what causes loss of 24 VAC at the thermostat (and furnace) ?

[ktnwin]

Correct, these transformers cannot draw up to 15A. I will also add a 3A fuse to the primary. So, two fuses for this transformer to protect it. It is very annoying when it is toasted.

Correct, these transformers cannot draw up to 15A. I will also add a 3A fuse to the primary. So, two fuses for this transformer to protect it. It is very annoying when it is toasted.

The new transformer cannot fit in the place where the old one is. So, I left the old one at the same place after disconnecting it from the 120V input, its outputs also disconnected. The new one is installed on the side of a metal switch box (where other transformers are), I ensure that its hot is only hot when the switch is ON. The 24VAC output goes thru a cable with its 2 wires connected to where the old transformer outputs are connected to. The hot 24V also has a 3A fuse inline. I have no room to add fuse for its 120V input.
Turn ON the breaker and test, my new Honeywell thermostat did control the heat as expected. But the fan is ONLY on if I set it to ON. It does not turn ON in AUTO mode even if the heat is ON (a new post will be created for this).
For now, this topic ends here.