I've had issues with my ac/heating for awhile and spent several thousand having various parts repaired/replaced including the evaporator coil, blower, circuit board, compressor and transformer. We also installed a new thermostat. I explained the problem to a building maint guy at work and he said it sounded like a thromostat. Told him that's what I initially thought too but replaced it so he gave me a transformer and said to try it. I tried the transformer after hooking the old one up to the multi meter and yup old one was bad, new one good. It blew about 5 minutes after hooking it up though. So any suggestion on what to try now?
We sunk so much money into it that I don't have a lot of money left. Had 4 different people out last year and they all gave a different story but it would only work for a week or so after each would fix it and come back again and again. It seemed like I was getting the run around and basically getting ripped off. I'm a mechanic in the Army so I'm pretty handy and should be able to handle an easy fix. I have a general understanding of mechanical/electrical so any help would be greatly appreciated.

We have two units in the house upstairs works but it has been cold for my family for the last 2 months so I would really like to get this fixed.

Thanks.

You most definitely have a short somewhere in your low voltage. Could be a bad tstat wire, loose wire nut, etc. If you had this problem before you changed your Tstat... you more than likely have a shorted wire... low voltage shorts are tricky... but if traced correctly can be done... if you have blown your transformer, replace it... but with one that has a reset, or install a inline fuse on your low voltage end of the transformer. That way you don't have to keep buying transformers! I would start in your t stat wire... find the bad one or just replace it! Good luck

Fuses are a lot cheaper. You can add one to the low voltage side or the high voltage side.
Use this as a suggested size in amps P/V * 1.5; P in VA usually 40, V 120 or 24.

If you do it right, it can stay there permanently.

Low voltage short, entirely likely. Replace the t-stat wire. If you are apprehensive, run it temporarily in the living space, etc and see if the problem goes away. It could also be wiring to the outside unit especially if it's a heat pump or only breaks in the summer.

You can likely troubleshoot by putting a 40 watt light bulb in SERIES with the primary. A light bulb is a current dependent resistor and is non-linear. A short will just make the bulb glow.

One of the first things you need to do , is check your old transformer with a ohms / continuity test to see which side actually blew. If you have an "open" winding on the primary side ( 115 volt feed side) your short or high draw was on the 24 volt side of the wiring. If your "open" winding is the 24 side of the transformer, you had a power surge on the 115 side of the transformer. Now that gives you the place to start looking for your problem. Even a bad gas valve that pulls a real high amp load will blow a transformer, but like your earlier posts stated, you can fuse the transformer to protect the new one. ( by testing your old one, you will now know which side to fuse.) Usually the short is on the 24 volt side, being the T'stat circuit... but the following is is also on the 24 volt side; gas valve, safety limits, controls such as relays, timers, and contactors at the cond. Unit, and all wiring / connections. By looking at the units wiring schematic, you can see what all is really on that low voltage side for YOUR unit. Do the above checks and post back your findings... As well, kindly post what model unit you are working on, it helps the people who know certain equipment better than other to help you...