Typical!
In AZ we have the Air Handler in the attic and the Heat Pump sitting on some square beside the building.
Shut off the AC last winter, and when went to turn it on this one time, nothing happened. Well, except the fan came on, but no big power hum, no fan blade inside the heat pump running.
Just as a check I measured all the voltages coming from the the thermostat out through to the unit, and they all seem correct. OFF no voltage, ON 26 Vac, yet unit doesn't come on. There is a distinct power hum, but nothing happens.
I turned OFF the 220Vac going to it, and measured the voltages around the huge relay right at the access and was disappointed to find almost nothing there. I expected the relay[whatever it's for] to be powered. Perhaps that's the key.
Inside the access panel lid is atype of schematic. One interesting item caught my attention 'low pressure switch' so I thought perhaps the unit has lost its charge and the swicth won't come on to prevent damage when it's not properly charged. But I don't know much about HVAC units. There are three access spigots for measuring/filling I think all right under the access panel.
Anybody out there with a bit of tutorial to help? Like what pressure range should be there, what to do if the pressure is too low, etc etc.
I've got two other identical, operating units to do comparisons to, but it still helps to know WHERE to look.