Heat Pump: A/C Button Matter?

Discussion in 'Prime Main Forum (2017-Current)' started by mr88cet, Nov 12, 2017 at 4:25 PM.

  1. mr88cet

    mr88cet Active Member

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    Since it only occasionally gets only somewhat cold here in Austin, there’s one thing about the heat pump that I’m a little unsure about:

    Best I can tell, it doesn’t matter whether the A/C switch is on or off; the heat pump will operate the same either way, right?

    Two possible interpretations of what that button does: Either “cooling on/off” or “climate control system, be it heating or cooling, on/off.” I gather it means the former — only cooling.
     
  2. bisco

    bisco cookie crumbler

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    assuming 4gen is the same, the a/c button is air conditioning. even the the manual calls hvac, ac.
     
  3. Tideland Prius

    Tideland Prius Moderator of the North
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    A/C = conditioning of the air.

    If it's in AUTO mode, the car will control whether the A/C compressor runs or not. I leave it on e-40 but it doesn't do anything even if the light is on.

    In the winter, it's usually used to dehumidify the air so that moisture doesn't build up inside the car (e.g. from the snow or rain you brought into the car with your shoes).

    If left off, then you're constantly having to turn on the front defroster to clear the windows; if you leave the A/C light on in AUTO mode, the car will run the compressor as needed to maintain a lower RH.
     
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  4. Mark57

    Mark57 Senior Member

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    Thanks. When it's 15° outside and I turn on the heat/auto mode it always freaks me out to see the A/C light on. Too many years in "stooopid" cars.
     
  5. mr88cet

    mr88cet Active Member

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    So, I’m not 100% sure I follow your answer. Checking...

    So then, the heat pump will not warm the cabin unless the A/C switch is on?

    From the very-limited cabin-warming needs I’ve had so far, it seemed like it has “pumped heat” regardless of whether that switch was on or off. Again though, it’s a little hard to tell for sure when the outdoor temperature hasn’t dropped much below 50 yet.

    “E-40” = ?

    So, such a system can only dehumidify the space where the evaporator is located. It dehumidifies air only by virtue of cooling it to where the air can no longer hold the moisture and that moisture condenses out of the air.

    For it to dehumidify the cabin and thereby defog the windows, it must cool the cabin, meaning that it’s functioning as an air conditioner in the usual sense of the term, and not as a heat pump, in the usual sense of the term.

    For it to defog the windows and also warm the cabin, it needs another source of heat, which means turning on the engine.

    I’ve attempted to prevent it turning on the engine by setting the temperature to “low” and the A/C switch off, and then occasionally hitting the windshield defroster button. My hope was that this would avoid turning on the engine by defogging the windshield by blowing *cold*, dry air across it, rather than warm, dry air. I’ve seen on-and-off evidence of it turning on the engine nevertheless even under those circumstances. Not completely sure though how often and under what circumstances it enlists engine heat even when the temperature is set to “low.”


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  6. CraigM

    CraigM New Member

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    The problem I’m having may be with terminology. In my mind A/C is for cooling, but it looks like Toyota considers cooling, or heating, as air conditioning (A/C)?

    I have my car set to ECO, and air conditioning ECO modes. In humid 40 degree weather, I set the the system to a comfortable 65 degrees and AUTO mode. Seems to defog the windshield within a minute, and the car gets warm. The gas engine does not come on.

    I’ve never pressed the button that looks like the standard defroster controller on other cars because I THINK that MAY allow the gas engine to start?

    Don’t know, still learning & playing!

    Craig
     
  7. bisco

    bisco cookie crumbler

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    toyota calls a/c what we call hvac, heating, ventilation and air conditioning. technically, they are right. hvac conditions the air, cooling is one part, as is heating and etc.
     
  8. Tideland Prius

    Tideland Prius Moderator of the North
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    Yeah, when I posted a picture of -40, one of my friends asked why the A/C light is on. Another replied "it probably warms the interior" haha.

    DSCN3783.jpg


    The heat pump runs in according to the outside temperature and the temperature setting of your climate control regardless of the indicator on the A/C button.

    Typo lol

    Well the condenser part of the HVAC unit takes care of the condensation of moisture of the air. The air that comes out is dry whether it's cold or warm. Also, the moisture content of the air coming out of the vents doesn't change with the temperature (since the condenser takes out "x" amount of moisture). The only thing that changes is the relative humidity of said air coming out of the vents. Hot air coming out will have lower relative humidity than cold air but the specific humidity (moisture content) remains the same.


