Furnace/Thermostat stopped working overnight

[TehXeno]

Hi all!

I need some help troubleshooting my furnace/thermostat.

 

My furnace and thermostat were working without issues yesterday (10/20).

I woke up this morning (10/21) to all my vents blowing cold air (no heat) and a thermostat with no power (as exhibited by the digital thermostat with a screen off). My home also has a humidifying system connected to the HVAC system and its display was also off.

I determined that both the thermostat and humidifier were both not receiving power from the furnace.
While inspecting the furnace, I found there to be power coming in to the transformer, being transformed, but *no power to the furnace terminals*.

I was able to *find and reset a thermal limiter*, and after doing this – I began to receive power at the furnace terminals again! At this time – my humidifier display began receiving power again.

*The problem I am still having is that there does not seem to be enough power being sent from the furnace and up to the thermostat that was running without problems no more than 36 hours ago*.
I am getting an electrical reading of at the correct AC voltage of 26v at the furnace terminals (basement), but when I read up at the thermostat (ground floor), all but one of my readings are at 14.5v.

I have attached a picture of the thermostat wires as well as the wires at the furnace. I did not touch any of the wiring at the thermostat, but I did interact with the wires at the furnace while performing my troubleshooting.

All of the wiring is as the units were installed since I moved in to the house. I moved in about 2 months ago and I have had no issues with the HVAC system until over last night.
In the pictures, you’ll notice than an “add a wire” was installed. There are a lot of wires because a humidifier is also installed.
You might also notice the loose brown wire at the bottom of the furnace configuration photo. This was labeled as “Green Thermostat” despite it being a brown coated white wire. I was able to trace this wire back to the humidifier, though.

 

*Any suggestions for troubleshooting would be much appreciated!*

Thanks for your help!

 

A couple of other notes that might be helpful…

HVAC Model: Carrier 58GS-100-161GA

Circuit Board Part #: HH84AA020

 

Thermostat connections:

White Wire connected to W1 (Reads at 26v when connected to R)
Yellow Wire connected to Y1 (Reads at 14.5v when connected to R)
Green Wire connected to G (Reads at 14.5v when connected to R)

Red Wire connected to R (Reads at 14.5v when connected to R)
Green Wire connected to C (Reads at 14.5v when connected to R)

 

[alamo1718]

Your first issue is the limit switch.  When it opens, it closes the gas valve and energized the fan on the board. Thats why it was blowing cold air, it thought the heat exchanger was too hot and needed to be cooled off. It is a safety to keep from damaging the system or your house. Usually this is an airflow issue,i.e. fan didn't start,fan stopped, blocked vents, dirty air filter etc. 

Before I went much further, I would suggest installing a low amp fuse in the low voltage circuit, if you are going to test it yourself further. At the thermostat, there should be 24V, your 26 is fine, between the Red and the C terminal, with no call for heat or cool. You can try jumper between the R and W at the thermostat sub base. This should turn on the heat. However, if you have a low voltage short, it will trip the fuse or burn up the transformer.

  You can eliminate the thermostat and wiring between the furnace and stat as a potential issue. At the furnace, note where each low voltage wire is connected. Remove all the low voltage wires from the furnace board and jump R to W. The heat should turn on. 

If might be too much amp draw from the humidifier. It might be a bad common. There might be an issue with the humidor. Will the heat work correctly with the humidifier and add a wire disconnected? 

Google Carrier 58gs , I like dms.hvacpartners site. There is a step by step troubleshooting guide. Remember, their troubleshooter does not have the humidifier  or adda wire installed. 

[TehXeno]

Thanks Alamo.

The filter was a bit dirty, so I replaced that.

I also removed the humidifier from the connections and the voltage gets up to the thermostat to turn it on and regulate the furnace.

 

I'm planning to leave the humidifier out of the connections for now to test if the system can run alright for a week before trying to troubleshoot adding it back in.

[alamo1718]

Transformers have a Volt Amp rating. I believe yours is 20VA. Its not just about the volts, its the amps too. If the system behaves without the humidifier, thats real good. At a guess, it would indicate a short or broken wire in the humidifier. My gut is saying a broken common in the humidifier. Also, the adda a wire is using the furnace transformer to power itself. It may be drawing too many amps, when combined with the furnace running. 

When you go back to troubleshooting, check your voltages at the stat sub base again, before you reconnect the humidifier.

The humidifier may be contributing to the limit switch tripping. It could be acting as an air flow obstruction that combined with a dirty air filter doesn't  allow enough air flow. When you look up the manual, it will have the temperature rise for your furnace. Measure that to give an indication whether the air flow is too low. 

[patrichardson]

It seems that your system needs maintenance. I would like to advise you to consult a professional technician and get maintained your system.