HELP!! I've pulling me hair out!! Carrier 40aq Air Handler. Original prob was air handler lost 24 volt output. I replaced transformer with a slightly undersized one... 40va instead of the spec'd 60va... and it ran for about 10 minutes.. then blew. Now I put in a new 75va transformer... and it ran for 10 minutes or so.. and then blew. I did check the wiring on the heat pump outside to see if any insulation had melted, creating a possible short circuit... but all seemed fine. New transformer has a reset in it.. and I have run through the cycle of it all working for just about 10 minutes every single time.. then the transformer blows again! Any ideas anyone? Could it be coming from the thermostat on the wall... or something outside on the heat pump.. because of occurring after 10 minutes each time? Thanks to all!!

Shorts are hard to trace. I would say if the system will run for 10 minutes before kicking out, The short is not in R and Y of the control circuit. I would think more of a short being in the W circuit. You can add a 3amp automotive fuse inline on one leg of the transformer(24v side) to keep it from blowing. If you turn your thermostat to emergency heat will it short out?

Hi there! Thanks for answering. The new transformer has a built in reset so it won't have to be replaced again.. I just have to track down the prob in the circuitry. An HVAC guy told me a few months ago that my EMERGENCY HEAT coils weren't working, but that wasn't crucial. I'm wondering if I should figure out which cable is the "in-house thermostat" and completely disconnect it to rule out it and it's wiring as the prob? It's funny how the system runs for almost the exact length of time (10 minutes approx) before it trips the transformer's breaker.. like it's some sort of cycle happening. Any clues there? Thanks so much!