In article , Jeff Thies wrote:
On 4/4/2011 4:22 PM, Steve Turner wrote:
On 4/4/2011 2:57 PM, wrote:
On Mon, 04 Apr 2011 09:50:54 -0400, Jeff
wrote:

On 4/4/2011 9:19 AM, Stormin Mormon wrote:
It blows because something is drawing too much current.

Probably not. More likely it just gave up as it was designed too near
the margins.

Jeff
I think it imay be safe to say something drew too much power for THAT
transformer - however - a primary could fail open even if it NEVER had
a load on the secondary (in which case, for THAT transformer, even NO
load was too much.

Lots of REAL crappy electrical "goods" on the market today.

Just picked up a new White Rodgers model 90-T40F3 (input 120/208/240V,
output 24V 40VA) from a local supplier that has a form factor very
similar to the original. White Rodgers also makes the control board into
which the 24V feeds, so hopefully bodes well for it being a proper
replacement.

Good.

Give a feel after it has run for a while to see how warm it gets, you
may wish to do that with other components also, and an IR thermometer
once over if you have one.

If the transformer has any bare metal, keep in mind that IR thermometers
usually do not get good readings on bare metal. Stick on a 3/4 inch
square of masking tape.
--
- Don Klipstein )

In article , bud-- wrote in small part:

I agree with looking at the secondary current.

Preferably with a "true RMS" meter.
--
- Don Klipstein )

Frequently as accessories are added the VA value of the transformer can be exceeded, or run at the top end for too long. Next time you replace the transformer increase the VA rating by 25VA. If it is a 50 VA go to 75VA.

When you turn the unit on, measure the voltage across the transformer. it should be above 20VAC when the unit is on and fully loaded. If it is not is in an indicator that there is a partial short in the 24VAC system. This can be traced down by eliminating some of the 24Volt accessories until the voltage returns above 20VAC.

. Next time you replace the transformer increase the VA rating by 25VA. If it is a 50 VA go to 75VA.


Next tme you reply to a post, check the date.


Mark

replying to Steve Turner, YosemiteDave wrote:
Check the contactor in the outside unit. These short out on the 24 volt side
and taking out the transformer.
But if the fan board on a XB80, the fuse should have blown if that was it.
But older ones did not have a fuse, I think.
If the fuse didn't blow, I'd say ya just got another bad transformer.


--
posted from
http://www.homeownershub.com/mainten...ne-628364-.htm

On Saturday, April 2, 2011 at 12:10:01 PM UTC-7, Steve Turner wrote:
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.

On Saturday, April 2, 2011 at 12:10:01 PM UTC-7, Steve Turner wrote:
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.

Check the control wire (low Voltage) connected to AC outside possible damage to wire by rodents.