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I’m looking to add a video doorbell to my home which was not wired for a doorbell when built. I can run the bell wire to my furnace closet and wondering if I can add a doorbell transformer here.

I’ve read that the HVAC system needs its own dedicated circuit, but I’ve also read that some people install their doorbell transformer on or inside the furnace, using its circuit. Some also wire the doorbell directly to the thermostat’s transformer, which I guess isn’t recommended. Either way, those solutions run counter to the dedicated circuit rule.

If my furnace has a service disconnect switch, will I be able to add my transformer there?

3 Answers 3

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The load for a typical doorbell or HVAC transformer is less than 1A of incoming 120V AC power. So the issue isn't the load, it is reliability, code compliance and ease of installation.

  • Reliability

If you install a transformer as part of your HVAC system then if something goes wrong with the HVAC system and it is temporarily (which can be anything from hours to days) shut down, or if the HVAC system is replaced (the HVAC installer likely knowing nothing about the doorbell transformer buried in the HVAC system) then you will lose doorbell functionality.

  • Code Compliance

Because the load is small and the output is low voltage, I don't think there are any code issues different from other methods of installing a transformer. But there may be something, somewhere.

  • Ease of Installation

You will need to run low voltage wires from the transformer to some part of the doorbell/camera/etc. system. Using the HVAC system as a starting point may be just as good as any other place. But there could be some place that is better - e.g., a general purpose circuit near the doorbell location.

My recommendation is to not use the HVAC system for this purpose. Pick a different circuit that has no special requirements. So that means avoiding:

  • Dedicated Laundry Room Circuit (a.k.a, washing machine)
  • Kitchen Countertop Circuits
  • Bathroom Receptacle Circuits

which typically leaves most lighting and/or receptacle circuits in other rooms - e.g., living room, bedrooms, hallways, basement, etc.

I would recommend a permanently installed (as opposed to plug-in) transformer that mounts on a standard junction box. The easiest thing (because it won't involve making it look nice) is if you have an available receptacle in an unfinished area (e.g., basement) or hidden area (e.g., closet) and can install a metal box next to it or replace existing box with a larger metal box. Metal box here is not just my general preference (though it is) - there are plenty of transformers designed to mount on a metal box, so it is often easier than with a plastic box.

Install the transformer on the box and you just have to figure out how to get the low-voltage wires to the doorbell. You can use any regular 120V/240V methods (NM cable, wires in conduit) or you can just run the wires "however it works" because that is generally allowed with low-voltage wiring.

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I don't think there would be any question about "can", since it is an acknowledged widespread practice. If the furnace disconnect metal switch box has the power running to it straight from the panel, that would seem to be an ideal place to mount the doorbell xformer.

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  • Thanks. I guess I’m more asking if I should wire it to the circuit (does it follow code, etc). Apr 15, 2023 at 18:35
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Why use a transformer at all. There are numerous wireless door bell systems now.

Just search for "wireless doorbells". No messing with a dedicated circuit, or any circuit for that matter. Problem solved!

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    Because battery doorbells don’t have the same features as wired ones. Apr 15, 2023 at 17:52
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    24/7 video recording, for example. I’m not talking about a simple doorbell. Apr 15, 2023 at 18:23
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    Then that should have been in your original question...we are not psychic here.
    – RMDman
    Apr 15, 2023 at 18:29
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    The question was about whether a doorbell transformer can be on an HVAC circuit, not what kind of doorbell I should use. My question was pretty clear about that. Apr 15, 2023 at 18:32
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    Your question was still lacking important information. Just a door bell will use power for a second or less. Video surveillance uses constant power.
    – RMDman
    Apr 15, 2023 at 18:34

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