Does heater use energy even with the fan off?

welcome!
no, the fan activates the heat pump and/or engine to bring the car to the set temp.

if she's using the guess o'meter to determin ev range, that may be part of the problem.

generally, as the weather cools, the actual range will suffer due to limited charging/discharging capacity, and more air and tire friction.

the best ways to measure range is to measure battery charge capacity and keep records, or have a set course to follow in similar weather conditions.

don't forget, the software reduces the range icon miles when you turn the fan on, as a warning that you may get less range due to hvac use.

 

You do not have to change the TEMP setting to turn off the heat or cool. The HVAC (both heating and cooling) is off when you touch the fan OFF button on the screen (the red dot in the pic below). At this stage, there is no energy drawn by the HVAC. As @bisco commented, the ambient temperature affects the EV range more than anything else. And the GOM indicated EV range is only an estimate, which can be wildly off depending on the environment and driving condition the car will see while driving.

That being said, the heating will usually use more traction battery power than cooling. Here is a good thread to read on the estimate of how much traction battery power the various HVAC setting uses.
Estimated EV penalties for various heating cooling and lights

 

You can always double check by pulling up the Energy Monitor
If an arrow is pointing to the fan, the HVAC is using battery/energy.

(photo credit this thread: ICE runs but does not show on Energy Monitor (2021 Prime) | PriusChat)

 

what do you mean by separately?you can run the fan for fresh air if you turn the temp to low

 

You can run the fan only without heating or cooling. As @bisco said, turn down the temp to low and make sure the AUTO and A/C are both OFF. In hotter climates, turning off both AUTO and A/C will shut down the cooling, but I think in colder climates A/C also runs the condenser occasionally if left to be ON.

 

Actually, the heater will run even with the fan off. Unless you set the temp to LO, if the inside of the vehicle is is cooler than the current temperature setting the heat pump will engage. It's a very poor design, forcing one to always set the temp to LO after using the A/C during the day.

I found this out the hard way one morning driving to work trying to nurse the last 20% of EV going to work. At one point my charge was at 20% and a mile or two later it had dropped about half that. Then I noticed the warm air coming through my vent (I had recirculation set to off, temp was set to about 74). Had to look down and manually set my temp all the way to LO to shut off the heat pump. It's something I noticed with my 2017 and also with my 2018.

One would think it is a simple software fix to have the vehicle disable the heat pump if the fan is OFF!

 

That's strange. I have never experienced what you describe. I keep the fan OFF in my PP most of the time. Even in the winter, I turn OFF the fan (thus HVAC) at the end of the trip so that the EV range display is at max. But the temp setting is almost always set to 70F and I never change it. I do not see any EV range reduction without turning on the fan and no warm air comes out of the vent. The Energy monitor and the energy does not flow to the fan (HVAC) when the fan is off. This has been true for all three models of PPs 2017, 2020, and 2021.

 

I have never experienced that either. Hitting the fan off button always killed the entire hvac system.

 

I first noticed it when I had left my temp setting at 74 during the day and was driving in early morning. It probably was about 65-70 degrees outside and I was wondering why the cabin was relatively warm. When I put my hand over the vents, I could feel warm air coming through even though the fan was off (recirc was off so outside cool air was blowing in).

This was something I noticed on both my 2017 and 2018.

 

Does a Prime have only a heat pump, or does it also have a conventional heater core?

 

AFAIK, it does not have a conventional electric resistance heater. If the temperature outside is too cold to extract enough heat by the heat pump operation (i.e. below 14F), then it will start the engine to use the engine waste heat for warming up the cabin.

 

That's what I meant by a conventional heater core.

Electric elements in the heater are not really conventional in cars. They have existed in earlier Prii, though with only negligible heating ability (around 700 watts, compared to 5300 or so from the hot engine coolant in the heater core).

 

The prime has an electric heat pump, and engine/exhaust heat.
No Electric heating elements in the Prime. Electric heating isn't nearly as efficient as a (newer) heat pump design.

Code:

https://www.youtube.com/watch?v=KUXZVg2AyiY
The Worlds SECOND most complicated car HVAC system Explained
Jun 19, 2021The Car Care Nut

I don't know if the center display, or trims, make a difference.
However, it seems like the "heated/cooled" fresh air through the vents is as semi-random as the screwed-up RSA (Road Sign Assistant) display.
Fwiw, I've twice had the RSA notifications work very nicely, and not being a constant PITA, on my HUD display. Note, I'm talking while driving the same exact routes to/from work. I've tried. It seems like it's multiple menus/sequences/settings dependent. So, now, I keep RSA off.


Back to the air through the vents:
If the Climate system is Off, and air recirculation is on, then, as I've noticed, there is air through the vents, and both the AC or heat-pump stay off.

However, even if the "Climate System is Off", if the air recirculation is set to "fresh air intake", then depending on the position of the stars, the 3rd lottery number chosen in States with an Even number of Vowels, and the 5th lottery number chosen in States with an Odd number of Consonant letters, then the Climate System will Psychically Know if you really really really do want to have heated or cooled air from the vents - regardless of what you may foolishly say.
Maybe the Climate System is most advanced AI system in the universe, and ready to take over the world? Or, maybe, stupidly designed/programmed.


It seems like a voice control of the Climate System may reset air recirculation setting?
Or, doing a Remote Climate System control may reset air recirculation setting?
I've also had the air recirculation setting reset to "fresh air", without using voice or remote.

What I used to do after shutting off the Climate System, was to make sure the air recirculation is off.
Now, since I'm using Climate pre-conditioning even more now, I typically keep Climate system always on.

 

even in the pip, with the engine cold and off, and the temp set to low, outside air usually feels warm coming out of the vents, even when it's much cooler outside.
i can't figure out why, i often have to turn the a/c on to cool the cabin

 

The first generation, 2012-2015