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Discussion Starter · #1 ·
I will be installing a new Trane XV80 furnace and 2 stage scroll condenser (no dedicated de-humidifier) this Saturday and reading over the manual has me a bit confused. I will be using my ecobee 3 which is two stage compatible with Y1 and Y2, but no BK.
The installation manual for the furnace has conflicting advice regarding wiring the second stage cooling. Page 20 Note 8. Reads, “For two step scroll system use Y for low speed and BK for high speed connection to the low voltage terminal board. Place a jumper between R and O and move S5-2 to the OFF position.
Page 23 of the manual, under note 7 reads "the Y2 terminal from the thermostat must be wired to the Y terminal of the furnace control for proper blower operation during cooling.”

Just trying to confirm the correct wiring should be thermostat-furnace-condenser should be Y2-BK-Y2 with a jumper at the furnace between R and O with the S5 Dip switch off (S5-2 OFF means de-humidifier = installed). I’m just worried because I thought BK was the proprietary variable speed blower wire that requires a specific Trane thermostat.

Furnace board has
YLO Y W [email protected] R G C O BK/DS/DHUM

Ecobee
4TTR7036A1 - XR17 Climate Tough Condenser
TUD2B080A9V3V - XV80 Furnace
4TXCB004DS3HC - 3 Ton Coil
 

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If bk sees a constant 24v it will allow the fan to run at 100%. 0v will run the fan at approximately 80%. The NEXIA thermostats use a PWM signal on bk to control fan speed from the stat.
With a 2stage unit with a unloading scroll like you have y1 turns the unit on and without a 24v on bk the fan will run at 80%. When the thermostat calls for y2 it will sent 24v to bk causing the fan to run at 100%.


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Discussion Starter · #3 ·
Well, I was hoping for 350 cfm per ton on stage 1. Stage 1 outdoor unit is 70 percent of 3 tons, so 2.1 tons. After setting the dip switches to 3 ton (close as I can figure, you match the furnace to fan not outdoor capacity) and 350 cfm per ton, total 3 ton speed would be 1050. With the BK wore set up at 80 percent, stage 1 would be 840 cfm not 700. Oh well. 2nd stage will be the correct 350cfm per ton.
 
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