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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22678
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I installed the RTH8500, screen is flashing and won't stay

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I installed the RTH8500, screen is flashing and won't stay on. I had to use the G wire for the C wire since I didn't have one with my existing system.
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: Just installed this morning. No power when using the extra wire so used the G wire for the C wire
JA: Do you plan on doing the work yourself?
Customer: would like to if i can
JA: Anything else we should know to help you best?
Customer: the system came on originally and was attempting to set up the wifi, but didn't have the message that it was in wifi set up...removed face plate for 10 seconds and then reinstalled...since then it has continued to flash not allowing me to set up


My name is Billy.

Do you still need assistance?

Customer: replied 26 days ago.
I do

Ok, can you tell me what color wire is on what terminal in thermostat...and did you move g wire in furnace to C and install jumper y to g?

Customer: replied 26 days ago.
I didn't jump the y to g...can try that now, just moved the g to c


G is moved to C and then a jumper is needed from Y to G...IF..there were 2 wires in the Y terminal in furnace...sometimes the 2 wires are just wire nutted together and not in the y terminal.

Let me know the status of yours

Customer: replied 26 days ago.
there is a blueb wire at the thermostat, but didn't give me power...jumped the y to g, moved g to c and turned power back on. thermostat flashes but doesn't stay on long enough to set up

Ok do you have Trane equipment or removed a Trane thermostat?

Customer: replied 26 days ago.
New one is Honeywell old was white-rodgers

What about the equipment is the outdoor unit rheem, ruud or Trane?

Customer: replied 26 days ago.
don t know

The blue wire on B is a concern.

Lets start old thermostat...

What wire was on what terminal? and do you have model number of old?

You see old B terminals can be a Com (C) or a reverse valve for rheem, ruud heat pumps. do you know if you have a gas furnace or heat pump?

Customer: replied 26 days ago.
I know it isn't gas, the wires matched the terminal, b wasn't attached, there was a w, g, y and r and a jump from r to rc

and what color wires?

Customer: replied 26 days ago.
the same, white, yellow, green and red

You said there was a blue wire also at thermostat?

Customer: replied 26 days ago.
it was loose, wrapped around wires but not connected...original model for thermostat was 1f78

Then we could use that unused blue as the C wire...and not move g wire?

Customer: replied 26 days ago.
most likely but I didn't get power when I used

Did you put the other end of the blue wire in furnace/airhandler to C?

Customer: replied 26 days ago.
I didn't, because wasn't sure how

Ok....lets do this...

First, can you tell me what the indoor blower unit is or take door off and post a pic for me?

Customer: replied 26 days ago.
thanks on my way to crawl space
Customer: replied 26 days ago.
here it is

thank you! I must ask for one more....there is a blower section to the right of the pic...that cover needs to come off then I can draw directions on that pic. It is a circuit board I need to see

Customer: replied 26 days ago.
there is a white wire connected to the c terminal
Customer: replied 26 days ago.
do i just add the blue to the connection

correct...that is the C wire from ac unit....we will add to that the blue C will have 2 wires..the white and the blue we will add...and thats it at furnace..

At furnace we should have from thermostat:

red to r

white to w

yellow to Y

green to g

blue to c

From ac unit are 2 wires in to Y with the yellow and white to c.

At thermostat red to r

white to w

yellow to y

green to g

blue to c

Can you try this now?

Customer: replied 26 days ago.
no power, do the panels need to be on the furnace

correct....the bottom panel over circuit board must be on to depress a sfety button

Customer: replied 26 days ago.
you rock...all is working now. thank you!

you are very welcome....just rate service (5 star system) to close..

Thank you!


BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 26 days ago.
Will do