All you’ve said is correct EXCEPT, a 20a circuit is not rated for a full 20a constant current. It’s only rated to carry 80% of its rating at 100% duty cycle. The 15a circuit he is running it on should only be used for 12a constant draw. A 20a circuit is good for 16a constant draw. So if a yong heng pulls 1800w at 120v that’s 15a constant draw. It easily overpowers a 15a circuit, and a 20a will just barely do it with the included water pump and maybe a small fan or 2. Not 5 extra amps worth. Take into account it’s an old house with fuses, which could mean it’s running aluminum wire or cloth coated copper that has deteriorated over the years, it’s very easy to overload a circuit and heat up that wire to the point of meltdown or fire.
I also cant condone the OP attitude to burning down his rental bc his landlord won’t “upgrade” his wiring. Like it’s his landlords responsibility to make sure he can run his yong heng compressor safely. I’m glad I’m not his landlord.