GP51MP-24B-045-1 C-wire

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Onkar Sharma

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Hi all,
Yesterday I went to install the Nest thermostat on my furnace and now I am completely puzzled. The events unfolded like this:

1. Nest wiring and back plate installed. Power turned back on. Checked for voltage across:
C and Y- 24V
C and G- 24 V
C and Rh- 1.9 V

2. Put Nest cover on. It did not boot up. Called customer service. Finally realized there was no power in the C-wire.

3. Funny thing is: There was a blue wire in the C-wire port at the thermostat. Went down to the furnace to check and did not find the blue wire hooked onto C. So why was I getting the voltage initially?

4. Checked voltage at the furnace, there was no voltage across the C-wire and Y-wire now.
The new voltages are:
Rh and Y- 24V
Rh and G- 24 V
C and Rh- 24 V
C and Y- 0V
C and G- 0 V

5. The nest is now running without the C-wire hooked on. However, this is not a desirable situation.

The furnace is still working for now.
Nest is going to pay for someone to come and have a look and potentially install a C-wire. I am just scratching my head. What is going on?
 

Dana

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When the wire isn't hooked up on either end it's just an antenna for alternating currents in the adjacent wiring, a weakly-coupled transformer. With a high-impedance voltage meter it can measure a voltage, but there isn't sufficient coupling to drive much current. As soon as one end or the other is tied to something with a defined potential relative to ground, the induced voltage won't be measurable with normal equipment.
 

Onkar Sharma

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When the wire isn't hooked up on either end it's just an antenna for alternating currents in the adjacent wiring, a weakly-coupled transformer. With a high-impedance voltage meter it can measure a voltage, but there isn't sufficient coupling to drive much current. As soon as one end or the other is tied to something with a defined potential relative to ground, the induced voltage won't be measurable with normal equipment.

Thanks Dana.
I looked at the control board wiring diagram. The 50M56U-843 control board says that the C wire should be on 24V relative to Y. There is no voltage across these two wires even at the furnace control board.

Is the control board somehow messed up?
 

Jadnashua

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To get power, you need a complete circuit. Power comes from a transformer. The only reliable 24vac is between the (typically) red wire and the C (could be any color), the two leads on the secondary of the 24vac control transformer. It's similar to a wall switch to control a lamp...power comes in on one lead, close the switch, and it goes out the other...neutral (similar to the C contact) is at the lamp, not normally needed at the switch. Many thermostats do not get wired up with the C terminal since they often do not need it there. Builders are cheap...they don't put in extra things unless they either have to or were asked. Extra wires cost money and time to them.

A thermostat is a fancy switch. When heat is needed, it connects the R to the W. That runs back to the furnace, goes through the control circuit, which has the C on the other side to complete the circuit.

Same idea with the Y...the thermostat connects R to Y which goes to the A/C unit to power the control circuits, and the C contact is on the other side to complete the circuit.

Same with the fan control.

Measuring between R and anything else but C will give you unknown values, and often, meaningless ones, some of them considerably determined by the type of meter you are using.

Many of the new, smart thermostats draw too much power to rely on using batteries for running them, so they need a complete power circuit inside. They all will have at least one side of the transformer (R), but you have to go back to the source to find where C is and ensure it gets run to the thermostat.

Luckily, when I installed my Nest thermostat a couple of years ago...while the C terminal wasn't used on the old one, there was an unused wire in the cable. I went down to the furnace, and connected it to C there, and at the thermostat, and it all works fine.
 

Onkar Sharma

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To get power, you need a complete circuit. Power comes from a transformer. The only reliable 24vac is between the (typically) red wire and the C (could be any color), the two leads on the secondary of the 24vac control transformer. It's similar to a wall switch to control a lamp...power comes in on one lead, close the switch, and it goes out the other...neutral (similar to the C contact) is at the lamp, not normally needed at the switch. Many thermostats do not get wired up with the C terminal since they often do not need it there. Builders are cheap...they don't put in extra things unless they either have to or were asked. Extra wires cost money and time to them.

A thermostat is a fancy switch. When heat is needed, it connects the R to the W. That runs back to the furnace, goes through the control circuit, which has the C on the other side to complete the circuit.

Same idea with the Y...the thermostat connects R to Y which goes to the A/C unit to power the control circuits, and the C contact is on the other side to complete the circuit.

Same with the fan control.

Measuring between R and anything else but C will give you unknown values, and often, meaningless ones, some of them considerably determined by the type of meter you are using.

Many of the new, smart thermostats draw too much power to rely on using batteries for running them, so they need a complete power circuit inside. They all will have at least one side of the transformer (R), but you have to go back to the source to find where C is and ensure it gets run to the thermostat.

Luckily, when I installed my Nest thermostat a couple of years ago...while the C terminal wasn't used on the old one, there was an unused wire in the cable. I went down to the furnace, and connected it to C there, and at the thermostat, and it all works fine.
Thanks Jadnashua. The nest is completely ok now. It has accepted the c-wire and the wiring looks perfect. However, when I ask it to cool, it sends a signal to the furnace, the fan at the furnace starts but the outside condenser does not start. How does one go around to troubleshoot this.
 

Onkar Sharma

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I took the c-wire off the furnace and reinstalled it. I also took all wires off the thermostat and reinstalled them. It then just worked.

However cannot celebrate too much. There is no cooling now.
 

Stuff

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Recheck the wiring. At the furnace there are two wires (normally a red and white) that go to the outdoor compressor. They should be attached to the Y and C terminals at the furnace. And each of the terminals also has a wire that goes to Nest.
 

Onkar Sharma

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Thanks Stuff. It is all fixed now. Turns out that the problem was not the 24 v circuit but the 240 v circuit. I bought this home in Nov 2017. Turns out that when the last owner upgraded the breaker panel, the electrician did not connect the negative terminal to the AC circuit breaker. This also explains why the last owner was so much against the house inspection. Unfortunately, my house inspector was pretty careless and missed most of the problems which I had to deal with. I am now laughing my head off because this would mean that the previous owner did not operate this AC for 5 years of his occupancy thinking that it is broken when it was not.

Nevertheless, I am happy that in the end it was a simple fix to the AC problem. Life is good now. :)

I also tightened the C-wire on the furnace and reattached the wires to the nest. The nest is also working well now. Alls well on the nest front too.

Now I am onto the attic furnace which is leaking water into the pan below. The condensate connections are all rusted. But thats a problem for a different day.
 
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WorthFlorida

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Hello Onkar, glad you worked out your problem and typically home inspectors do not remove the breaker panel cover. They'll open it and may switch off/on all breakers I hope the diagram belows help explain what the c wire is all about. After you reconnected all the wires for a second time and it started to work it probably was the c wire or A wire (red) wire not making contact. As it happen to me ,you push the wire into the terminal too far and you end up tighten the screw on top of the insulation, not the copper.

Here is a basic simple thermostat wiring and I added the C wire and an electronic board. The C wire at the thermostat is only there to power the thermostat. Before WiFi, digital thermostats usually had three AA batteries and they could last for years. But with the intelligent thermostats (CPU & memory chips), color screens and especially the WiFi connection just takes more power. The batteries alone would not last very long.

This picture shows when the heating and cooling equipment are one unit such as an air handler for AC with electric heat. When there are two separate systems, an AC unit and a furnace there will be two 24v transformers. This is when you remove the jumper between Rc (cool) and Rh (heat).

upload_2018-5-6_23-13-34.png
 
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