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A few questions/concerns centered around the transformer in pictures that I am pretty sure has failed. I'm not an electrician so I'll do my best to provide information but it's been a struggle for me so far just to understand what's actually there and etc., and I am a bit limited by what the person that built home left behind, e.g. unique wiring choices and "iffy" electric panel labeling. So please bear with me...note I am happy to break this into different posts/question if that makes sense.

The transformer is in my attic and attached directly to a junction box that has two live feeds (white romex-style wires) into it, and the switch right above it is the cutoff for my furnace located up in attic as well. A voltage tester pen-thing lights up when passed over all of the wires leading into the j-box, but when I test the output of the transformer with a Klein multi-meter for either (AC) voltage or current, I get nothing on the readout, or more correctly only when I touch the leads to the transformer contacts and then the meter registers something for just a moment but then goes to zero.

So I am pretty sure I need to replace the transformer...I wish I was 100% positive that it is the problem instead of just 90%, though. One thing that makes me confused and anxious is I am not exactly sure what this transformer is to...my doorbell stopped working several years ago but I now want to fix it (which is why I started investigating and DIYing). But I can't fathom why a doorbell transformer would be mounted directly to a j-box like this, and to the same box/circuit that the furnace cut-off is at/on? And it then begs the question...if this transformer (which I believe is bad) isn't the one for the house/HVAC system thermostat, which I believe also has lower voltage requirement and is clearly working, i.e. getting voltage/current from somewhere, then where is that transformer located?

So it seems the only way for me to know for sure if this transformer is bad, and that it is the one for the doorbell, is to replace it with a known good transformer and see if doorbell then works. So an additional question is making sure I understand how to remove and replace it...it looks like there is a screw on the inside of the j-box that tightens a hold-down thingy for transformer (hard to see from pic...poor lighting up there). I guess I can deal that pretty simply once I figure out how to make sure all current is off for the wires leading into j-box. But I noticed that the replacement transformers I see (Amazon, Lowes Depot) don't look like they have similar mounting configuration...should I care?

Transformer circuit overview

Transformer info

J-box internal wiring

Chime Box Pics:

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2 Answers 2

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I'd disagree with @ecnerwal's response above

  1. Use a multimeter to see if the 120v side of the transformer has power. You can also use one of those pens to check if the wires leading to the transformer, or an old fashioned test light.

  2. It was very common years ago to put the transformer next to the furnace, or at least I've seen lots of houses that did that. I just added a c-wire to a transformer that powered both the thermostats and the doorbell. IF you want to see where those wires go, disconnect one and put an electrical tone generator on it and follow it around.

  3. Try removing both wires from the transformer and re-testing it.

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  • "Use a multimeter to see if the 120v side of the transformer has power. You can also use one of those pens to check if the wires leading to the transformer, or an old fashioned test light." I did this with tester pen...there is clearly voltage running along the hot (black) wires in that metal j-box and into transformer. Thanks...
    – AA040371
    Commented Oct 24, 2023 at 15:03
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The co-location and being on the same circuit as your furnace strongly suggest this one is for the thermostat and potentially other low voltage furnace controls, NOT your doorbell. However, those are more commonly 24VAC and that one is 16 VAC per its label.

If this isn't the thermostat, it may just be that someone considered it a handy spot. The thermostat low voltage supply is also commonly inside the furnace itself. If you disconnect these wires and the thermostat still works, you can bet it's not the thermostat supply.

The multimeter test report sounds too much like "user error" to be a definitive result one way or the other as reported. Make sure the multimeter test leads are plugged in to the correct ports for measuring voltage, and set the meter to a suitable voltage range if it's not "auto" - 0-20 or 0-25 VAC should work here. Try testing it while you have the low-voltage wires disconnected - it's possible the doorbell problem is a short in the low-voltage wiring.

If you have a doorbell chime, the doorbell transformer is usually/often hiding behind it.

Mounting a transformer directly to a junction box is quite normal, as it makes it easy to keep the line-voltage wires properly protected and the junctions accessible as is required. The more typical mounting method is a threaded nipple and nut - so long as it's intended to mount to a hole in a junction box, it need not match the way the old one did that.

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  • "If you disconnect these wires and the thermostat still works, you can bet it's not the thermostat supply." I should have thought of this...the ravages of old age...sigh. But when I disconnect the two thin wires on exterior of transformer in pic, thermostat still worked, so that appears to answer that question. Thanks...
    – AA040371
    Commented Oct 24, 2023 at 14:11
  • "If you have a doorbell chime, the doorbell transformer is usually/often hiding behind it." Yes...I had previously checked this several days ago before posting here, based on previous online research. There is only a small j-box with thinner wires coming in, stripped to allow wires to connect to chime box. If there is anything else behind the drywall, I am not aware of it. When voltage tester pen was passed over/around the wires in that chime/j-box, nothing registered. I've added those previous pics I took of chime box area to OP in case they help.
    – AA040371
    Commented Oct 24, 2023 at 14:56
  • "Mounting a transformer directly to a junction box is quite normal..." That makes sense, but I can't seem to identify/find a transformer that looks like it will mount to the metal j-box in same way as 1st pic. Is there a particular type of transformer or search term I should be using other than "doorbell chime transformer"?
    – AA040371
    Commented Oct 24, 2023 at 15:01
  • 1
    As already noted in the sentence after the one you quoted, it does not need to to use the same style of mount to a knockout (and typically won't these days.) A threaded nipple that fills the knockout and a nut to secure the nipple to the box is more common now.
    – Ecnerwal
    Commented Oct 24, 2023 at 15:24
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    You have 3 other pairs of wires in that cable. Try connecting to a different pair. Debug it one step at a time. Spare pairs can and do save the day.
    – Ecnerwal
    Commented Oct 24, 2023 at 15:59

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