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BillyHvac, Journeyman HVAC Tech
Category: HVAC
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My furnace will not turn on after changing the thermostat,

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My furnace will not turn on after changing the thermostat, but will operate manually. I changed the thermostat because the old one was cycling the furnace too rapidly (1 -5 min runs) . Wiring pics can be found here:



are you ready?

Customer: replied 3 years ago.

Sorry? Am I ready for what?

To proceed, I didnt see yu were online.

At the furnace I see there is no wire on W terminal. The wire on W in thermostat...what does it go to in furnace?

Customer: replied 3 years ago.

Ah.. definitely.. W on the stat connect to Y on the furnace (unchanged from old stat)

Customer: replied 3 years ago.

Sorry.. i was incorrect... W on the stat connect to the terminal seen in the second pic.


It's labeled TH-TR (beside black wire)

Sorry just saw the black wire.

Now on Y...are those from Y at thermostat and the other goes outside to ac unit?

Customer: replied 3 years ago.

There are two wires at the Y terminal at the furnace, The orange goes to the Y at the stat, and the white wire leaves the furnace and goes to the AC compressor

The black wire on the gas valve left that coming out of the thermostat wire cable?

Customer: replied 3 years ago.

yes, it connects to Rc on the stat.

OK that is not where we want it.

Please bear with me as this wiring is not the norm for a heat/cool system.

we know the following:

red from furnace r goes to thermostat r
yellow (your orange) goes from furnace y to thermostat y
then we have a green Or what your color is) goes from furnace g to thermostat g

so far so good?

Customer: replied 3 years ago.

Yes, this is accurate so far.

ok...what terminal was the black wire on previously in old stat?
Customer: replied 3 years ago.

Also on Rc.

you see rc is 24v for cooling from a separate trans former. rc has nothing to do with the heat about red...? was it on a r terminal as well?

do you have the model of the old stat


update...I think I have it now...


old stat have 2 different r's?



Customer: replied 3 years ago.

Old Stat had R and Rc, The new one (honeywell rth6400) also has both. By default the R and Rc were jumpered, but the instructions said if the old stat had distinct wire for both to remove the jumper.

Ok I believe your heat was wired backwards worked but took a backwards path instead of forward.

Try this...and I hope we have not fried one of the trans formers.

White from furnace to R (no jumper)
Red from furnace to Rc
yellow (orange) from furnace Y to thermostat Y
Furnace green to thermostat G
Black from the gas valve left terminal to thermostat W

Try this and let me know.

Customer: replied 3 years ago.

Apologies for the delay.. my fathers friend (who does hvac) just called and agreed that the circuits appear to be miswired.


Here's what he had me do (while breaker off): disconnect the Red wire from the stat, still at the stat, take black wire from Rc and put it into R (without jumper)


So now the red wire is not on anything in the stat, and is on R in the furnace. Black wire is on left valve terminal and R on the stat. The stat is set to heat, and the fan is on auto and the furnace appears to be responding to the stat.




You see they added a fan control That is what has the r, g, y, c...before the heat only need just 2 wires from the gas valve.

So your red wire is 24v for goes on Rc. That will control cooling and fan only.

The 2 wires from the gas valve go on r and w...that is all for the heat.

Rc = cooling circuit
R = heat circuit.

hook red to rc and you are all set.


BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 3 years ago.

Wiring unchanged form previous post the fan will not manually turn on, Connecting red wire to Rc as you suggested allows for manual fan control.

Customer: replied 3 years ago.


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