If I hit AUTO on climate, typically A/C indicator also turns on. I'm guessing that if it's cold out, that does nothing and is harmless?
if it is calling for heat, it depends on whether the engine is running or not. it likes to use dry air to delist the windshield, but if only the heat pump is running, it can't heat and cool at the same time.
the A/C is actually a pump, works both ways, as a heat pump or a cool pump. and as @bisco always says, there's a thread for that Heat Pump: A/C Button Matter? | PriusChat
Correct. It's unsettling to see the A/C light on but in the cold it's drying the air, not cooling it. That's a good thing for defogging. Your profile shows you don't drive a Prime. Is your vehicle type in your profile correct? It's easier for us to help people if we know what they drive.
I would not say it "does nothing". The A/C function will dehumidify the air, keeping the windows from fogging the inside surfaces.
I think maybe there is some misunderstanding. On an ordinary car, A/C will dehumidify. On the prime, we have a heat pump for producing heat. I would be almost certain that this is the A/C run in reverse, there is 1 compressor. It can't act as a dehumidifier while producing heat as in an ordinary car. Am I mistaken?
you are correct. one unit to heat and cool. if you're calling for heat, and the engine is on, the heat pump can do nothing but produce heat. no cooling, no dehumidifying. again, to the japanese, a/c does not mean cooling, as we typically understand it. they are right thought, air conditioning means 'conditioning the air' in some way or another.
In the Prime, there is an evaporator and condenser in the cabin, as well as a coil under the hood which which can function as either an evaporator or condenser, depending on the state of expansion valves. The inside evaporator is always cold, and the inside condenser is always hot. There is also a traditional heating coil for the engine coolant in the cabin. Dampers control whether air is being blown across them or not. Unlike in home heat pumps, which usually reverse the flow of refrigerant, the refrigerant in the Prime always goes along the same path, whether in heat or cool mode. The only difference is that,when in cool mode, the expansion valve AFTER the outside coil is constricted and the one BEFORE it is wide open, so the outside coil acts as a condenser. It is the other way around in heat pump mode.To dehumidify without cooling, the system runs air across both the inside condenser and the inside evaporator.
True heating the air lowers the relative humidity, but it doesn't remove moisture from the air. An air conditioner however can remove moisture from the air, then when that air is heated using engine heat it will be drier than if you just heated it. That's why a cars air conditioning compressor runs when you defog the windshield even in winter
