Why does the 115V->24V transformer keep blowing on my Trane XB80?

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A couple of months ago the heater quit coming on in my upstairs Trane XB80 HVAC unit. It's been off warranty for about 2 years, and I am enough of a do-it-yourselfer that I'm comfortable with simple repairs. I found that the LED on the control board was not lighting up, and subsequently found that I was not getting 24V output from the 115V->24V transformer. Sure enough, the 24V circuit was open, so I found a local replacement transformer (not the exact form factor, but same specs) and things worked fine after I replaced it. Now, I have the very same problem (only this time it's the A/C that's trying to come on; not that it matters), and once again 24V is missing on the output side of the transformer. This time the 24V circuit has continuity, but the 115V circuit is open. Of course, I can replace the transformer AGAIN, but I'd sure like to know what could be causing this problem... Any ideas would be appreciated. Thanks.
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On 4/2/2011 2:10 PM, Steve Turner wrote:

Cheap transformers comes to mind???
Had other indications of transients? Any chance there's something on the load side that's drawing excess current?
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On 4/2/2011 3:23 PM, dpb wrote:

That is what I think also. Google yields a slew of hits for 24 Trane xformers.
Transformers can short inside (winding to winding) and they will wind up smelling burnt. That may burn one of the leads leading in. The fact that you have one with a primary failure, and another with a secondary leads to thinking they are designed at the margin.
Someone that knows Trane might come along and offer an opinion. I rarely saw opened transformers, and when I did it was on the input side. Sometimes they put in fusible links in lieu of a fuse elsewhere, but this would be audio gear...
Jeff
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On Sat, 02 Apr 2011 14:10:01 -0500, Steve Turner wrote:

Probably should measure the power consumption and see if it is in or out of spec with the transformer if you can find the specs. If it's in watts the formula is volts x amps. 24 volts x 500 miliamps = 12 watts and so forth. If the draw is higher than the specs of the transformer then something on the 24 volt circuit is drawing too much current. You might be able to spec the transformer by calling Trane.
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We have a winner folks. My vote for the best answer. Instead of speculating on transformers, I'd put an amp meter on the transformer and find out what the load really is. It's not going to do any good changing transformers if there is a failing component somewhere else that is drawing a large current.. Or if the load side has been overloaded by wiring something else into it that should not be.
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On 4/3/2011 8:18 AM, snipped-for-privacy@optonline.net wrote:

You do know that the common lead (often blue colored) for the 24volts is usually grounded to the metal cabinet of the air handler. A bit of insulation skinned off a thermostat wire could cause a short circuit when the bare copper comes in contact with the air handler cabinet.
TDD
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On Sat, 02 Apr 2011 14:10:01 -0500, Steve Turner

You didn;t say how long the original one lasted. It could be that the replacement was not strong enough. Ratings are not always everything, they may mean PEAK output, which is only for short periods of time. Besides that, they're likely something made in China or some other foreign country and are cheaply made. If the original lasted for years, try to get another one of them. Otherwise find a replacement with a higher amperage. They're more durable.
Of course check your AC input (line) voltage. You may have a loose connection on a neutral causing higher than normal voltages. And of course lighting can cause surges to destroy things like this too.
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On 4/2/2011 3:37 PM, snipped-for-privacy@myplace.com wrote:

The original lasted about 6 years. It was made in Mexico by Basler (model BE25214009 class 2 XFMR; it also has C340041P04 printed on it). The primary leg says "115V 60HZ 115V", and the secondary says "24V 35VA 24V". I can't seem to find a replacement for it online anywhere.
The replacement was indeed made in China. It's an Edwards model 599:
http://www.edwards-signals.com/index.cfm?pid 4&level
Perhaps (as you say) it's just cheaply made an not up to the task, and its subsequent failure has nothing to do with the failure of the first.
Any suggestions on what to do about securing a replacement? When the first one failed I made quite a few calls to local electrical supply houses with no luck (and I live in Austin; it shouldn't be hard to find a suitable replacement) before I found this Edwards transformer. We were having a hard freeze at the time and I was just happy to find something I thought would work. This time I'd rather be sure I get something that's up to the task.
Thanks for the help.
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On Sat, 02 Apr 2011 15:03:52 -0500, Steve Turner

