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Video Doorbell Transformer and Heat

 
 
illusha
 
Reply Wed 30 Jan, 2019 07:10 pm
I am installing a Nest Hello video doorbell. I’ve installed all new thick-jacket low-voltage wiring and a new 24V-20VA transformer to support 2 mechanical chimes. The transformer is installed in an attic full of deep blown-in cellulose insulation (same location where the old one was).

A regular doorbell only energizes the transformer "momentarily" when the button is pressed. My understanding is that video doorbells require "constant" power. But most transformers I've met start getting hot under constant power.

I've heard of cases when a regular doorbell button gets stuck, which continuously energizes the transformer, burning it up within hours. That tells me that these transformers were not designed for "continuous" duty cycle. Yet, Nest tech support is telling me that it's "perfectly safe", they do not expect the transformer to heat up much more than ambient temp and there are thousands of people using these setups nowadays.

So, my question is, if running under a constant power from a Video Doorbell will make the transformer run much hotter than with a Regular doorbell button?

Are there any factors, such as output wire gauge for example, that would help me keep the transformer temperatures down? Any suggestions on fire-proofing the install location, I can't move the transformer without much rewiring, but at least I did not cover it with insulation like the old one was. Should I find some fire-resistant materials (like HardiBacker) to surround the junction box + transformer to isolate them from contacting the cellulose insulation or is that too much?

Am I overthinking all this? I know I could just hook it up and see what happens. I just prefer not to experiment on things that could start a smoldering fire in an attic : )

I mainly don’t want to constantly worry about how hot the transformer gets in that attic full of loose insulation because it’s being used "continuously" rather than "momentarily".
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engineer
 
  2  
Reply Thu 31 Jan, 2019 07:23 am
@illusha,
Transformers get hot when you pull a lot of current through them, but under low loads they are fine. I haven't looked up the specs on the Nest, but if it is just in standby mode, I doubt it is pulling much current. I think you can trust the tech support people on this one.
illusha
 
  1  
Reply Thu 31 Jan, 2019 04:32 pm
@engineer,
I am interested in learning the science of it all due to the reports of transformers dying from overheating when a mechanical doorbell button gets stuck and a possible resulting fire hazard.

My particular transformer outputs 0.83A (20VA/24V) max. Nest recommends a minimum of 16V-10VA (=0.62A) transformer and I chose a slightly higher rated one as I have 2 chimes. There are no power consumption specs I could find for any video doorbells anywhere. But there was a guy who measured power draw of a similar Ring Doorbell on a blog or a forum, I can’t find the source at the moment, but his measured readings were something like 0.4A in standby, 0.6A when video triggered, and 0.9A when button pressed (I could be slightly off on the numbers, but that’s the ball park). If those numbers are correct, I guess Nest draws slightly less than Ring doorbell, but it would still be pretty close to the transformer’s maximum rated capacity.

I’m trying to understand how this continuous draw from a digital doorbell is different from a continuous draw from a mechanical button getting stuck and overheating the transformer.
engineer
 
  2  
Reply Fri 1 Feb, 2019 07:19 am
@illusha,
The continuous draw is likely milliamps. It is no where near what is required to energize your chimes. Think of the Nest like a cell phone and infrequently is called on to be a doorbell.
illusha
 
  1  
Reply Fri 1 Feb, 2019 06:38 pm
@engineer,
Here’s that post I was referring to with Ring doorbell, guy measured 0.14A in standby, 0.57A motion triggered, and 0.92A when button pressed, although I think that Nest version may draw slightly less based on their stated requirements - https://www.reddit.com/comments/7uxhfl - I might have to perform similar measurements myself just out of curiosity.
engineer
 
  2  
Reply Fri 1 Feb, 2019 08:55 pm
@illusha,
From a heat point of view, the amount of heat is going to be related to the current squared, so you get 43 times the heat when the button is pressed that you do in standby.
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taopathway
 
  1  
Reply Mon 3 Jun, 2019 12:16 am
@illusha,
Just make sure it is in the proper junction box with a cover. Transformers will not get hot unless you hook the wires incorrectly. I mis- wired the doorbell transformer once and my boss was with me and we thought it might blow. However a transformer is just a coil of wires creating magnetic fields so no combustibles.
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