Question about tstat wiring for two-stage cooling

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  #1  
Old 08-23-19, 12:20 PM
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Question about tstat wiring for two-stage cooling

My church has a HVAC system with a commercial rooftop unit (Rheem RKKB-A180..., with what looks 4 condensers built in).

The tstat for that unit was previously a Braeburn 2000NC, with a yellow and blue wire BOTH going into the same "Y" terminal. This indicates that the unit has two-stage cooling to me. See attached picture.

I replaced the tstat with a Venstar T8850 wifi one, which has separate Y1 and Y2 terminals. I put the Yellow on Y1 and Blue on Y2. (I used the previously unused brown wire as common). See attached picture.

My questions:
- It looks like the 2000NC is a one-stage cooling tstat. Does that mean for all these years, our system was wired to just either be "off" or "max", since both wires were at the same Y terminal?
- In a two-stage cooling system, does it matter which wire is on Y1 or Y2? I don't have easy access to the rooftop unit to see how it is wired.
- Is there an easy way to tell if the unit is running on stage 1 or stage 2?

thanks.

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Last edited by PJmax; 08-27-19 at 04:31 PM. Reason: resized pictures
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Old 08-23-19, 07:47 PM
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Rheem RKKB-A180 is a group. To know exactly what you have we'd need the full model number.
It looks like the entire series uses 4 condenser fans and either 2 or 4 compressors.
They all appear to be two stage controllable.

Did you just assume the brown wire was common ?

According to the system description...... it looks like Y1 and Y2 need to be kept in the proper order.
See page 41/ cooling section operation.

Rheem RKKB/RKNB series (pdf)
 
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Old 08-24-19, 03:58 AM
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Thanks PJ. When I was up on the roof a few weeks ago with an HVAC tech, I took a picture of the information sticker but the full model # was obscured by paint. See here: https://imgur.com/a/aQwnbRR

When he looked at the wiring, the tech said it was a two stage system, and that the brown wire was wired to common. Unfortunately I did not get a picture of the wiring setup to confirm the yellow vs. blue connection for Y1 vs. Y2.

The manual you linked to also indicates this unit has two-stage heat, but we only have a white wire for W2. Nothing connected to W2.
 

Last edited by petesamprs; 08-24-19 at 04:21 AM.
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Old 08-24-19, 01:29 PM
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Hi, the Tstat should call for 1st cooling and as the demand increases the 2nd stage should kick in. Or Y1 and then Y2 or not to confuse the issue Y1 could be used as an economizer to take in outside air if it was cool enough.
Really should see the RTU wiring Dia. If that unit is close to the ones in the link then it is a pretty simple hook up.
Was the Brown conductor connected at the RTU?
Geo
 

Last edited by Geochurchi; 08-24-19 at 01:49 PM.
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Old 08-24-19, 01:52 PM
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That's probably...... RKKB-A180CL25 ..... for 25 ton.
 
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Old 08-24-19, 02:01 PM
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Thanks, isn't the CL25 only 15 tons? It says net cooling capacity 180k BTU which I calc as 15 tons. Anyway, does that help with the info you need for the questions above?
 
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Old 08-24-19, 04:57 PM
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Hi, the wiring Dia should be the same ,tonnage should not matter.
Geo
 
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Old 08-25-19, 11:36 AM
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RKKB-A180CL25E.... 15 ton..... 4 compressors and 4 fans. Three phase powered.
Two stage controllable.
 
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