Get yourself an industrial grade control transformer, like a triad f107z or f108u available from Newark for about $19 or $34 respectively. The 107 is 48va, the 108 is 96. Personally I'd go for the 108 because of it's form factor and it has leads, not solder terminals.
A Hammond166l24 fir about $25 or a 166N24 for about $36 might be even better (simpler connections) from the same source
What you do NOT want is another "home depot" bell or thermostat transformer.
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On 4/2/2011 3:28 PM, snipped-for-privacy@snyder.on.ca wrote:

Whatever you use must be "Class 2". That means there is not enough power available on the secondary to cause a fire. Using a class 2 transformer allows using light weight thermostat wire instead of power wiring.
Class 2 heating system transformers can be easily installed to be safe, with primary wiring not exposed - may or may not be true of the transformers above.
Modifying a class 2 transformer by replacing internal fuses with external fuses makes the source not class 2. If you knew what you were doing you could probably protect the secondary with a fuse. I wouldn't.
I agree with looking at the secondary current.
--
bud--


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Preferably with a "true RMS" meter.
--
- Don Klipstein ( snipped-for-privacy@misty.com)

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On Sat, 02 Apr 2011 15:03:52 -0500, Steve Turner

If you need it fast, call some heating supply companies. Austin being a large city should have several heating supply stores.
If you can wait to mailorder it, go to Graingers. http://www.grainger.com They have all of that electrical stuff, motors, etc.
If I'm not mistaken, both doorbell and thermostat transformers operate at 24v. Go to a large hardware store like Ace, and see if they have anything. If physical mounting is not a big issue, any transformer with 115vac input and 24vac output should work as long as the current is the same or higher. You need 35va, I'd find one with the highest (VA) rating as possible. Just make sure the in/out are the same voltage. By the way, 35va is pretty flimsy, and made in China means it's as minimal as possible on the specs.
One other thing, have you contacted the Trane Company? http://www.trane.com Maybe they can use next day shipping (of course that is pricey).
You're having a HARD FREEZE in Texas right now? Geezzzz, I'd think it would be 80 deg or higher now.....
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On 4/2/2011 4:29 PM, snipped-for-privacy@myplace.com wrote:

I would have, but having discovered this on a Saturday afternoon everything was closed.

Yep, that solution has crossed my mind. There's a Grainger's about 15 miles away from my house.

No, the hard freeze was a few months back when the first transformer failed. This time it's the air conditioner I want to run but cannot...
Thanks for the advice.
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wrote:

What you got seems to be for door bells. That is for a short on time. The control board needs one rated for continious duty. You should be able to just ask for a transformer that is rated for 115 or 120 volts input and 24 volts output. The volt/amp (VA ) rating should be 35. A higher V/A rating will be ok also. It will just cost more and may be larger in physical size.
There is one on ebay that will work if you don't mind wires insteadof screw terminals. Trane Transformer 35806009 TRN-101
It has a 40 VA secondary which should be just slightly higher than needed and it will run slightly cooler and may last longer than the origional. It is less than $ 25 including shipping.
I would think there would be a Trane dealer in town that would have something that would work for you in a large town.
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wrote:

Smitty... All the transformers like that type that I have opened have the thermo fuse underneath the taping over the windings. I carefully get to it and add a 1 amp fuse in place of it. WW
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Why a 1 amp fuse ? If on the 120 volt size, it would be over twice the needed value and if on the secondary side, it would be too small. Also it would not be a thermal overload,but a current overload fuse.
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On Sat, 02 Apr 2011 13:46:05 -0700, Smitty Two

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On Sat, 02 Apr 2011 20:50:41 -0700, Smitty Two

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On Sat, 02 Apr 2011 14:10:01 -0500, Steve Turner

rate. Try using one that is rated 50% to 100% higher than the original and has a brand name on it
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wrote:

Per other post...

the problem may be on the supply side rather than the load side.
I would suggest a surge protector of some sort on the supply side.
and as others have suggested, a better / continuos duty higher rating transformer from Newark.
cheers Bob
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