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Existing Transformer

This is the only doorbell looking transformer that I found in my home. It is right behind the wall that connects to the thermostat “below”. This is inside the garage close to the HVAC.

It does say load 16V 10VA which is what you’d expect from a doorbell transformer. But no idea if it’s solely for the thermostat or is also being shared by the doorbell. It says “Smarthome 1591” in the picture; which on Googling returns results for a doorbell transformer). It's wired right around the HVAC/thermostat so I have no idea if it's a shared one or not.

I attached a Ring doorbell to existing wires and it doesn’t seem to power up. Neither touching the doorbell wires seems to cause the chime to ring. May need to swap out the transformer but not sure it is the right one?

I am planning to install Nest and I may have to replace this one in any case. I just don’t know if I need to find another one for the bell or is it shared? How to be sure?

PS: the existing thermostat is working.

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    Use a multimeter to check if it is really getting 16V. As far as determining what it controls, for anything that is currently working, see if it works after you disconnect the red wires. Jun 15 at 20:46
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    Trying to recall, but Nest's doorbell does specify their product works with a range of volts and amps (don't have that handy and may not be true of newer models). Can't speak for Ring. If you have a multimeter, you can determine if a sufficient range of volts & amps are provided. You can disconnect one of those wires to see if it affects the doorbell (and/or anything else). If it is both the doorbell transformer and correct range, then it should work. If it is the doorbell transformer and not the correct range, then you can easily replace it. Nest's docs should indicate how to use multimeter.
    – Sam
    Jun 15 at 20:47
  • 10VA seems small for a modern smart doorbell-type device. IIRC, Ring cams suggest 30-40VA transformers. From a vendor site: "The Ring Doorbell Pro requires a constant power supply with a voltage between 16V-24V and at least 30VA of amperage"
    – Armand
    Jun 15 at 21:23
  • Did the doorbell work before? If it did and now you touch the wires and it doesn't then you need to do some troubleshooting before trying to install the Ring doorbell. Jun 15 at 21:37
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    Unhook it and find out. This is the low voltage side so should be OK to touch, but turn off breaker anyway. It's 16V so too low-voltage for a thermostat. Also 10 VA is awfully low to power controls on a furnace and A/C unit. Jun 16 at 0:15

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Use a voltmeter (multimeter) and test voltage at the doorbell wires, voltage at the transformer terminals, and continuity of wires to make sure they go where they appear to go.

Ring has very good documentation, you should be able to find instruction manuals and articles for any Ring model. Fitting those to your situation may be tricky.

Consider this graphic from Ring.com:

doorbell, transformer, chime wiring diagram

The doorbell, transformer, and chime make a loop that's broken at each device. You may find that you need a jumper cable at the chime to complete your loop and charge your doorbell. This removes the chime's ability to sound, though.

Closing the loop at the doorbell by touching the wires together (without the jumper between chime terminals) should sound the chime, and you indicated it doesn't. That may mean these wires/this transformer don't belong to the doorbell, or that the doorbell wires are broken at some point. Continuity testing will tell you whether that's the case, and you may be able to replace wires to fix it.

Worst case, some Ring doorbells (and other brands as well) can run on batteries. You have to disconnect occasionally to plug it in and charge it back up, but if your wiring won't cooperate and you really want video of visitors, it can be useful.

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