Do I need to turn on AC for heat?

The car thinks for you.

 

So, in the winter does it make any difference if the AC light is on? The heat works without it, but does it do anything different with it on? Does it actually dehumidify?

 

Wait... if you have A/C OFF, would it be cooling in this example? You said, A/C ON is for cooling in the summer or for dehumidification in the winter... I thought I had this sorted out. But now, I am confused again.

 

With a heat pump, that is a very good question. For cars without a heat pump, turning off the A/C tells the system not to use the compressor. Since a heat pump is basically A/C running in reverse, does turning off A/C tell the system not use the compressor even for heat?

Assuming the engine isn't on, I guess that answers the about question.

The heat pump design in the Prime can do some dehumidifying. It can cool down incoming air to dry it, and them heat it back up, as there are two condensing/expansion coils in the dash instead of just one. Revisiting my question, does having the A/C light off mean the system will bypass the drying coil to provide more heat? Normally the heat pump has to heat air it just cooled down. I think there is a humidity sensor, so it is probably all done automatically.

The dehumidifying by heat pump is limited though. Which is why when the defroster is turned on, engine comes on. During that operation all the heat pumps efforts go into drying the air, and the engine is needed for heat. In another car, turning on defrost cancels the A/C off, and turning it off again, usually means foggy windows.

 

On third gen at least, monitoring the AC compressor usage with a ScanGuage: when vent mode was set to Heat/Defog, the AC would sometimes activate (even though AC light was not on, stealth AC).

I believe the cabin temp sensor incorporates a humidity sensor as well? I believe the car's monitoring humidity and cabin temp? It would commonly run AC when temps were moderate and it was raining (the main time I'd use Heat/Defog). If temps dropped to near freezing and/or the air was fairly dry, it wouldn't activate AC.

 

I think there is supposedly a sensor near the forward camera that checks if the windshield is foggy.

 

Yeah, from 4th Gen Prius Repair Manual:

 

For the last two years, I kept my car with AUTO 70F with A/C ON all the time throughout the year. I never had to fiddle with any of the manual buttons to adjust my temperature or defogging function except occasionally turning on the front defroster in a morning. This took care of heating, cooling and dehumidification year around. However, it caused using quite bit of traction battery SOC for HVAC functions. In winter, my A/C load ratio could get as high as 30-40%.

Then @Tideland Prius told me that A/C ON was not needed for the heat pump. And I discovered, even with A/C OFF, if the direction of the airflow is to WINDOW, then the electric compressor would turn ON automatically either in AUTO setting or Manual setting. I have been using manual A/C OFF with fan speed set appropriately and switch airflow to Window (compressor ON for dehumidification) or to Cabin (compressor OFF but heat kept on). With this setting so far, my A/C load ratio has been below 10%. But it has not been super cold yet. I will have to see how it works in middle of winter.

 

it's good to have simple, user friendly systems

 

Do we know if it actually dehumidifies while heating in EV mode with the AC light on. You describe the way we expect it to work, but my question remains does it actually operate differently? Is there any way to tell?

I wish I could control when the heat is on, because often if it's cold outside and warm in the car I want cool air but no heat. Usually it seems to do that, but I don't trust it, and heating would be a waste of energy when I don't want it. I know I can set the temperature to Low, but that's a lot of clicks, and then I have to set it back when I park so the remote AC button works for heating.

Dehimidifying when it's not needed would be a big energy penalty, given how much energy the heat pump seems to use in defroster mode. Maybe the humidity sensor works so well that it's perfectly seamless and I've never noticed it. But Toyota software and climate control isn't that way. Things like the primitive way it calculates range of the "climate score" don't give me high confidence.

 

Yes, I want that too.

At least, for cooling, we know A/C OFF means OFF. Thanks for clarifying this @Tideland Prius

What we need is Heat ON/OFF button. In fact not just Heat but Heat Pump ON/OFF button. During winter, I want to heat only from ICE not from the heat pump if I can select that mode.

 

I know this is not for everyone. But since Toyota designed the PRIME (and entire lines of Prius) with a focus on fuel efficiency, the car tris to keep the ICE off as much as possible. Many PRIME owners want that even that means using battery power to heat the cabin using the heat pump which results in reduced EV range. In my case, for my daily drives during winter, ICE comes on anyway because it is too cold and I have too long to drive or I have to use defroster, etc, etc. On top of it, in my case, running PRIME in HV mode costs less than running in EV mode especially in winter. So, if ICE is going to run anyway and it is actually going to save money by doing so, I want to utilize as much of wasted heat as possible by using only the ICE generated heat and not waste valuable battery power on heat pump which I know it to be not very effective in heating the cabin in our winter temperature. If I had a heat pump ON/OFF switch, I think I can control this process more effectively than how the car handles it in the AUTO setting.

 

I assumed the car uses ICE heat if it is available. What I don't know is if it ever switches to the heat pump after ICE cools down or when I switch to EV mode. There is no indicator I can check on the heat pump operation. From reading this thread, it seems I may be able to get that info from Scan Guage. I don't think Hybrid Assistance has that information displayed.

Yap. I have been saving the EV mode for my afternoon commute back home. This is when the heat pump operates and shortens the range. I had my HVAC on 70F AUTO with A/C ON all the time for those winters. This year, I am planning to switch EV/HV during both morning and afternoon to see if that decreases the heat pump use and extends the EV range despite not in one contiguous drive.

 

Yes, I think it will turn the heat pump back on if you switch to EV mode once the heater core cools down. If you're in HV mode and it goes to EV for a while, it will use residual heat and then turn the engine on again when it needs more heat, so using the heat in HV mode will cause the engine to run more than it might otherwise need to.