Pro Tip: Setting the climate 71F or lower uses less energy in hot weather

[Gasaraki]

yes, when it gets cooler (or warmer if you live somewhere cold) it will start drawing in air. i believe this is mostly just based on outside temperature and maybe humidity. For me it essentially never autoswitches to draw in air outside because its too humid and hot here even at night. However, in winter at nighttime it does often switch to outside air being drawn in. The magic cutoff seems around 23C or so. I guess at those temperatures it is more efficient to take the outside air and condition it than to use inside air - as the car needs to then turn on the heater to warm the air back up again.

The car afaik cannot mix outside with inside air. You either draw air in via the outside intake or via the recirc intake inside the cars.

Yesterday it was 87 outside and I had it set inside to 72 and it was drawing in outside air (not blue). The air was still cool (probably around 72F) so YES cars can mix outside air and inside air to maintain the proper temperature inside the cabin. If it can't mix air why bother drawing in hot humid air when it could just use the inside air which is already cool...

People need to understand that the compressor is either ON or OFF, it can't control the temperature. A HVAC unit controls temperature by controlling the FAN speed and drawing in (mixing) hot outside air when needed.

 

[Candleflame]

Yesterday it was 87 outside and I had it set inside to 72 and it was drawing in outside air (not blue). The air was still cool (probably around 72F) so YES cars can mix outside air and inside air to maintain the proper temperature inside the cabin. If it can't mix air why bother drawing in hot humid air when it could just use the inside air which is already cool...

People need to understand that the compressor is either ON or OFF, it can't control the temperature. A HVAC unit controls temperature by controlling the FAN speed and drawing in (mixing) hot outside air when needed.

Yes thats what I am saying. the compressor cools down the air and then the car heats the air back up with the heater.

the reason the car switched to draw in outside air at 87 was because it was dry outside. The outside vent cant mix with the inside vent.

 

[techlogik]

In the South/Florida, we have heat pumps. As explained above. We don't have both an A/C unit to cool in Summer and a separate Furnace to provide heat in the Winter. It is the same compressor outside, and the air handler (inside) that does both functions. Since the temp drops not much in Winter, well, it can go down into 32F and below during Winter, but will usually go into the 50Fs during the day during cold time, the system indeed reverses the flow of cooling/heating refrigerant in the system based on the settings of Cool or Heat requested. So it is either 1 or the other, heating or cooling never both at the same time. This is much more efficient than having two complete systems for cooling and heating and saves money.

Since Tesla is using a heat pump, and it is set in Auto mode, meaning, it will either circulate refrigerant to cool, or heat as needed in the proper direction so the handler/fan outside can cool the coils/refrigerant being circulated back inside, or remove it as needed, the answer to one poster is yes, Tesla would indeed heat the air, even if it is 95F outside to keep the temperature at the point you need in the car set by the temp on the screen. This is how a heat pump works.

Important note, heat pumps are very inefficient at heating. They are great at cooling, since in FL in Summer, your A/C runs about 22-23hrs per day. Why our power bills jump 2-3x in the Summer compared to Winter. All cooling. You really don't want to use them in the Winter much, your power bill will worse than Winter due to the way they function. They are intended to cool, not heat efficiently. So the theory that you are setting the temp around 71-72F in the car, and it uses more energy makes sense completely to me. Tesla is using a heat pump to heat the air as needed based on temp request, which is less efficient than A/C operations using the same heat pump system.

It costs less money/uses less energy to cool with a heat pump than to heat with a heat pump. So, the TS observation would be correct if you want to save some battery/energy on your car. Either have the system off, or use the A/C, not heat unless necessary and really cold.

 

[MDElectric]

Important note, heat pumps are very inefficient at heating.

No, actually resistant heat is “very inefficient”. The heat pump (A/C is a heat pump) efficiency can be up to 300%, but is driven by temperature differences between inside and outside. 95 vs 75 (20 deg) in summer compared to 35 vs 75 in winter (40) and larger temp delta in winter also increases heat loss through walls.

 

[Big Earl]

There is a lot of inaccurate information about the Tesla HVAC system in this thread.

Yes the heat pump system can heat and cool simultaneously - it’s necessary to both dehumidify and heat the air.

