Is it more efficient to say run it for 30 minutes at 30 degrees heating or for 2 hours at 26 degrees?
In: 3
You mean an A/C unit?
Go for the continuous work. Always the less the temperature difference, the better.
how do you efficiently run an old split system heater/cooler?
Split systems are more effective (power wise), the smaller difference in temperature is on both sides. So for heating – as low inside temperature as comfortable, for cooling – as high inside temperarure as comfortable.