Nest power connector doesn't seem to do anything useful

[jonbrewster42]

Just put a new Nest thermostat on our furnace, courtesy of our energy supplier.  No C-wire connection on the old thermostat (just R and W wires) so I installed the Nest Power Connector (hereafter NPC 🙂 .  No joy there either.  Eventually I decided maybe it was bad, so I ordered a second NPC, but no change.  Eventually I bought a little transformer billed as a C-wire replacement and voila! the Nest thermostat works.  (Connected it to the R and C terminals on the thermostat.)

What I'm hoping for here is some enlightenment about this (mis)adventure.

The furnace  is a Burnham Alpine furnace, about 10 yrs old.  I don't have much in the way of diagnostic equipment, just a digital multimeter, but from what i can see,  the only source of power for the NPC is either the R and W pair o connections to the furnace controls or one of those and the C connection.  However, it looks like there is only a pulse train across the R-W pair, 20-ish volts with a 3 or 4 second period, and a 3 volt or so pulse train across the W-Common pair.  My conjecture is that the device perhaps isn't a universal fix.  Am I wrong?

voila

[CoolingWizard · Accepted solution]

Jonbrewster42,

You need to understand how the heating system control works.   The R terminal is the input power to the thermostat from the transformer.  This power is connected to the W terminal when heat is being called for.  This sends the 24 volts to the furnace and that starts the heating cycle.  

not having said this, there are some older systems that do not operate on 24 V AC. They operate on a small current of about 500 mA to 700 mA. In this case, what’s happening is you are completing a circuit between the R and the W, or in most cases between T1 and T2 on the furnace. This connection signifies to the gas valve to be initialize to start the gas burners.

Now, if your system was one of the low-voltage type, you’re adding the step down transformer in essence converted it to run on higher voltage however, you must be careful that you don’t try to send 24 V to the gas valve that’s expecting to have 500 to 700 mA of current only. That will damage the gas valve. You need to verify that your standard heater uses a 24 V AC control system before you added that transformer.

The AC Cooling Wizard

[CoolingWizard · Accepted solution]

Jonbrewster42,

You need to understand how the heating system control works.   The R terminal is the input power to the thermostat from the transformer.  This power is connected to the W terminal when heat is being called for.  This sends the 24 volts to the furnace and that starts the heating cycle.  

not having said this, there are some older systems that do not operate on 24 V AC. They operate on a small current of about 500 mA to 700 mA. In this case, what’s happening is you are completing a circuit between the R and the W, or in most cases between T1 and T2 on the furnace. This connection signifies to the gas valve to be initialize to start the gas burners.

Now, if your system was one of the low-voltage type, you’re adding the step down transformer in essence converted it to run on higher voltage however, you must be careful that you don’t try to send 24 V to the gas valve that’s expecting to have 500 to 700 mA of current only. That will damage the gas valve. You need to verify that your standard heater uses a 24 V AC control system before you added that transformer.

The AC Cooling Wizard

[Patrick_Caezza · Accepted solution]

How did you wire the Google Nest Thermostat?

The correct way to use an external 24Vac transform is using the Google Nest Power Connector. This is how it should be wired

[jonbrewster42]

Thank you; that has resolved my issue.  There is a C-terminal in the furnace controls, but apparently it has nothing to do with the thermostat wiring.  Wiring the Nest Power Connector  per the diagram you posted did the trick.

[Jeran]

Thank you for helping out on this thread, CoolingWizard and Patrick_Caezza!

Hey there jonbrewster42,

I'm just checking up on this thread, and I'm glad to hear that your issue was resolved. In that case, do you have any more questions or need any additional help? If not, I'll go ahead and lock the thread in 24 hours.

Best regards,
Jeran