Yes the compressor is variable output. It is not just on or off. That said, there are control strategies that ensure the system performs in accordance with best practices. One of those best practices is to maintain an evaporator temperature that is below the dew point when cooling so that the air is dehumidified, thus it can only unload so much before it must blend in heat to maintain occupant comfort, particularly in humid conditions.

Here is an excellent three part video series about how the Tesla heat pump system works. There are numerous system modes - far more than your standard home heat pump system.

Part 1:

Part 2:

Part 3:

 

[joelliot]

Yesterday it was 87 outside and I had it set inside to 72 and it was drawing in outside air (not blue). The air was still cool (probably around 72F) so YES cars can mix outside air and inside air to maintain the proper temperature inside the cabin. If it can't mix air why bother drawing in hot humid air when it could just use the inside air which is already cool...

People need to understand that the compressor is either ON or OFF, it can't control the temperature. A HVAC unit controls temperature by controlling the FAN speed and drawing in (mixing) hot outside air when needed.

My heat pump and my model 3 both have variable speed compressors. If you start the a/c and listen you can actually hear the compressor slow down as the interior temperature gets closer to the set point.

 

[rjpjnk]

yeah heat pumps dont really exist here. There are dual cycle ACs which i guess do something similar but we dont have those here as it doesnt get cold enough. (i.e. here you wake up maybe once or twice in winter freezing for that odd cold night we get. and by cold i mean i.e. 15C. Feels worse in europe though because houses have no isolation). Weather here is sorta like Havana in Cuba with the difference that we get a colder winter I guess... Cant wait until I move down south next year haha. #los angles climate yas.

when you go into the desert here it can go down to 0 at night (and bloody 30 during daytime). When I lived there I did have a reverse cycle A/C but tbh I didnt use it much... just use a thick blanket in winter.


Anyway all of this doesnt explain how the newer model 3s use the heatpump to condition the air. I think what happens is that the car has a seperate air conditioner/dryer before the heatpump so it dries the air to the desired % and then just uses the heatpump to either warm or cool it, taking into account the humidity %.

hm i dont have a heat pump but i would find it strange if the heat pump cant heat or cool at the same time.
from what i was told the model 3 does not have an auxillary heater. So if you drive in the humid tropics and its 23C at night and the air has to be dehumidified you need to warm it up otherwise itll become really cold.

Good example is my room right now. Nice and comfortable 23.5C in winter here, but its like 70% humidity. If i put on my AC to make this more bareable in dehumidify mode the temp goes down to 20.5-21C. Thats kinda cold but when I go to bed in a minute I will put it on as its just too stuffy sleeping.

Very interesting topic and data! I have been trying to understand why my AC turns warm sometimes and have been studying the hvac system in my Model Y.

I don’t know if the Model 3 discussed here has a heat pump or if it uses electric resistance heat, but I can tell you that the Model Y has a very complex system unlike anything I’ve ever heard of in a car or house. I don’t have a diagram now but can post later. This system has no resistance heat. The cabin air handler contains both an evaporator coil and a condensed coil and an octovalve system that appears to be able to use them at the same time or is quick succession. Likewise there are multiple external condenser and evaporator options including a liquid to liquid heat exchanger.

Even with all this complication it does sometimes cause heated air to come from the vents when I’m trying to “run the air conditioner “ whatever that means now. I do think the system has trouble slightly cooling when outside temp is not much higher than interior. It seems to over cool the air then heat it back up again. Still trying to figure it out.

 

[rjpjnk]

There is a lot of inaccurate information about the Tesla HVAC system in this thread.

Yes the heat pump system can heat and cool simultaneously - it’s necessary to both dehumidify and heat the air.

Yes the compressor is variable output. It is not just on or off. That said, there are control strategies that ensure the system performs in accordance with best practices. One of those best practices is to maintain an evaporator temperature that is below the dew point when cooling so that the air is dehumidified, thus it can only unload so much before it must blend in heat to maintain occupant comfort, particularly in humid conditions.

Here is an excellent three part video series about how the Tesla heat pump system works. There are numerous system modes - far more than your standard home heat pump system.

Part 1:

Part 2:

Part 3:

Ah, looks like Big Earl beat me to it while I was typing! Yes, these are the videos that have the diagram I was referring to. They are excellent.

Here is a screenshot from one of these showing the components. Just take a look at the air handler box in red dashed lines on the left. Two coils in it. Heating and cooling. What a design!