until we turned on exhaust fan, I cant figure out how or why but it seems like the exaust fan is some how priority and is stealing the neutral path.

Could phasing to building be messed up? maybe B phase isnt 180 degree offset?

any way i gave the swamp cooler its own neutral from buss and it works fine now, just stumped on the MWBC should share a neutral.

All wire is THHN #12 and sq d QO 20 double pole breaker feeding MWBC

 

[480sparky]

.......stealing the neutral path....

"Stealing the neutral path"?

....Could phasing to building be messed up? maybe B phase isnt 180 degree offset?....

Um.... it's not 180° off..... they're the same phase. Hence the name: single phase.

....a just stumped on the MWBC should share a neutral.....

That's the definition of a MWBC...... more than one circuit share the neurtal.

 

[LarryFine]

I have wired and rewired many commercial hood systems. I can't answer with the info given. Can you show us a wiring diagram?

 

[hbiss]

I have no idea what you are talking about either. The exhaust and makeup air fans (it's NOT a swamp cooler!) have to operate independently.

I can tell you that CaptiveAire has a very complicated microprocessor control system. Bottom line is to follow their wiring instructions exactly. If they show separate hots and neutrals to each blower from specified terminals do it.

-Hal

 

[Strombea]

"Stealing the neutral path"?



Um.... it's not 180° off..... they're the same phase. Hence the name: single phase.



That's the definition of a MWBC...... more than one circuit share the neurtal.

Yes my point... so why is it that bringing in the second neutral fixed it? forget captive air box, pretend you have two fans on a MWBC and when you turn the second one on the first one stops ( lost neutral because I still had 120 to ground) and when I turn the second one off the first one works fine.



FYI the make up air fans ARE swamp coolers here, without water hooked up, don't ask me why.

 

[480sparky]

So you're saying 'stealing the neutral' means you identified an open neutral in the MWBC?

 

[gadfly56]

Yes my point... so why is it that bringing in the second neutral fixed it? forget captive air box, pretend you have two fans on a MWBC and when you turn the second one on the first one stops ( lost neutral because I still had 120 to ground) and when I turn the second one off the first one works fine.



FYI the make up air fans ARE swamp coolers here, without water hooked up, don't ask me why.

Sounds like someone is switching the neutral instead of the hots.

 

[ptonsparky]

Sounds like someone is switching the neutral instead of the hots.

Something I was pondering, but I haven't figured out how that would make a difference.

I think the OP is missing one of those pesky little details that seem to get in the way.

 

[growler]

I think the OP is missing one of those pesky little details that seem to get in the way.

That's what I was thinking.

 

[hbiss]

Me three.
Looking at the drawing I have from CaptiveAire for a job I bid on awhile back, there are two contactors in the control panel that are supposed to interface with either starters or direct to the fan motors. Each shows T1, T2 and Gnd. T1= hot, T2 =neutral. So obviously T2 is switched.

I suspect that if 208 or 240V motors were used (and the panel configured as such) both T1 and T2 would be hots. That's why they are both switched.

problem: when we fired up the swamp cooler it ran fine until we turned on exhaust fan,

You would have to look at the pages and pages of programming and the interaction with room temperature sensors, etc to see why that is happening. (The makeup air is supposed to shut down while the exhaust stays on if the system thinks there is a fire.) I suspect the EC is only responsible for wiring these things. Like a fire system, the factory sends a tech on site to turn it on, set it up and see that it's operating properly.

-Hal