24vac transformer frenquently burns out
 

the 24vac transformer in my inside unit of A/C system was burned out 3 times in this winter. one month ago, the unit did not work, no heating, no blowing. we found out the transformer burned out and replaced it. one week ago, burned out and replaced it again. yesterday (worked for 5 days) it was burned out again.

there is no fuse on the inside unit board. this is a electric heating system. any suggestion to figure out this issue is appreciated. thanks.

Are you replacing it with the properly rated transformer?
See if you can reference the ratings for the original and compare it with what has been previously installed.

thanks for suggestion. I replaced it with the same transformer previously installed. the main issue is why it worked for only a few days and then burns out?

Anytime a transformer burns out and you go to change it, like you have been told , use the right size VA rating transformer. You also have to assume that there is a reason that it burned out. so the very first thing you need to do is take a amp draw when its calling for heat and blower start. Lets say you have a 40 VA transformer and its secondary power is 24 volts. Divide the voltage into the VA and that will tell you how many amps the transformer will handle. 40 divided by 24 is 1.6 amps. that's how much that transformer is good for. If you try to use 3 amps through it, it will burn up.
Do you have a amp probe?
Most amps probes don't have a small enough scale to check amperage for 1 amp or less. Take your amp probe that has maybe 6 amp scale and use a wire with 10 loops in a circle, so 10 wires are going through the amp probe. This way you readings are divided by 10, so if you get a reading of 4 that would .4 amps.
Most relays only use about .2 amps when powered . You might have to check all your 24 volt loads one at a time to see which one is drawing too much power. Check all low volt wires for bare spots and lose connections. Paul

A problem that I sometimes run across is that if someone did not have the exact rated part but, something close to it, they will install what they have on the truck at the time of service. If this is done several times, it eventually gets to the point where someone thinks they are replacing with like rated parts when the part is actually way different from the original.
For example: if a 5amp part goes bad and this is replaced with a 4 amp part and then again replaced with a 3 amp part, thinking that the replacement part is only 1 amp less than the failed part, you are now actually 2 amps of the mark.
This is why it's always best practice to find the rating of the OEM part before replacing what you find installed in the unit.

Mine was wired up wrong at the initial installation and every time the emergency heat came on the transformer shorted out. It might be a week, a month or once a year between times the emer. heat came on but each time the transformer would go out. The problem was discovered when I had to have a new compressor in.They discovered it and corrected the problem.

thanks for all your response. it is my rental property and I do not have detail information in my hand. I will go to check tomorrow.

check wiring as well
 

I have seen a few repeat transformer problems that are caused by stripped wiring or bad connections. Copper wire will heat up and start drawing current and smoke a 24v secondary side of the transformer pretty quick. You can also wire in a fuse of say 5 amps to the 24v side to save the transformer while you look at wires and trace them back to the thermostat. Fuses are cheap compared to repeat transformers.

Repeat transformer failures have always been in the wiring somehow. If you have a blower motor that works (green wire) then a call for the compressor kicks in at the thermostat and that wire is touching something it shouldnt or stripped it will pop it (audible pop) followed by nasty electrical smell.

(Repeat transformer failures have always been in the wiring somehow) A good many times the problem is in one of the loads not just wires I have seen a lot of coils over amp. In fact if a relay is getting bad it will cause the wires to burn. If you put a 5 amp fuse on a 40 Va transformer. that would still be two times the amps that transformer can handle and it will still burn out.
He does not have a dead short because he said the transformer did last a few days. Paul

It could be an intermittent short to ground. Measure voltage at r-c on board while furnace runs. If 24V then it is an intermittent short to ground. Check all wiring even tstat wires leading to tstat. I had on once that shorted to ground when homeowner cooked dinner and stood on floor near stove which slightly pressed down on floorboard where a thermostat ran underneath but on top of a copper water pipe and over the years the insulation on the wire was damaged to the point where the pressing on it by the floorboard made electrical contact with the copper pipe. This happened every night at the same time which I happened to be diagnosing at the moment she stepped on that exact spot on the floor above.