So, the home AC's relay contactors started to vibrate, like bursts from a machinegun.  These control current to the blower motor.  This is a symptom of under voltage to the coil that operates the relay, I think.

The transformer changes 120V ac into 24 v, don't know if it's ac or dc.

I guess the transformer had an internal short, it overheated and then started burning.

I shut down the power and gave it some CO2, after it cooled I removed it, so we are without AC.

Is it probably just the transformer or is it likely that something else caused it to fail?  The transformer failed about ten years ago if that's any help.

Also seeking advice on getting the 28 year old system replaced.  This is a two story home, it has one AC/Heat unit with a motor controlled damper that sends air upstairs or downstairs.  The damper motor unit keeps failing because it has plastic gears forced over steel shafts and the gears split when they age and then slip on the shaft.  At present I just have an allen wrench attached to a tiller and this sets the damper to send some air upstairs and some downstairs.  This AC can barely keep up with the heat load into the home.  We have to get the home down to 70 F by morning and the unit runs continuously until early the next morning.  The temp upstairs goes up to 80 F during the day, way too hot.

I'm thinking adding a second unit is the plan.  Seperate units for upstairs and downstairs would avoid repeated failures of the gear train that drive the damper since there wouldn't be a damper if there are two units.

Who should I see to get a trouble free AC setup?  How much should it cost?  Any Arfcom AC guys, in San Antonio, who would like to do the job?

Thanks

PS, if this sounds incoherent it's cause I've got a cold at present.

I would first change the transformer to buy me some time on the cheap.  It sounds like you have a bad one.  



I assume that this is in the air handler inside, and not the compressor.  It should be 120v.

First off transformer failure is pretty common so replace it and forget it. If you're worried about it get one with a resetable overload on the side. As for new A/C units you may be able to go with one system because modern A/C fan units have higher horsepower blowers in them = much better airflow. You also might want somebody to check out the duct work.

I would avoid Trane or Carrier units. While they are good, they break like anything else. Parts are specific and very expensive.

The problem is in the air handler not the compressor.  

The compressor is around 7 or 8 years old, the air handler stuff is 29 years old.  I had a new plenum box made for the air handler and the ducts were all replaced about three years ago.

Monday I'll get a new transformer, one with overload protection.  (that's a circuit breaker, right?)

4:30 am it's sort of attention getting to find a fire in the attic, but at least the relay chattering woke me up so I found the fire.  I was sleeping downstairs because of having a cold or the fire would have gotten much bigger before we found it.  As it was it was only the transformer that was on fire.

PS, clutchsmoke, is that a Styer 9mm?  The really high capacity one?  If so how does it shoot?

Got a transformer with a resetable overload.  Foam cleaned the condenser and evaporator coils.  The AC is doing it's thing, life is good, eh?

Thermal runaway.

Thermal runaway only occurs on silicon-based devices such as transistors.   Transformers don't go into thermal runaway because their resistance increase with temperature, so they are self-limiting.     Transformers fail due to insulation breakdown,  which can be caused by overheating due to a hard short on the windings and can be cause by mechanical

or age-related damage to the winding insulation.





CJ





 

The problem is the allspark…

Now that you've fixed the problem, you have some breathing room.(Bad joke, you have a cold)
As a contractor ( God, I'm getting tired of typing that) I would recommend that you talk to neighbors and friends that you trust to get referrals for a HVAC guy. Also, take some time and research if you are eligible for some rebates if you upgrade your system.
Glad to hear you caught the transformer fire in time!

First off good job on the A/C repair, it pays to handy.  The pistol is a HK P9S single stack 9mm, it's roller locked like the rifles, gotta be the softest recoiling 9mm pistol I've ever fired.  It's almost as accurate as my Sig P210.