I am replacing my ring doorbell with a Unifi G4 Doorbell and when I hooked it up, I was alerted that the power was too low. Through some searching I discovered that I need to replace my transformer.

I spent a long time looking for my doorbell transformer and was unable to locate it. I found one attached to my furnace, but I assumed that transformer was specifically for the furnace. See the picture below for how this is hooked up. But I went ahead and flipped off the breaker for the furnace and the doorbell stopped working as well.

So my first question is whether this is the doorbell transformer, or if it's potentially shared as both the doorbell and thermostat transformer and how I could further test that. If not, are there any tips on locating the doorbell transformer?

If it is the right transformer, how do I go about replacing this?


I went ant tested the voltage with a multimeter and it came in as 21.2V

Update 2

I tested flipping off the power to the furnace (The furnace switch is attached to the box in the picture). My thermostat reported no power on the red wire, but this transformer reported 21.2V still. This feels like more confirmation that this is for the doorbell.

Transformer tied to Thermostat

  • 1
    Disconnect those wires from the transformer. Test to see if the doorbell and/or furnace continue to operate. That will conclusively answer the question.
    – user71659
    Jan 1, 2023 at 23:44
  • @user71659 Of course! I tried everything except that, that did the trick and confirmed that this is specifically the transformer for the doorbell. Now to determine how to replace it. It seems to be screwed in and there's just not enough room to unscrew it as the furnace intake is in the way. I'll probaly have to remove the whole junction box
    – Dan Drews
    Jan 1, 2023 at 23:53
  • @DanDrews you'll have to get the junction box open (with the breaker off, of course!) to remove the transformer Jan 2, 2023 at 0:13
  • Also, what is the bulgy shiny thing that forms the cover of the box? (I suspect its a fuse holder) Jan 2, 2023 at 1:36

2 Answers 2


Sounds like this is doorbell-only and the furnace has its own transformer inside it. (the normal situation).

Why not just ditch this transformer and use the furnace's transformer? It is honest 24V and It's Right Here. Go inside the furnace and find the "R" and "C" wires. Connect the doorbell red to R and doorbell white to "C". That should give you honest 24VAC across the chime/switch.

However, I am skeptical of the "power too low" indication. That does not mean "voltage too low". If your doorbell is wired up "2-wire", what's actually happening is the doorbell is wired in series with the chime. The doorbell leaks some power through the chime to power itself (not enough power to actually ring the chime). So your "power too low" may mean perfectly good voltage but too high impedance (resistance) of a chime, which would be typical of a modern electronic chime.

That can be corrected by placing a shunt "resistor" across the chime to lower its resistance. Since it's AC power, a capacitor may be a better choice. Lutron makes the LUT-MLC to solve a similar problem with dimmers; it's made for higher voltage and higher current but it may work. I'd try it. I recommend a product made for such uses rather than a Radio Shack component which is not made for open use like this.


You appear to have two transformers in the picture, fed by the same circuit. I don't think there's anything horrifically wrong with that although it might not be allowed in your jurisdiction since the furnace is meand to be on a "dedicated" circuit. Whether a second transformer fed by the same line makes it not dedicated I do not know.

The best approach


  • One of the metal clad cables entering the box supplies power
  • the bulgy shiny thing is the furnace transformer and the other metal clad cable carries 24V to the furnace OR the bulgy thing is a fuse, the second metal clad cable carries 120V to the furnace and the furnace transformer is further along.
  • the yellow transformer is for the doorbell and the white phone wire carries 24V to the bell.
  • the yellow transformer is mounted to a knockout in the box, it has a threaded mounting post and is held into the box by a box nut.


  1. You need to confirm all these assumptions opening the box, tracing and/or disconnecting wires.
  2. If I'm wrong, update the question with more detailed facts. If I'm right you need to look at the power capacity and voltage of the yellow transformer and the power and voltage requirement of your new doorbell. If the transformer has adequate specs you need to measure and see if it's performing to spec.
  3. Let's assume it's under-spec'd or not working, the best approach would be to simply replace it with one that is up to the task. The new one must also be knockout-mounted, so the 120V side is inside the box and the 24V side is outside, just like the existing one. You can't replace it with one that requires either 120V or 24V wires to dangle out of the box.

Another less likely possibility is that the existing wiring includes series-wired old neon-lit doorbell buttons and somehow your wiring of the new doorbell is incorrect so that these lit buttons are dropping the voltage seen at the bell or limiting the power available.

  • The bulgy shiny thing is a fuse holder (probably the old-timey screw-in lamp base type fuse); the furnace transformer is buried somewhere inside the furnace. The two metal-clad cables would both be for high voltage AC.
    – Greg Hill
    Jan 2, 2023 at 1:37
  • @GregHill sounds reasonable, I'll change the assumptions but the steps stay the same.
    – jay613
    Jan 2, 2023 at 1:44
  • Yep, 100% agree. Well written. BTW if you enlarge the photo and squint carefully, it's possible to make out the words "fuse holder" stamped in that cover.
    – Greg Hill
    Jan 2, 2023 at 1:50

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