Nest 3rd Gen Thermostat says it’s heating, but no heat

[MyNameIsBrad]

Installed a 3rd gen Nest thermostat in September, seemed to be working fine, but now it lights up orange as if calling for heat, and the blower turns on but the furnace doesn’t kick in.  Shorting Rh and W1 will ignite the burner.

I have wires at Y1, G, W1, Rh.  I’ve done a factory reset on the thermostat and also tried setting safety temps to off (a post somewhere said that could be the issue).  It’s running software version 6.2-27

Any ideas? 

[CoolingWizard]

@MyNameIsBrad 

I always instruct people to verify the call for heat and to make sure the Forced Air blower is not turned on. Specifically let the gas furnace control the blower fan.  

Now get a Volt Ohm Multimeter and electrically verify the call for heat is being made. To do this verify voltage is arriving on W1 wire at the Furnace.  

AC Cooling Wizard

[MyNameIsBrad]

Thanks for the reply. Should I be metering between W1 and Rh at the furnace, or between W1 and ground?

[CoolingWizard]

@MyNameIsBrad , 

No. the W1 wire will be carrying the voltage from R. Measure W1 wire and bare metal frame of the cabinet or the C terminal on the furnace control board.  Keep on mind there is a safety switch that opens when you take cabinet panel off.  Use a piece of tape to keep it closed.

AC Cooling Wizard 

[MyNameIsBrad]

Measuring from W1 to the chassis of the blower fan gives me 27.7 VAC when the Nest is calling for heat. No heat but my control board is giving me a pressure sensor error.

I've tested the sensors NO continuity and closed continuity once the inducer is going and they're fine.

Shorting Rh and W1 at either the Nest base station or at the board terminals does activate heating. 

My system doesn't have a C wire, but it's worked  these past few months. I'll pull a C wire and report back

[CoolingWizard]

@MyNameIsBrad ,

You need to understand the gas furnace heating cycle.  
The thermostat calls for heat by connecting Rh to W1 thus sending 24VAC to the Integrated Furnace Control-board ( IFC).  The IFC will turn on the induction fan motor and wait for the pressure switch to close thus proving the combustion chamber has no leaks.  
Next the IFC sends power to the igniter which can be a spark igniter or hot surface igniter.  
Once the igniter is on the IFC turns on the gas valve and checks the flame detector sensor. If no flame is detected the IFC closes the gas valve and purges for 5-minutes then restarts the cycle. 
If flame is detected the gas remains on and IFC looks for the rollout sensor and if present will turn off the gas valve and purges then enters a hard lockout state that will require a technician to come service the furnace.  

If you perform a continuity test across the vacuum sensor and it does not close when the induction motor is running then that causes the IFC to suspend the heating cycle and reset. 

AC Cooling Wizard 

[MyNameIsBrad]

@CoolingWizard Don’t worry, I understand the chain of events, I spent most of the day watching troubleshooting YouTube video, studying the manual for flash codes and testing various parts of my furnace.

When my Nest called for heat it didn’t work no matter what I did (factory reset, power cycle, safety temps, etc) and would give me a pressure switch error code.  Testing the pressure switches with a meter showed no errors.

But manually connecting Rh to W1 worked no problem with no error codes flashing

It’s a brand new control board, so that leads me to believe the issue is with the Nest.  I have some 2 conductor 16 AWG left over from my lighting control system days so I used that to run a C wire, and knock on wood the system seems to be working, Nest is saying make heat and there is burning gas. 

For anyone in the future running across this thread, if you don’t have a C wire, pull a C wire.

[CoolingWizard]

@MyNameIsBrad ,

with the nest learning thermostat sometimes if the battery voltage gets low, it cannot send enough voltage to actually start the system up. So yes by adding the C wire you maintain a high voltage and current available in the nest battery and it should all work fine now. 

AC Cooling Wizard