For how many years? It could be that the 15A breaker is barely enough,
and after 8 years of operating at, say, 14A, it is just worn out. The
easiest thing to try first is replacing the 15A breaker with a new 15A.
Are these window units? Maybe you can find a higher efficiency AC to
install upstairs that will use less amps for the same (or more) BTU/hr.
You should be able to run a 12000 BTU unit on a 15A dedicated circuit
if the EER is at least 10.0. You might also look for a 240V AC and
switch it over to 240V.
Don't up the breaker to 20A unless you are *sure* all the wire is 12
gauge. But you might can make this work with 15A and 14 ga wire.
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