An air conditioner.
In the bathroom.
If it is tripping a GCFI not a circuit breaker it most likely has a fault to ground somewhere.
Did you make sure the drains were clean on the AC?
My work in electronics has shown me that even imperceptible amounts of dust cause major strains on equipment.
The fan is the biggest load, and as it gets older and dustier the load increases.
GCFI breakers get old and trip. They need replaced from time to time.
If the condenser is dirty it will raise pressures in the unit and cause it to pull more amps.
I replaced a GFCI in my basement a while ago, but it turns out the problem was with a device on an outside outlet of the circuit that the GFCI was in series with.
If it only kicks out when you have the air conditioner plugged in, it is my non-expert opinion that the GFCI is fine and the air conditioner is the problem.
I frequently need to replace the Chinese made GFI’s in my rentals. They seem to degrade over time. (The older American made GFI’s in my house have never needed replacement.)
Replace the GFCI. Cheapest first step.
Couple things.
1. Yes a space heater does use less than an air conditioner.
2. It could be cycling more often than it used to in order to do the same job. Could need the condenser and Evaporator blown out with compressed air because of dust and lint. Cycling more will heat the GFI more.
3. GFIs are famous for getting weaker after they are tripped a few times. Meaning it takes less and less juice to trip them. I have had some that became quite a bit weaker after only being tripped once.
I had a GFCI once, it was awful! General fudge candy increments have nothing on real candy! Those damn acronyms again!
Yes, esp. if like 10 yrs old. But EPA regs drive AC prices higher. https://goldenrulephc.com/blog/air-conditioning-changes-2025/
Also I've seen GFCI's getting fooled thinking a motor and compressor starting, as on your old air conditioner, as a ground fault.
Get a new A/C
Retired electrical contractor.
Breakers and GFCI breakers go out after a while too.
Moisture has gotten somewhere it shouldn’t have, or a loose wire, or bad insulation and is probably creating a return path for the current that is NOT the neutral (the ground wire). Not a safe condition. Only 5 mA difference in current between the hot and neutral lines can trip a GFCI. Unless you want to rewire the unit and hope that does the trick, it’s probably not worth the cost and effort.
If the GFI trips when the compressor kicks in there most likely is a ground fault in the air conditioner motor or its associated wiring.
Clean it thoroughly
GFCI is not a circuit breaker for excessive current. It senses a current difference between the hot and neutral i.e. some current is going where it isn’t supposed to. It is a safety feature.
Does the breaker trip when the air conditioner first comes on?
A space heater rated at 1500 watts may be the same rating as your air conditioner. However, start up load on the air unit can be much higher, thus tripping a weakened breaker.
It’s more likely your GFCI has failed. They do fail and don’t have a long life. I’ve had to change one of our that was tripping our Christmas lights all the time. That’s a far cheaper fix to try than anything else.