Do I need to turn on AC for heat?

Discussion in 'Prime Main Forum (2017-Current)' started by mister2cool, Nov 11, 2019.

  1. Salamander_King

    Salamander_King Senior Member

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    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?
     
  2. m8547

    m8547 Senior Member

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

    Salamander_King Senior Member

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    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.
     
  4. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    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.
     
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  5. m8547

    m8547 Senior Member

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    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.
     
  6. m8547

    m8547 Senior Member

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    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.
     
  7. Toyo_Tom

    Toyo_Tom New Member

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

    PiPLosAngeles Active Member

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    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.
     
  9. mister2cool

    mister2cool Junior Member

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    wow what a can of worm I opened..
    In non-prime Prius, at the Gen 2 I had.. one or two very weak electrical heater runs until ICE is warmed up. After that electrical heater is only used for supplemental heat since ICE is going to be running most of the time and coolant will be warm for a while when ICE is off. I would not expect Prime to work differently in HV mode, other than heat pump replacing the electric heater.

    In the Prime, I have driven many trips in EV only, temp set to AUTO 68F, and AC OFF. There is definitely heat in the cabin, but I can't recall for sure if dehumidifying is taking place. Easy way to tell is have heat on & AC off on a cold rainy day, and see if the windows fog up. Perhaps the button labeled AC is really only referring to cooling as it was traditionally used.
     
  10. Salamander_King

    Salamander_King Senior Member

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    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.
     
  11. mister2cool

    mister2cool Junior Member

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    My confusion would be "compressor on". Driving in EV, with heat pump, compressor will be as long as it's producing heat, regardless if AC is ON or OFF. right?

    Front windsheild defrost will turn on ICE for max heat? I didn't know that one.. what if it's not cold enough for heat? I thought that Front windsheild defrost is MAX fan with AC ON.
     
  12. Salamander_King

    Salamander_King Senior Member

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    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.
     
  13. Mendel Leisk

    Mendel Leisk Senior Member

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    Could you guys sub "engine" for "ICE" for a while. I know I'm being persnickety, but...
     
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  14. mister2cool

    mister2cool Junior Member

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    I tested a little in rainy drive home. In EV mode, AUTO is ON @ 68 F and AC is OFF. Outside temp was about 50F. There was definitely heat coming out so heat pump compressor was definitely on. It was definitely not removing any moisture as all windows were getting foggy so AC compressor is not on or compressor not running in AC mode. Once I switch on AC (not window defrost), air coming out was noticeably cooler but fogging started to clear out on all the windows.

    I have never had a head pump system, and can't seem to figure out how exactly it works even though I have read up on it a lot. Does our system have only one compressor? My understanding for heat pump to work is that it literally reverse the direction of the flow of the refrigerant so I am not sure how one compressor can do heat and AC at the same time, or does it switch back and forth?
     
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  15. Mendel Leisk

    Mendel Leisk Senior Member

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    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.
     
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  16. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    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.
     
    #36 CharlesH, Dec 11, 2019
    Last edited: Dec 11, 2019
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  17. mister2cool

    mister2cool Junior Member

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    Ah! that makes a lot of sense! So the AC\Heat pump system in the Prius is somewhat different from what we usually see in a residential system. I sort of understand how residential heat pump works and was having a real hard time wrapping my head around how that system would work in our car.
    Thanks for the education!
     
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  18. Tideland Prius

    Tideland Prius Moderator of the North
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    You seem technically inclined. Would this thread help you learn more? It's too complicated for me but I was able to follow @Mendel Leisk 's explanation above.

    How does the heat pump work in a Toyota Prius Prime plug-in hybrid? | PriusChat
     
  19. mister2cool

    mister2cool Junior Member

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  20. Tideland Prius

    Tideland Prius Moderator of the North
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    Np! Yeah I'd love to figure out how the vapour injected heat pump works and how it's an advantage over the other systems in other EVs/PHEVs. All I can say is that it's more efficient when I mention that the Prime has the vapour injected heat pump but can't explain the vapour injected part that differentiates it from their regular heat pump.

    All I know is that my Prime is more efficient than their EVs even now that the weather is cold.
     
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