Replacement transformer

[tcianci]

Hello All,

My central AC has crapped out. I removed the thermostat to jumper the correct leads to see if it was the thermostat. That experiment yielded no results. Since there was no common wire running down to the thermostat, I went to the air handler and started reading voltages. There was 122V at the input of the transformer that supplies power to the thermostat and 0 at the output. Then I disconnected the transformer secondary to be sure there was nothing loading it. Still no voltage. I didn't have time to actually ohm out the secondary without power applied but I'm going to assume it's open.

This is an old Comfort-Aire air handler built by Heat Controller Inc. I'm not having any luck locating an exact replacement transformer but I do have a 50 VA 120/24 VAC transformer and a quick calculation shows it's good for a couple of amps.

There's plenty of room in the enclosure for this transformer, it's actually just a little larger than the existing unit. There's not much to go on on the wiring diagram although there may be some sort of part number on the transformer itself once I remove it. My question is, does anyone know offhand what a typical transformer in this application would be rated for. From what I can see, it supplies the power to the thermostat and probably a contactor for the blower motor and one for the compressor.

I know I can call the service department of the guys who installed it but if they're only going to install a generic transformer, I can do that myself.

Any comments?

 

[mrobins297aaa]

I would think it would be fine, years ago when we did AC on a system that wasn't prepped for AC we used 40va transformers

 

[brewchief]

Most modern air handlers are 240v, verify that yours is 120v before replaceing the transformer, it's possible to lose one leg of the input power. If it's a 120v unit then yes the 50va transformer will be fine, in all probability it came with a 40va from the factory.


Sent from my XT1254 using Tapatalk

 

[Trey T]

50VA/24V is ~2A, which is typical.

That amount of current should be sufficient for powering thermostat, circuit boards, and contactor.

 

[bwringer]

Just installed a universal transformer from Amazon in my neighbor's system. As noted, just make sure you have the voltages correct so you get 24VAC. Most take 240VAC in; the universal I installed had extra wires so it could take 240 or 120 in and output 12 or 24VAC.

The draw should be less than 2 amps.

 

[tcianci]

Thanks so much. Yeah it’s actually 120 volts on the primary. It’s wired into a circuit that supplies the attic lights. I remember that the contractor had to order a 120v air handler. I’ll probably replace it tonight

 

[Climatecreator]

This is what I carry and use for most non OEM repairs. https://amzn.to/2MG3IfI it covers most everything.

Also add in an inline fuse while you're in there.
I use two female disconnect crimp connector like these https://amzn.to/2I8SOeU and plug them into a 3 amp automotive fuse like these https://amzn.to/2tozDIM

Good luck

CC

 

[tcianci]

Don’t you hate it when you get into a thread and you never find out what the outcome was?
Me too!!! And there is still no outcome.
I did install the 50 VA transformer, without a fuse, of course. I ran it open circuit and read a little over 26 VAC, and about a minute into my adventure, the replacement transformer went pop. But, not before I got a chance to read the secondary voltage under load, which was about 10 volts. Not good. Fortunately, I sourced another transformer in a few minutes and started checking things in a little more orderly manner. Turns out it was the contactor coil out at the compressor that was shorted. I couldn’t pull one of those out of a hat on a Saturday, so I ordered a new one online, I should have it in a few days.

 

[Trey T]

Stock extra contactor and caps

 

[Fixin'Stuff]

What Trey said. I always keep a spare contactor and cap on hand. They always seem to fail on Friday at 5:30 PM of a 3-day weekend.

 

[bonneyman]

Sorry I didn't get to your post fast enough, I might have saved you a burned transformer. Whenever I swap out a trans I install a fused connector and fuse in the secondary side. Just in case there's a hidden or intermittent short - it'll take out the fuse and not your new trans.

http://www.littelfuse.com/products/fuse-blocks-fuseholders-and-fuse-accessories/fuseholders/150.aspx

I've even made-do with an auto blade fuse and in-line fuse holder. Being that alot of units use 3-5 amp blade fuses on their control boards, I have a stash of those on hand already.

 

[Trey T]

^that's a great advise, which reminds me of a fuse kit I bought for my other house needs to be installed.

 

[tcianci]

My new contactor arrived today. I installed it and livened everything up and hit cool on the thermostat and Lo and Behold... COOL!

Thanks guys!