A few questions about the cabin temperature control!

The heated air in the Prius is just like other ICE only cars in that the heat comes from the engine coolant loop through a heater coil.

The AC compressor for cooling, though, is a variable speed electric motor and is driven by the HV battery (through the Inverter module) which in turn is recharged by MG1 and the ICE.

The AC compressor is very efficient though in that it can run the compressor and the fan at variable speed to meet the cooling needs. But it does take energy to do this and will result in slightly lower mpg while in operation.

 

It's also worth adding that, because the heat comes from the engine coolant, and the car uses extra gas until the coolant has warmed to 70°C (to avoid extra wear and tear by running on a cold engine), it effectively is wasting a little bit of gas if you run the heat while the car is warming up. After the first 10 or 20 minutes in the city (or more, or less, depending on the outside temperature), or a fairly short jaunt on a highway, the car is probably fully warmed up, and any heat for the climate control would otherwise be wasted by sending it to the radiator.

It's never the case that it uses electric power for heat; but it may use additional gas as it struggles to get to 70°C. (Other cars will probably have a fuel economy penalty for using the heat before the engine is warm, but it's probably not nearly as severe as in the Prius.)

 

I suppose another advantage of using A/C to heat air is that the A/C will dehumidify the air. I will do this when windows fog.

I prefer not to use A/C to heat cabin if possible, as I want to maximize MPGs.

 

If you are talking about having the AC work as a reversible heat pump, then it won't dehumidify the air, since the dehumidification works by moving moist air across cool coils, condensing moisture.

If you are talking about running the AC while also heating the air, then it will dehumidify (at the cost of more energy usage).

If something else, pardon me.

 

This all leads me to another question, should the air conditioner automatically come on when the temperature is lowered or does the AC button have to be pushed on manually?

 

If you just lower the temperature setpoint the AC will not come on but the fan speed will increase to try to satisfy the new setpoint. You will need to manually push the AC button to turn the AC on.

On the other hand, if you turn on the window defogger or use a Mode with the defogger the AC will come on automatically to help remove moisture without having to push the AC button.

 

I think it was designed so you can just leave the A/C button on all the time; it will run the air conditioning if it needs to, for example to lower the humidity or temperature inside the car. If it doesn't need to, then the A/C will be off, even though the light is on.

On the other hand, the climate control is typically optimized for comfort, not fuel efficiency. So if you want to squeeze the most miles from a gallon of gas, you might do slightly better by leaving the A/C off except when you decide you need it.

But if you just lower the temperature and the A/C light is off, it will just act as a vent, increasing fan speed to cool you off.

 

Excellent point about the AC not coming on unless you manually turn it on. I found this out the hard way when it was 87 degrees by moving the temp to "LO" and waited for the AC to come on. I drove 20 miles with hot air blowing in my face. The Prius climate control does not work like others in this respect....most of them activate the AC automatically if it is needed to cool the car down.

 

Yep, you have to turn the AC on manually - confirmed by the dealer where I get my service. Thanks for the replies!