Today was the hottest day in Phoenix, i think it hit 114 degrees. My ac has been running since 4 pm continuously. It was set for 84 since no one was home, and since i am freezing entire day in my office, i left it at 84 when i came home. Around 5:40 pm i realized that AC didn't shut off once, it just kept going on and on, even though thermostat displayed the temp of 84. I raised the temp to 85 just to see if it will shut off, it did, but 3 -4 minutes later it came on again.
So i went ahead and lowered the temperature to 81. It's been running non stop since 4 pm, now is 10-45pm, and thermostat is displaying 81 degree temp for a long time now. For some reason, it just doesn't shut off. The Ac Only shut off twice for couple of minutes today.
Is this happening because it's so hot outside or is it time for new thermostat?
This is a older RUUD roof top unit, which was serviced last year. Thermostat is about 10-12 years old, digital programmable Honeywell.
Also, last year i played with internal settings on the thermostat and might have changed the temperature sensitivity settings. Since then, i misplaced the manual i was unable to find info on how to access this setting again.
Any insight? I am dreading the bill next month. Our bill for month of June was the highest we have ever gotten in ten years.

 

Not the A/C guy but you should be able to google your thermostat and download a manual. I would guess your A/C is at almost max capacity and that's why running all the time. Lived in Phoenix for 10 years remember those days well. Parking lots are a killer.
Still wish I was back west.

 

What is the delta-T ? (the difference between input cold air temperature and return air temperature )

 

The cooling design temperature for Phoenix is 107*. So yes it will run continuously until the temp drops to 107*. This depends on your system running properly ( Clean filter, clean coils and the correct refrigerant level).