Nest Thermostat Cycling Inducer


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Old 06-14-22, 10:55 AM
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Unhappy Nest Thermostat Cycling Inducer

Hey Guys,

So I installed the base model Nest Thermostat to my DC90 Tempstar Gas, Forced Air System.
I have C,Y,W,R,G wires and they are all for sure wired correctly and hooked up.

For whatever reason my furnace inducer keeps firing up for 10 seconds and then turning off, the only time it doesn't do this repeatedly is when I run a Heating Cycle. What the forums across the internet are telling me is that it has to do with the power stealing from the nest thermostat; however, i have a C wire, which seems to fix that problem for everyone else.

Help would be appreciated quickly because right now I'm just running AC and unplugging Heat.
 
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Old 06-14-22, 12:37 PM
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Welcome to the forums.

Here's the problem.....
When a wire is actually in the Y terminal.... the nest uses Y and C for power.
In your case.... your Y terminal may not be referenced to the furnace C.
Can you post a pic of your furnace board/wiring ?

 
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  #3  
Old 06-14-22, 07:19 PM
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Verify that the C wire is actually hooked up at the furnace end. The newest Nest will try to steal power from both Y and W if C doesn't work. Some furnaces really don't like that.
 
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Old 06-15-22, 06:43 AM
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Okay okay so I do have a C wire that I stripped and hooked up on both ends. My nest registers that it sees the C connected, that's not the problem. This pulling from Y seems like it could be it. Here is a picture of my setup, it's not accurate because I have restripped and reconnected all the wires. Returning G to G and now have a Common wire on C, also there is no Jumper now. Wish I had a better picture but I'm at work!

P.S. I believe the additional Pink and White wire lead to my AC unit.
 
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Old 06-15-22, 11:06 AM
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If you disconnect the W wire at the furnace and run for a while does it stop the inducer cycling? If so the solution may be to install an isolation relay. Another easier solution that sometimes works is to install a resistor between W and C.
 
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Old 06-15-22, 11:15 AM
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Yes when I disconnect the W wire the inducer stops cycling. What does a resistor do and what does an isolation relay do?
 
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Old 06-15-22, 08:54 PM
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The theory when this happens is that Nest is leaking current which causes the furnace to see voltage above 0 vac on its W connector, maybe 6 vac. Most systems don't care if they see a small voltage and will only react and activate if they see 18 volts or so (based on 24vac standard). Your furnace appears to be more sensitive to activate the inducer but not sensitive enough to also activate the burner.

A resistor between W and C would bleed off some of the current which would reduce the voltage seen by the furnace. It wouldn't go down to zero but close and should be enough to not activate the furnace's inducer.

A relay would isolate the furnace from Nest's current leakage by sending that current through its coil. The relay should ignore that small current (causing it to see that 6 volts) and only activate if it sees 18vac or so. When it would activate it would connect the furnace's W to R. So the furnace would see 0 vac without the relay active and 24 vac when the relay was active.
 
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Old 06-16-22, 06:19 AM
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Okay and do I just install the resistor as I would a jumper?
 
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Old 06-16-22, 07:51 AM
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Yes, install the resistor just like a jumper. One end on W and one end on C. Leave the other wires attached under the screws.
 
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Old 06-17-22, 09:33 PM
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A resistor will work but 220 ohms will get very hot.

You're using green for C and you added a G jumper to Y.
Did you do a re-setup after connecting the C wire ?
If not.... it may still be using the W for power.
 
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Old 06-18-22, 02:01 PM
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Hi, have you measured the voltage between R & C at the stat location? I love all of this M M stuff to gat a Nest to work!
Geo🇺🇸
 
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Old 06-18-22, 03:15 PM
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That won't help.

What makes things challenging is that the thermostat actually knows which terminal has a wire in it.... whether it's dead or alive. When you do a setup ..... the nest looks at all the connected wires and firstly tries to ID what type of system it is. It's not always correct. If you change or add a new wire..... you need to do a re-setup so that the thermostat sees the new wiring. It will then prompt you thru the programming stages again.

It was first set up with no C which means it's trying to get power thru W.
 
 

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