MPG Impact of Heater

Discussion in 'Prius v Fuel Economy' started by TomNat, Oct 11, 2015.

  1. TomNat

    TomNat Junior Member

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    We are new owners as of July of a Prius v model 5, and as it is getting colder, would like to know about the heater operation options & impacts on MPG. Our prior cars were non-hybrid/EV with manual analog temperature heater/AC controls (no Auto mode or digital temperature displays). From the manual, the prius ICE will run to provide heat, even if the ICE is not needed for driving or battery charging, while the AC runs from the battery.

    We assume that the heater will run when the inside temperature setting number is more than the outside temperature number, and the fan is running. Does air from the outside come in when the fan is off & car is moving, as our other cars? Will the heat work when recirculating button is pressed? How does the heat work with Auto pressed or not pressed? How do you suggest operating the heater for best MPG with reasonable comfort? Thanks for your help!
     
  2. Mendel Leisk

    Mendel Leisk Underfoot

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    The Prius heats the cabin with engine coolant, just like any other car. And as long as you're not using AC, not much of a dent.
     
  3. dorunron

    dorunron Senior Member

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    You will notice a slight drop in MPG during the winter months, but that is really due to the winter fuel blending. All vehicles suffer from this, some worse than others.

    I usually set it around 76 or so and let auto do it's thing. Everyone has their own temperature they like. Play with it until you are comfortable. If you use no heat and or no a/c you will get the best MPG. But where you are heat will be a given just like A/C is a given here.

    Best of luck to you and enjoy your v
     
  4. rdgrimes

    rdgrimes Senior Member

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    This is correct. When the weather is sub-freezing the ICE can run much of the time purely to make heat. The higher you set the inside temp, the higher the system will keep the coolant temp, so lowering the temp saves some fuel.

    Best choice is to use auto mode with the hybrid system in ECO mode. This will limit some less economical HVAC functions, will lock out bi-level vent mode too, and may also change how the defroster responds (in terms of when and how much A/C is called for).
    Mostly, the auto system control is smarter than the average driver, it really is a set it and forget it system. Just keep the temp setting low in the winter and higher in the summer.
     
  5. bisco

    bisco cookie crumbler

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    agree with all above, plus, the engine has to run longer to warm up, and more often to stay up to minimum temp. so even without heat, there is some drop in mpg's. i used to notice about a 20% drop from ideal conditions to the coldest days, all things considered.
     
  6. cproaudio

    cproaudio Speedlock Overrider

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    Stop beating around the bush guys. Winter weather and heater kill your MPG as much as 30%. Your 44mpg city can get as low as 30mpg. In winter, with heater on, you will get worse mpg than summer with AC on. The Prius engine is very efficient. It produces less heat than any other cars. Plus the engine shuts off when you cost and/or come to a complete stop. Engine runs longer and uses more gas just to warm up and maintain the heat. You can block the grill and seal the hood/fender gap with foam tubes to trap the heat in the engine compartment. Even with all that, you'll only gain 2mpg back, 5mpg if you're really careful. It won't be nearly enough to compensate for the heat loss of cold winter and heater on. I get 50mpg with 50% upper/ 95% lower grill block and 100% fender/hood gap block without heater in 35-45f weather. 44-48mpg with heater. My area never get below freezing. In spring, I get 55-60mpg in 75-80f weather with cracked window. In summer, I get 48-52mpg in 90f-115f weather with AC on. These are calculated MPG not what the display says.
     
  7. TomNat

    TomNat Junior Member

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    Thanks for all the info.We snowbird to S. FL in the winter; so do not need grill blocking.

    We verified that the Prius heat/AC seems to operate in the non-auto mode like our prior non-hybrid cars, e.g. recirc with fan to block fumes, outside air when moving with fan off, etc. We have been OK using ECO mode with AC in manual & auto([email protected]) modes in CO; so hope that heat works as well in auto & ECO modes.

    Since it is still very warm lately here, we have not tried heat in the auto mode. For good MPG, many suggest using ECO & auto([email protected]) for heat. Any auto heat suggestions would be appreciated.
     
  8. Mendel Leisk

    Mendel Leisk Underfoot

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    I can reliably turn the engine on/off at a stop, by ticking the cabin heat setting up/down. The threshold's around 21.5C I think.
     
  9. rdgrimes

    rdgrimes Senior Member

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    Unless you use BOTH auto and eco modes, you don't get HVAC energy savings associated with eco mode. Keep the heat set as low as you can tolerate is the main thing. But if the blower is showing 3 or fewer pips you won't be using much energy regardless.
     
  10. TomNat

    TomNat Junior Member

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    Thanks for new info that you need to use both auto AC/heat & eco modes to get eco HVAC MPG savings, i.e. no MPG savings with eco & manual AC.