    Also, turning on the windshield defroster in any modern car (~2000 or newer depending on when the model was redesigned and whether the manufacturer opted to add that feature in early or late 2000s) will automatically turn on the A/C compressor.
     
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  9. mr88cet

    mr88cet Active Member

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    So, “thanks” regarding the A/C switch and heat-pump operation. That’s what I thought I was seeing, but wasn’t sure...

    FWIW, the evaporator side — the low-temperature/low-pressure side — the side that evaporates the freon the — dehumidifies. A heat-pump being reversible, the evaporator could be inside or outside.

    But anyway...

    Yes, I too have seen that hitting the windshield defroster generally automatically turns on the air conditioner for its dehumidifying. However, I have not seen that it necessarily adds heat. I’ve usually seen that controlled independently.

    I was hoping that, if I keep the temperature at “low,” it would just blow cold, dry air over the windshield, and not turn on the engine...


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  10. Trollbait

    Trollbait It's a D&D thing

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    To dehumidify air for the defroster, the heat pump needs to run in the cooling cycle; no heating is possible. That's why the ICE comes on normally with the defroster in cold temps. So what you propose could work as long as the temperature is set to lower than ambient. Note that while cold dry air will remove fogging, it won't work as well as warm dry air, and if the windshield is chilled, condensation can increase on either side.
     
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  11. mr88cet

    mr88cet Active Member

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    OK, yeah, so, when I’ve tried that — temperature “low” and windshield defroster on — I’ve seen at least one case where the engine went on anyway, but not sure whether that’s always what it does...


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  12. Trollbait

    Trollbait It's a D&D thing

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    The ICE coming on with defrost could be controlled by ambient temperature; under a certain temp it turns on no matter what the defrost temp is set too.
     
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  13. mr88cet

    mr88cet Active Member

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    I’d be surprised if Austin has gotten that low so far this winter, but...


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  14. Trollbait

    Trollbait It's a D&D thing

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    Then the ICE coming on might have nothing to do with the defroster at that time.
     
  15. bisco

    bisco cookie crumbler

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    i thought the ice always comes on with the defroster.
     
  16. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    I saw on one of the drawings of the Prime HVAC system that there is a both a condenser AND an evaporator inside the cabin, as well as an outside coil. How does the heat pump work in a Toyota Prius Prime plug-in hybrid? | PriusChat This allows one to get dehumidification without cooling the cabin and without requiring use of the ICE to reheat the air. It is way more complex than a typical home heat pump, which just reverses the flow of the refrigerant to reverse heat/cool roles. There is always hot, compressed refrigerant in the inside condenser, and cold refrigerant gas in the inside evaporator, and dampers control the air flow through them as appropriate. There is also a conventional ICE heating core inside the car with engine coolant. For ordinary dehumidification, the heat pump works fine, but when you need serious heat to defrost the windshield, that is when the ICE kicks in. There are no resistive heating elements in a Prime.
     
    #16 CharlesH, Nov 14, 2017 at 10:58 PM
    Last edited: Nov 15, 2017 at 12:08 AM
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  17. DavidA

    DavidA Prius owner since July 2009

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    OK, so I want to get this straight. For heat, hopefully without the ICE turning on, and to dehumidify the interior, we need to turn on the A/C button on when using the heat pump heater? And without A/C button on, it won't dehumidify?
     
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  18. Elektroingenieur

    Elektroingenieur Active Member

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    It’s unfortunate that the Owner’s Manual doesn’t really explain what the A/C switch does.

    Toyota’s New Car Features book for the Prius Prime (available by subscription to techinfo.toyota.com), however, has an entire section (“Air Conditioning Control”) on this topic. It’s too much material to post here, but I can share two relevant quotes:
     
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  19. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    ScanGauge has a programable XGauge for ACWatts, can be used to monitor conclusively if the compressor is running or not. This is on 3rd gen, not sure if it also works on 4th and/or Prime.

    My experience on a 2010 Canadian car:

    The air conditioning compressor (old-school, american definition of AC) typically only runs when the AC button is pushed, it's light is on. The one exception: it will sometimes run when mode is set to Heat/Defog.
     
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