No power to the C-wire, suggestions?

[250]

We had our first cool evening here in a while, so we turned the AC off via the thermostat, opened the windows, and went to bed. When we got up the next morning my wife noticed that the thermostat wasn't displaying any information, and she couldn't access it with her app. This system is brand spanking new, as in less than two weeks. We called first thing monday morning, and their first appointment is a week and a half out.

I'm no multi-meter wizard, but I was able to determine that there was no power on the C wire coming in. The main power coming into the condenser is good, but the two wires highlighted don't have any power. Shouldn't there be some juice leaving here and going to the thermostat or are these only powered when the condenser is on? Anything else I can check?

 

[Jim greengo]

Common wire shouldn't have power,if you dont show 24v between the 2 low voltage wires ,check the board in the furnace 1st.
Maybe blown fuse on furnace board?

 

[Mzungu]

Those two wires are incoming to energize the relay that they are attached to in the outdoor condenser. Normally one would be terminated on c on the furnace board and the other terminated on y on the furnace board. Check your thermostat to see if you have 24v coming in on the R & C terminals.

 

[Mzungu]

Hopefully this helps.

 

[fitter30]

Did u check r to c at thermostat? At furnace terminal strip in blower compartment will have to press door switch for power to furnace. Should have 24vac.

 

[250]

I did check the r to c at the thermostat. No power.

It looks like the wire all run through the furnace/air handler in the attic, which I don't think I understood completely before. I did go up there on sunday, but didn't see an access hatch, just the brown hvac wires coming in and out. I'll have to go back up and look in the more inconvenient places for the hatch.

 

[dogdog]

Check at the transformer output(24Vac) / input (120Vac)...

 

[RPH]

Check at the transformer output(24Vac) / input (120Vac)...

Kill the power on the secondary side of xformer. Tie the pair of secondary wires to gather and confirm continuity between the pair of wire. Sounds like an open.

 

[dogdog]

Kill the power on the secondary side of xformer. Tie the pair of secondary wires to gather and confirm continuity between the pair of wire. Sounds like an open.

Where did you see the op mention this ?

Sorry, don’t even know what this means


“Tie the pair of secondary wires to gather and confirm continuity between the pair of wire”

Is there suppose to have continuity between the primary and secondary ?

 

[Aileron]

My money is on the c wired not connected in the furnace or air handler section properly if its there at all and the stat had been running on its battery backup power. Change the batteries in the stat and it may power itself back up until the service guys come out and repair their screwup.

 

[Mzungu]

My money is on the c wired not connected in the furnace or air handler section properly if its there at all and the stat had been running on its battery backup power. Change the batteries in the stat and it may power itself back up until the service guys come out and repair their screwup.

Pretty sure OP said he shut off the A/C the night before, so it was running. If the C wire was never connected the A/C never would have worked at all.

 

[fitter30]

Take a pic of the furnace or air handler terminal strip and wiring diagram on blower door.

 

[Jim greengo]

Where did you see the op mention this ?

Sorry, don’t even know what this means


“Tie the pair of secondary wires to gather and confirm continuity between the pair of wire”

Is there suppose to have continuity between the primary and secondary ?

Hes just saying to check and make sure wires are good between the furnace and condensing unit is all.

 

[Jim greengo]

I did check the r to c at the thermostat. No power.

It looks like the wire all run through the furnace/air handler in the attic, which I don't think I understood completely before. I did go up there on sunday, but didn't see an access hatch, just the brown hvac wires coming in and out. I'll have to go back up and look in the more inconvenient places for the hatch.

Check the fuse on board inside of furnace.

 

[Aileron]

Pretty sure OP said he shut off the A/C the night before, so it was running. If the C wire was never connected the A/C never would have worked at all.

Nope, it will run. All the stat is is a switch. Old stats don't need a c wire and battery operate stats don't either. I'm not talking at the condensing unit, I'm talking about the common from the transformer in the furnace to the stat. If it wasn't connected correctly to keep the stat powered it could have ran off battery backup for the two weeks and coincidentally the batteries died.
I had to explain this one night over the phone to a electrician when his went dead. I finally told him , change the batteries and bingo its back on. His stat did not use a common, only batteries.

 

[Barnabas]

Yes, dead batteries in the thermostat is my first guess.

 

[oldwino]

Did u check r to c at thermostat? At furnace terminal strip in blower compartment will have to press door switch for power to furnace. Should have 24vac.

I got caught by the door switch myself. No power to furnace. Figured I’d tackle it in the morning. Woke up about 2AM when it hit me, the access door was open when I was testing circuit. Felt like an idiot