Where does the heat come from (When AC is off)

Where does the heat come from (When AC is off)

In gas car, if you turn off the A/C and turn the temperature to Max, you still get some heat from the engine...

But in a Tesla, you can still do the same trick and get heat... without turning on the AC...
So where the heat is from? The battery waste heat?

andy.connor.e | 09. Dezember 2019

An ICEV is taking waste heat from the radiator which is cooling the engine. Essentially, the heater taps into the cooling system to use waste heat for the cabin heater. Which is also why in an ICEV, the heat is slow to respond and usually takes until the engine warms up for the heat to really pump out.

EVs will typically use a resistive heating system, which does not produce heat unless the heat is turned on, which is equivalent to running a space heater plugged into the wall in your house.

Bighorn | 09. Dezember 2019

Resistive element.

ColoDriver | 09. Dezember 2019

It uses toaster oven technology.

WardT | 09. Dezember 2019

AC units are sometimes called heat pumps. You can buy a heat pump to heat or cool your house or your pool. I am under the impression Tesla uses a heat pump compressor to cool the interior. I wonder why Tesla doesn’t use the heat pump to heat the interior? An ICEV has a massive amount of waste heat to warm the interior as hot as you want. I expect when it gets really cold the heat pump would be too inefficient or not big enough to heat the interior. I suppose I answered my own question, but it helped for me to type this out and think about it a little more. Thanks for listening!

donharvey2323 | 09. Dezember 2019

Yes, Heat pumps don't work well as heaters in cold weather. Resistivity heat works right away but will affect mileage. Pre-warm when it is still plugged in.

finman100 | 09. Dezember 2019

no heat pump in any Tesla. yet.

Patronus | 09. Dezember 2019

It is a lot like a hair drier. Not very efficient.

Kylelam | 11. Dezember 2019

I see! The reason I was asking is that...

when the "AC" is off, I feel the air does not heat up right away (until I drive a bit), and it doesn't get as hot (it's like.. 70F), vs turning the AC on and delivering heat right away.
It's either the software making it as if there were a heat pump.. or some magic is happening?

jallred | 11. Dezember 2019

It's either the software making it as if there were a heat pump.. or some magic is happening? -kylelam

Not sure why you believe those are the only two possibilities.

There is no designed path for heat flow from motor, inverter, or batteries to the inside cabin. Possible ways to heat the cabin without turning on the heat is 1) heat transfer from the occupants and 2) heat transfer from the motor cooling/heating system by physical proximity (not by design). Keep in mind though that the temperature of the various systems in the car is statically nothing like the temperatures found in an ICE.

jallred | 11. Dezember 2019

It is a lot like a hair drier. Not very efficient. - Patronus

Electric resistance heating is 100% efficient.

WEST TEX EV | 11. Dezember 2019

Patronus | December 9, 2019
“It is a lot like a hair drier. Not very efficient.”
Actually quite efficient. Like 100%

But does use ALOT of power/energy/range....

andy.connor.e | 11. Dezember 2019

Correct on the technicality @WEST

Resistive heaters are 100% efficient. Although not the most effective way to heat a room since it is not taking heat from one place to another like a heat pump.

jallred | 11. Dezember 2019

A heat pump needs a lot of components. Condenser, valve, evaporator, compressor, refrigerant. Lots of points of failures, lots of maintenance. Also it adds volume and weight in the system. Add all that to the fact that there is a handful of environmental conditions where heat pumps either don't work or don't work very well.

Lots of trade-offs. The engineers seem to have made a decision to go with resistive heating elements instead of devising a system to move "free" heat from somewhere else to the cabin.

Sometimes the cost isn't just measured in energy consumption.

Varricks | 14. Dezember 2019

Sandy Munro said in one of his Autoline Detroit After Hours appearances that the Tesla stalls the drive motor to generate cabin heat. I have no details.
Apparently the Superbottle of the Tesla is one of the biggest breakthroughs in vehicle temperature management in decades.

jallred | 14. Dezember 2019

Super bottle has no loop that goes to cabin heat. It only has two loops, one for battery and one for drivetrain.

The only connection between super bottle loops and HVAC is that the AC can route to a chiller that helps cool battery and drive train loops.

Cabin heat is with resistance heater.

vmulla | 14. Dezember 2019

"There is no designed path for heat flow from motor, inverter, or batteries to the inside cabin." @jallred
Maybe so, but still there's plenty of heat from the battery that bleeds into the cabin. This is quite easy to observe after a long drive. Just put your hand over the small openings behind the front seat rails - the opening that leads to va recessed area over the battery. You can feel a warm draft.
Similar situation happens near the foot well, but I haven't figured out where the bleed is happening from.

jallred | 14. Dezember 2019

@vmulla. I agree.

Environmental heat from the car's systems can migrate naturally to the cabin. But it doesn't happen through a controlled heat transfer management system. | 14. Dezember 2019

@vmulla - There is no heat bleed from the battery to the cabin. The HVAC system will blow air from the cabin heater to the floor vents under the front seats to warm rear passenger's feet. The battery pack is thermally isolated from the cabin. You can test it out by turning on the heat to HI, set to the floor and max the fan. You'll feel the heat coming out of these vents.

Bighorn | 14. Dezember 2019


You're feeling the heat vent under the seat.

vmulla | 14. Dezember 2019

@bighorn, @teslatap,
No, I’m not getting confused with the rear floor vent. I’m talking about the small hole behind the seat through which you can feel bare metal of the battery casing. It’s the same hole through which small objects fall in and make a metallic rattle.
I emphasized that the heat bleed happens only after longer drives. I see similar situation happening near front footwell, but in that situation I haven't figured out where the secondary heat is coming from. | 14. Dezember 2019

@vmulla Perhaps the heat from the vent is heating up the metal? There shouldn't be a way to touch the battery case from the cabin. The cabin floor looks to be a single sheet of metal looking at factory construction. The battery has it's own top and the pack can be removed without affecting the floor. Still, you could be on to something - maybe there are holes in the bottom of the cabin floor - I haven't looked that carefully.

vmulla | 14. Dezember 2019

I'll post a video to show what I'm talking about.

Check for mentos or m&ms making a rattling sound in your handydandy search utility ;) you'll understand the recessed area I'm talking about. I've lost my contact lens and a sim card into that space. I've also researched rear vent heat because I experienced rear vent heating bug (Tesla acknowledged that it's a known bug BTW)

M-A-B-MCMLXXX | 14. Dezember 2019


"Resistivity heat works right away but will affect mileage. Pre-warm when it is still plugged in."

Anyone note that you can't remotely engage the seat heaters while charging?

Bighorn | 14. Dezember 2019

I just did

M-A-B-MCMLXXX | 14. Dezember 2019

^ I’ll try tomorrow. Previously, I’ve been able to turn on cabin heat, but if charging, pressing the seat icon didn’t do anything.

M-A-B-MCMLXXX | 16. Dezember 2019

Same build, when charging the seat heat icons don’t show up and pressing the seats doesn’t do anything.

derotam | 16. Dezember 2019

I'll note that if you have scheduled charging for departure, the seat heaters don't show up. I saw that this morning...The car was not charging anymore but the cabin was in it's scheduled pre-conditioning mode. I turned off climate from the app and turned it back on and the seat heaters showed up again.

M-A-B-MCMLXXX | 16. Dezember 2019

Derotam -

Yes, I was also in scheduled charging mode. Good observation of a strange restriction.