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Furnace not running in proper sequence

DB413MA
Community Member

I just installed a Nest Learning gen 3 thermostat to my oil FHA furnace (no a/c or other accessories). The thermostat is messing up the operation sequence of the furnace. PROPER operation is: stat calls for heat -> burner fires and preheats furnace -> blower comes on in about 90 seconds -> blower and burner continue to run until room is at temperature -> burner shuts down -> blower runs for additional 2-4 minutes, then shuts down.

After I installed the Nest, the sequence changed to: stat calls for heat -> burner fires and runs for 4-5 minutes, then shuts down -> then blower comes on and runs for 4-5 minutes, shuts down.

This sequence repeats 2-3 times until the room eventually heats up, but the sequence is clearly wrong (and possibly dangerous because the heat exchanger in the furnace is overheating since the burner runs so long without the blower switching on?).

This is not a problem with the furnace. I re-installed the old thermostat, and the furnace returned to normal sequencing. I hooked up the Nest for a second time, and the problem happened again.

Can anyone help? THANKS!

1 Recommended Answer

DB413MA
Community Member

Thank you for your interest in helping!

I read many threads and posts suggesting that if the thermostat is installed without a "C" wire feed, the voltage may be too low to properly trip the necessary relays/etc. on the furnace control board.

Because I only have a 3-wire cable running from the furnace to the Nest, I re-purposed the blower control wire to become a "C" wire. This solved the issue immediately.

The good news is the furnace is sequencing correctly again. The bad news is I've lost the ability to run just the blower independently of firing the furnace up without running a new 4 wire cable or buying the proper power adapters to enable this.

I think Nest is being a LITTLE deceptive by marketing this unit as compatible with systems that don't have an active "C" wire. As I've worked through this issue, I discovered dozens and dozens of comments and threads that all have a low voltage issue/resolved with C wire connection theme.

Luckily, I'm handy and comfortable enough with electricity and changing wires to work through the issue and solve this by trying suggestions I found in the forums. Otherwise, I'd have had to pay for a furnace repairman service call to resolve an issue that should have maybe been made more clear upfront!

Thank you again for checking in!

View Recommended Answer in original post

8 REPLIES 8

EdwardT
Community Specialist
Community Specialist

Hi DB413MA,

 

Thanks for reaching out. I'm sorry to hear that you're having an issue with your Nest Thermostat. I'd love to know more about this and I have a few questions:

 

  • What type of Nest Thermostat do you have?
  • How is your Nest Thermostat wired?
  • What is the power reading of your Nest Thermostat? You can find this by going to Settings > Technical Info > Power.

Looking forward to your response.

 

Thanks,

Edward

EdwardT
Community Specialist
Community Specialist

Hi DB413MA,

 

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do. 

 

Thanks,

Edward

DB413MA
Community Member

Thank you for your interest in helping!

I read many threads and posts suggesting that if the thermostat is installed without a "C" wire feed, the voltage may be too low to properly trip the necessary relays/etc. on the furnace control board.

Because I only have a 3-wire cable running from the furnace to the Nest, I re-purposed the blower control wire to become a "C" wire. This solved the issue immediately.

The good news is the furnace is sequencing correctly again. The bad news is I've lost the ability to run just the blower independently of firing the furnace up without running a new 4 wire cable or buying the proper power adapters to enable this.

I think Nest is being a LITTLE deceptive by marketing this unit as compatible with systems that don't have an active "C" wire. As I've worked through this issue, I discovered dozens and dozens of comments and threads that all have a low voltage issue/resolved with C wire connection theme.

Luckily, I'm handy and comfortable enough with electricity and changing wires to work through the issue and solve this by trying suggestions I found in the forums. Otherwise, I'd have had to pay for a furnace repairman service call to resolve an issue that should have maybe been made more clear upfront!

Thank you again for checking in!

EdwardT
Community Specialist
Community Specialist

Hi DB413MA,

 

Thanks for the update. I'm glad to hear that you were able to get it working. You could also check the Nest Power Connector so you don't have to run a new wire on your system.

 

Thanks,

Edward

EdwardT
Community Specialist
Community Specialist

Hey DB413MA,

 

It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do. 

 

Thanks,

Edward

 

EdwardT
Community Specialist
Community Specialist

Hi DB413MA,

 

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

 

Thanks,

Edward

DB413MA
Community Member

Hello,

I resolved the issue by repurposing the blower control wire to become the C wire. The issue is resolved.

EdwardT
Community Specialist
Community Specialist

HI DB413MA,

 

Thanks for the update. I'm glad to hear that you were able to get it working.  I will be locking this thread now. Please feel free to create a new thread should you have some more questions or concerns in the near future and we'll be more than glad to help.

 

Thanks,

Edward