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Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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I have a Ruud ugdg-12earjr furnace and tried to install a new

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I have a Ruud ugdg-12earjr furnace and tried to install a new thermostat ( to control it. Hooked up the Red, White, Green, Yellow wires to the new thermostat and all is good. After a bout a week the thermostat tells me it needs a common wire to power it (guess it wasn't getting enough power when the furnace was running). So I go back, pull the un-used blue wire out of the wire bundle at the thermostat end, hook it up to the thermostat terminal, and the "C" post on the furnace control board, where it was also unused (there was a white wire already attached to the C post that I THINK went to the air conditioning unit so I just doubled up the wires on the same post). Anyway, the furnace ignites properly, but now the fan won't come on.

So I pull the blue wire from the thermostat and furnace starts working again as expected (but thermostat goes back to saying it needs a common wire to power it).

Any ideas what I've done wrong?
Submitted: 5 years ago.
Category: HVAC
Expert:  Douglas replied 5 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

Which thermostat do you have?
Customer: replied 5 years ago.
It is the "Nest" thermostat (found at
Expert:  Douglas replied 5 years ago.
ok let me look at it. on moment.
Expert:  Douglas replied 5 years ago.
Neat thermostat, I never saw one of those, none the less, lets try something.

On your C terminal in the furnace you have two wires. the new blue and the white leading to the outdoor unit. Just to eliminate any bacfeed issues, lets take the white off of C at the furnace and reconnect the blue at the furnace and the thermostat on C and see what happens.
Customer: replied 5 years ago.
I am at work at the moment, but will try that tonight and see if it helps. If that does correct the problem (and the fan goes on after ignition), then what's next? The white wire is obviously there for some reason.
Expert:  Douglas replied 5 years ago.
well, yes the white wire is the common to the outdoor unit, its not going to be used for now (in heating mode unless this is a dual fuel system in which case the NEST is not compatable), but will be needed at some point, I just need to eliminate that as a source for backfeeding for this test. If we have to isolate the common we'll get to that.
Expert:  Douglas replied 5 years ago.
You really shouldnt need the common to the thermostat unless the thermostat wires are too small.
Customer: replied 5 years ago.
The wires are very small. Almost like the wires of a telephone box.

Single fuel system.
Expert:  Douglas replied 5 years ago.
that may be an issue. they really need to be 18 guage wires which are small, but not as small as phone wires.. phone wires are too small and may be an issue.

One way around the common wire maybe by isolating the W wire by installing an isolation relay OR by installing a resistor in the furnace between the W and C wire.

You see its using the W wire as a source of power through the circuit but some control boards will react if the power needed to power the thermostat is too great.

What we do in this case is install a relay in the furnace. On the coil we wire C from the furnace and W from the thermostat, then we connect the R from the thermostat to one side of the normally open contacts and then to W for the other side, this allows current to flow to power the thermostat through the relay coil, rather than the control board on the furnace,

The other option is to go to radio shack and get a 250 ohm 10 watt resistor to put between W and C on the furnace. This is called a shunt resistor for the terms of what we are doing here.

Douglas and other HVAC Specialists are ready to help you
Customer: replied 5 years ago.
tonight I will remove the white wire from the air conditioning unit and see if that solves the problem.

I also bought 50 feet of 5wire/18gauge wire. If the first solution doesn't work, I'll try wiring the furnace using this wire (outside the walls to begin with - then snake it through the walls if it solves the problem.) I'll get back to you and let you know what happens.

Beyond that it sounds like it's more complicated a problem than I want to tackle. I don't want to fry the control board by doing something stupid. I'll just have to get an HVAC guy to come look at it if these don't work.
Expert:  Douglas replied 5 years ago.
No, the resistor is an old trick used to overcome applications where we need to pull higher current through the white wire.
Customer: replied 5 years ago.
So just that I understand... I would put the resistor between the W and the C terminals on the furnace control board and NOT use the blue wire at all with the thermostat, instead, boosting the power to the already connected White wire via the shunt?
Expert:  Douglas replied 5 years ago.
correct, and you could do this with the white wire connected to C that leads outside.

Well it doesnt boost the power, it just doesnt steal it from the furnace control internals, it allows it to come more directly from the transformer.
Customer: replied 5 years ago.
Got it. I'll give that a try tonight as well and let you know tomorrow what did/didn't work.

Anything I should be especially care of while doing any of this so as not to screw things up or make matters worse?
Expert:  Douglas replied 5 years ago.
no, the power will be off when you connect it because the door is off, just make sure the resistor is only connected to those two and is not contacting any metal otherwise.
Customer: replied 5 years ago.

I wasn't able to track down a 200 ohm, 10w resistor tonight (radio shack was out) but I can find one tomorrow. In the mean time, I did try removing the white wire from the contact on the furnace control panel, leaving only the blue wire connected to it, and to the thermostat on the other end. No change.

One mistake that I did make that may influence your response is that I originally said that the furnace would fire up but the blower (squirrel cage) wouldn't come on. On closer look, it turns out that the only thing that was, in fact turning on, was the small blower fan (exhaust fan?). The igniter did not ignite nor did the gas flow.

So... to recap... with the common terminal connected to the blue wire (and ONLY the blue wire) at the furnace and to the thermostat, the furnace will only start up the small blower fan. It does not ignite, or turn on the large blower-squirrel-cage fan.

Without the blue wire connected to the thermostat, it will work properly for a period of time until it eventually throws an error message that says it needs a common wire to operate properly.

Do you still think that a resistor between the W and C terminals will solve the problem?

I have photos of the entier furnace panel if you think that would help.

Expert:  Douglas replied 5 years ago.
Like I said, its probably using all the current it can for now. Ultimately I think the heavier wiring will do it, but for the love of 79 cents for a resistor, I would try that, at least temporary. Then later, if that holds better, you can pull the new wire.
Customer: replied 5 years ago.
I'll track down the resistor today at lunch and put it in and let you know what became of it. Also, the 18 gauge wire that I bought last night is MUCH thicker wire than the existing one, so it ultimately may help as well.

I will keep you posted.

Expert:  Douglas replied 5 years ago.
great. I'll be in and out, had to run to DC last night and onto NY this morning.
Customer: replied 5 years ago.
so all that i can find in town is a 260 ohm/5w or a 200 ohm/8w resistor. Will either of these work? No one seems to have a 250/10 resistor.
Expert:  Douglas replied 5 years ago.
Oh they probably will., if worse comes to worse, you could parallel two of the 260 5w ones.