[xa1200]

Hi, I got a 3rd gen nest for Christmas and I am having a heck of a time making it work. Without the C wire, Nest reports an error with the R wire having no power. When the C wire was connected, it would cause my furnace to constantly turn on and off. From my research, the furnace coming on and off is a power sharing issue that should have been resolved with a C wire. But in my cause, the C wire was causing the issue..

After a few tries I went back to my old thermostat, a Honeywell, that did not require a C wire because it runs on battery. However, since I already connected the C wire I decided to hook it up just in-case the battery runs out and it caused my transformer to blow up.

1. So my question is, why did the transformer malfunction when the C wire was connected? Looking at the furnace diagram, it only shows G-Y-W-R connecting to the thermostat but leaves the C wire terminal empty. Is it possible that it wasn't design for powering a thermostat? The wires are not shorted *tested using continuity with my DDM.

2. I want to run a secondary transformer to power the Nest independently. Can anyone tell me how to do this? I did my best to look on google and youtube but I never found a conclusive answer..

3. To prevent any damages in the future, I decided to rig up a fuse for my replacement transformer.

Should the fuse be placed on the primary side or secondary?I currently have it on the secondary side but i wanted to make sure

How many amps should my fuse be? I currently have a 5 Amp but i believe it should be either 2-3.


My Current setup:
R ------ Thermostat
W ------- Thermostat
Y ------- Thermostat And to the condenser Contactor
G ------- Thermostat
C -------- To the Condenser Contactor and *just recently to Thermostat

Honeywell HQ1009836HW Control Board
120V 24VAC 40VA Transformer

 

[user_12345a]

maybe the va rating of the old transformer wasn't high enough or there's a problem in the honeywell, some internal short on the 24v side?

nest? likely far more trouble than it's worth. you can get a wifi honeywell for less and it's probably a better quality product that switches stuff on and off with proper relays, not triacs or whatever.

 

[jimn]

Try calling Nest. I have three (no C wire, but in my case just R/W. ). I did have to use customer support once and I found them excellent

 

[supers05]

That 40VA transformer should be fused at about 1.5A. We commonly install 75VA or larger when replacing.

40VA is probably too small for a fancy thermostat. My cell phone takes 10VA by itself, just for comparison. Use your DMM to measure the current draw without the t-stat and the furnace running. Add another 2-6VA for your AC contactor coil.

Cheers!

 

[Bob Sanders]

Looking at the furnace diagram, it only shows G-Y-W-R connecting to the thermostat but leaves the C wire terminal empty.

I'm a little confused here. Was there originally a C wire running out to your condenser from the air handler? (In other words does your condenser have its own transformer?)

 

[xa1200]

maybe the va rating of the old transformer wasn't high enough or there's a problem in the honeywell, some internal short on the 24v side?

Its probable. However, I read that Nest should only take about 100ma ;\

That 40VA transformer should be fused at about 1.5A. We commonly install 75VA or larger when replacing.

40VA is probably too small for a fancy thermostat. My cell phone takes 10VA by itself, just for comparison. Use your DMM to measure the current draw without the t-stat and the furnace running. Add another 2-6VA for your AC contactor coil.

Cheers!

I actually ordered a 40VA because it was the only one available for next-day shipping on Amazon. We had this issue on Christmas and we were freezing Friday and Saturday! But I did place an order for a 75VA coming next week.

I'm a little confused here. Was there originally a C wire running out to your condenser from the air handler? (In other words does your condenser have its own transformer?)

The condenser was sharing the same transformer from the furnace by connecting to Y and C terminal on the control board.