    Still too warm here to test out the heat options! Is there a way to get heat manually, or do you have to press auto with temperature set more than outside temp?? We assume the Prius v AC/heat is the same as that on the Prius Gen 3. Some long threads on Prius Gen 3 AC/heat, that are helping us understand how clever the auto AC/heat is.
     
  11. farmecologist

    farmecologist Senior Member

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    Gee it should be interesting to see how our 2010 handles southern Minnesota winters. Frankly, ANY car gets substantially less MPG around here in winter due to winter blend and warm up. I'm not too worried about it.

    I have read differing opinions on the Prius in extreme cold. Some folks seem to think the car warms up *faster* than a traditional car..some don't. Of course, this is subjective. I'll chip in my opinion sometime this winter.

    From what I'm reading, It does sound like grill blocking is a necessity though..I'll have to get on that soon.
     
  12. TomNat

    TomNat Junior Member

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    It finally has got cold enough (45-50F during day) to try our Prius v Model 5 in the Auto heat mode. As mentioned previously, the manual heat mode operates as our prior analog heat control cars, when the Set Temp is greater than the Outside Temp with & without Recirc pressed.

    In ECO mode with Set Temp (70F) greater than Outside Temp (50F) with floor mode air flow & lowest fan speed, when we press Auto, the AC comes on, air flow changes to the face & fan speed increases, rather than staying on Heat, floor air flow mode & lowest fan speed. We have to press AC to manually switch back to Heat, which seems like a bad design. When we manually change the air flow from face to floor or to floor/face, the Auto light goes off, which the manual says should happen, but "automatic mode functions other than that operated are maintained".

    Switching on AC when Auto is pressed, when we are in manual Heat mode, seems like bad human factors. What are we doing wrong or not understand??
     
    #12 TomNat, Oct 29, 2015
    Last edited: Oct 29, 2015
  13. rdgrimes

    rdgrimes Senior Member

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    Ignore the green light on the A/C button, it means nothing. The A/C compressor only runs when humidity is high enough to trigger it. For some reason the engineers decided to have it on all the time. If you manually force it off, you can't get the benefit if its super-humid.
    If the system isn't keeping things warm enough for your taste, just turn the temp up.
     
  14. Mendel Leisk

    Mendel Leisk Underfoot

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    When the AC light is on, the compressor runs, maybe 50/50 on/off. At least with a regular Prius.
     
  15. rdgrimes

    rdgrimes Senior Member

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    That's entirely up to the conditions. In cooler weather it may not run at all. (in auto mode)
     
  16. Mendel Leisk

    Mendel Leisk Underfoot

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    The only time* Ive seen the compressor stay right off is when I've got the AC button off (no light) and set Mode to Heat/Defog. Normally this will run AC, at somewhat lower wattage. But at temps maybe lower than 5C, it stops doing so. But with AC button depressed, unless temps are very near freezing, the compressor's running, off and on.

    I'm monitoring AC wattage with Scangauge. It shows zero when the compressor's off, various wattages when it's running.

    * Well, also, if you've got it on any mode but Heat/Defog, and the AC button off. Again, then the compressor will not run.
     
  17. ChapmanF

    ChapmanF Senior Member

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    The New Car Features Manual for both the Gen 1 and Gen 2 (the latest ones I have, sorry) both rate the heat output identically at 5,300 watts. That refers to the amount of heat it can extract from the hot engine coolant when the heater is running full out, and by the TANSTAAFL principle, all of that heat will be replaced by burning fuel in the engine. On the other hand, if you weren't using it, a lot of it would just be gotten rid of at the radiator anyway.

    5,300 watts is about 7.1 horsepower. If the engine's thermal efficiency is around 38%, then the engine is producing at least that much waste heat anyway, any time it is running above about 4.4 HP output - that's a bit above 5% of max output.

    Of course the heater doesn't often run full out - it runs at whatever fraction of full out it needs to maintain the settings you prefer. Also, before the engine is totally warmed up, if the heater needs to run full out, it also kicks in the electric elements, which add about 700 watts in Gen 1 and Gen 2 (I don't know about other models). Ultimately, that power also comes from fuel in the engine, just by a different, mechanical-to-electrical route.

    -Chap
     
  18. Shahid Jan

    Shahid Jan New Member

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    Sir , it's second time when I turned on the heater , immediately the tank dropped to zero. It's JDM Prius 2010. I am uncertain whether it is the gauge faking it while heater is turned on or it's the real tank which drained as result
     
    #18 Shahid Jan, Nov 7, 2022
    Last edited by a moderator: Nov 7, 2022
  19. Air_Boss

    Air_Boss Senior Member

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    A seven year old thread, revived?
     
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