Honeywell Wi-Fi thermostat Transformer?

RedWireFebruary 23, 2014

I am about to replace my old 2 wire (red, white) thermostat with this Wi-Fi device but need some input on what transformer to use. I chatted with Honeywell TS and was told the correct method for wiring but was given info on purchasing the 110-24VAC. They had me order this what seems to me massive one that is rated as 24Vac 40VA. Now if I'm not mistaken the 40VA means 40 amps. I'm no electrical genius but it seems to me this thermostat will no way require 40 amps. My question is how small can I go to get where I want to be. The way it looks now I have to mount this sucker into some metal box and maybe add a fan and UPS. The cable The Home Depot sold me looks like it will support a fridge. Bigger than what runs into my A/C.

Anyone with anything to add that will seem more reasonable?

Thanks in advance.


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I believe the formula for this is-
40VA means that at an effective voltage of 24V it can handle an effective current of 40VA / 24V = 1.67A

    Bookmark   February 23, 2014 at 1:01PM
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Is the transformer just a plug-in transformer type? As steve_fl said, the 110 current draw will certainly be less than an amp.

    Bookmark   February 23, 2014 at 1:15PM
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That's more like 40watts - not 40amps.

Not unreasonable at all.

Here is a link that might be useful: 120/24v Universal Mount Transformer (40VA)

    Bookmark   February 23, 2014 at 2:08PM
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I'm trying to figure out how to reply to each response but I'm just not seeing it so.

Thanks Steve for the formula.

jreagan this is a bare bones AC transformer that needs an enclosure and maybe a ground somewhere. That brings me to the power cord that has 3 wires. I was hoping for a simpler solution like a RadioShack device with built in plastic case.

dennisgli this link shows something like what I have. Model Number PF42440 Home Depot

    Bookmark   February 23, 2014 at 2:43PM
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How are you planning on mounting/enclosing that Home Depot transformer? Why do you want a llastic case?

    Bookmark   February 23, 2014 at 3:38PM
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Not sure why they sold you a bare transformer when you can easily find a plug-in transformer that produces that voltage and wattage. Here's one for $9.50

Here is a link that might be useful: Wall Transformer

    Bookmark   February 23, 2014 at 5:26PM
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dennisgli I'm not trying to find a plastic case for this transformer, I'm trying to find an alternative power source that has it's own case. Like so many of the others I have. All are plastic.

I am sure now I was mistaken by ordering the exact one Honeywell tech support recommended. At the time I was not aware this would be the one I received.

Thanks for the link jreagan. This one looks perfect. Just what I was hoping.


    Bookmark   February 23, 2014 at 6:44PM
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This transformer looks like it could mount to a standard electrical box through a knock-out on the side. The black and white wires would connect (wire nuts) inside the box. The red/green would be 24v.

    Bookmark   February 24, 2014 at 8:51PM
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That's what I thought. I've decided to go with the one in the link denisgli provided. I

t does seem od that this rather large transformer is only a little over 1 amp but I'll just toss it back in the box and stick in on a shelf somewhere with the rest of my collection of maybe useful trinkets.

Thanks weedmeister.

    Bookmark   February 25, 2014 at 3:35AM
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I decided to go with the device in the link provided by jreagan.

    Bookmark   February 25, 2014 at 3:40AM
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Typically transformers for things like doorbells or HVAC systems are hard wired so you don't have to worry about somebody unplugging them - or the plug falling out - etc. The one that I linked mounts on the knockout on a standard electrical box or circuit breaker panel.

If you want one that can be unplugged then the other one should be fine. But do note the reviews that say that it has an internal fuse - so if you short the output you will have to buy a new one.

This post was edited by dennisgli on Sun, Mar 2, 14 at 12:58

    Bookmark   March 2, 2014 at 9:31AM
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BTW - when I replaced my thermostat with one that needed power I just replaced my ancient control relay box with one that has a built-in transformer and fuse for powering the thermostat. It was under $50 and made for a clean installation.

Here is a link that might be useful: Universal Switching Relay w/ Internal Transformer

    Bookmark   March 2, 2014 at 9:41AM
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