Do I need to turn on AC for heat?

So, what you are saying is in EV mode, the heat pump is ON if no residual heat is available from ICE and heating requirement does not call for the ICE to turn back on, but under HV mode the heat pump is always off. Have you confirmed this using Scan Gauge?

 

No, that's not what I meant. I don't know if the heat pump is always off in HV mode.

 

OK, I will have to do more investigation to find out when the heat pump is operating on my car. I hope there is an easy way.

 

I would think that if you are in HV mode, the ICE would be running regularly, and thus the coolant will be at least warm, so the heat pump won't be needed.Just my thought without actual empirical evidence. I don't live in an area where freezing weather and snow are an issue.

 

Usually that is true, but the question is, does it ever run while in HV mode? There might be a situation where it does, but I don't know. The most likely times would be before the engine is warmed up when you first switch to HV mode, if you're parked for a few minutes with the heat on, or if the AC light is on and it runs the AC to dehumidify.

 

Usually that is true, but the question is, does it ever run while in HV mode? There might be a situation where it does, but I don't know. The most likely times would be before the engine is warmed up when you first switch to HV mode, if you're parked for a few minutes with the heat on, or if the AC light is on and it runs the AC to dehumidify.

 

It's too bad Glenn Frey is gone. Now we will never know when the heat is on.

 

My unfogged windows every morning confirm this. I am dubious about the system doing any sensing of window fogging because I've seen it get so bad that I had to pull over and the ventilation system never took any steps to address it. When I manually pop the A/C button on the windows clear up in under a minute.

 

With AUTO and A/C ON, PRIME is definitely dehumidifying. I had my car set like that with no fogging problem ever, except in the very beginning of my morning driving when not enough heat is available. My understanding is that with A/C OFF, there is no dehumidification except when the direction of air is to WINDOW. In winter AC ON is for electric compressor for dehumidification, but even with AC OFF, if the air is directed to WINDOW the car turns on the compressor dehumidifying the front window. This is also true when the front windsheild defrost button is pushed. In this case regardless of AC button, the HVAC will turn on ICE for max heat and compressor for the max dehumidification.

 

Correct. I once thought AC was meant to be both cooling and heating by use of electric compressor similar to the remote A/C button on the fob, but as it turned out it was only meant to be traditional air conditioning for cooling at higher temp and dehumidification at lower temp. The heat pump will come on regardless of A/C button if you are EV and temperature setting allows it.

We had some discussion on this topic on another tread: Window Fogging in Cold | PriusChat
I have not played with my ScanGauge yet, so I don't have definitive answer, but there are six different modes PRIME's HVAC system operates.

1 = Cooling
2 = Series Heating/dehumidification (40-60° F/4-16° C for serial)
3 = Parallel Heating/dehumidification (32-40° F/0-4° C for parallel)
4 = Heating (I believe this is conventional heating using ICE)
5 = Gas Injection Heating (This must be the "heat pump")
6 = Defrosting

I think the pure heat pump mode is 5 and I think this mode is only used under 32F down to 14F. At this freezing temperature, defumidification does not work. What I don't know for sure is mode 2 and 3. Both heat pump and dehumidification can work at the same time or work at somewhat different timing. The term "Series" seems to suggest they work in tandem, but the term "Parallel" seems to suggest it work simultaneously.

Since the front defrost switch will force the ICE to turn on, I thought it needs the heat from ICE. But now as you pointed out, I am not sure if that is only MAX fan and AC (compressor) with out heat? I am sure I could feel the warm air coming out of the vent when Front Defrost is turned on which must be coming from the ICE. This is the mode 6. With this setting both ICE and compressor are running, but I believe the heat pump is not on running as primary source of the heat.

 

Could you guys sub "engine" for "ICE" for a while. I know I'm being persnickety, but...

 

The basics of any AC system is to increase and decrease the pressure of the AC "fluid", with a compressor. When it's a low pressure it's cold, and capable of absorbing heat from it's surrounding. When you compress it it becomes much hotter, and begins to radiate heat to it's surroundings

In a traditional AC system the low pressure occurs when the fluid is in a "radiator" within the the cabin, it's cold, and absorbs heat from the air being pumped through. High pressure occurs, on the other side of the compressor, at another "radiator", out at the front grill of the vehicle.

It's somewhat analogous to a dishrag, blotting up water while loose and open, then going somewhere else and being rung out, aka compressed, and now the rag gets much wetter, starts dripping the collected water. Substitute AC fluid for the rag, and heat for water, and that's the system.

A traditional AC system. I don't really know the ins-and-outs, but I'd speculate a heat pump works similarly, but also finds away to reverse this, if needed: have the high pressure heat radiator in the cabin, and the low pressure heat absorber radiator outside of the cabin, at the front grill.

 

In residential systems, heat pumps usually just reverse the flow of the refrigerant, but the Prime does it very differently. The refrigerant always flows in the same direction. There are two refrigerant coils in the cabin, and one always has hot high pressure refrigerant, but dampers control whether or not the cabin air is heated by it. In cooling mode, a pressure valve makes the external coil in the engine compartment have high pressure (and be hot, acting as a condensor), the low pressure "evaporator" coil in the cabin is cold and cools the cabin air, and the damper on the hot inside coil is closed, so it doesn't heat the cabin air. In heat pump mode, the damper on the hot interior coil is opened to heat cabin air, refrigerant pressure in the external coil is low (cold) and acts as an evaporator, and the evaporator coil in the cabin is bypassed. In dehumidification mode, the system is basically in cooling mode, but the damper on the hot interior coil is opened, so cabin air goes over both the cold evaporator coil and is dehumidified, and is then reheated by the hot coil.

Separately from all this, there is also a traditional engine coolant heater core in the loop which can heat the cabin air if the gasoline engine has been running. This is typically only used when it is very cold or you really need serious heat to do de-icing and such, or the engine has been running anyway, like in HV mode, so you might as well use that heat rather than running the heat pump.