Is Air Conditioning less effective at high altitude?

[Barry]

Usually I keep my AC set on 67. I just spent a few days in the mountains of Colorado (9300+ ft ASL) and found cooling to be insufficient at 67. Pushed it down a degree to 66 and it was fine. On my way home, coming down the big hill from the Eisenhower tunnel, I started to feel cooler at about 8000 ft ASL and pushed it back up to 67.

Just curious about the physics behind this. Lower air pressure = less efficiency?

 

[Armadillos]

That is a good question. Usually for me, when I get that high up in the mountains, I just open up the window, as it never really gets above 80 at that high of elevation.

 

[Barry]

Yesterday was pretty warm up there, especially with the strong sun beating in (no, I don't have tint). Also, I typically don't open windows on the freeways - too noisy.

 

[Ingineer]

Yes it is, but that's probably not your problem. The software Tesla is running in the HVAC controller is not tuned properly. They have a Solar radiation sensor and take into account the exterior temperature when calculating the setpoint and they haven't done a great job. I've noticed when it's sunny and close to 100F out the system tracks your setpoint well, but if it's less sunny or cooler, you have to really lower the temperature to get the car to hold the same temp.

In other words, I wanted the car at 69F, and it was according to an external thermometer, but only when 100F outside. When it dropped to 80F outside, I had to lower the setting to 65F to maintain 69F inside.

 

[tinm]

I just set it to LO permanently and adjust fan. It takes a long time to cool the car down. After about an hour on a long drive where it's 100F out, the car is comfortable. But at LO and 7 or 8 on fan speed.

 

[mknox]

They have a Solar radiation sensor and take into account the exterior temperature when calculating the setpoint and they haven't done a great job.

I'm not sure the early cars have a solar radiation sensor. I don't believe mine does. Mine also doesn't have the interior air temp aspirator that is now found on the center armrest near the USB ports (looks like a tiny speaker grille). I just have a static sensor near the steering wheel that is susceptible to "heat soak" from the surrounding materials.

 

[Ingineer]

If you don't have the solar sensor, then how do your automatic headlights work? (Or do you not have those either?) Regardless, you aren't missing anything, as they don't work properly anyway.

 

[Max*]

Regardless, you aren't missing anything, as they don't work properly anyway.

In what sense? Mine work pretty well.

The only areas I can comment on where they're not perfect are:
1. They're a little sensitive (like when I'm going through an underpass, they sometimes turn on).
2. If I had to manually turn them on, occasionally I'd do it a little earlier than they do it on their own (i.e. I'd turn them on before it gets as dark as it does), but with DRL's I just leave it on full auto.

 

[mknox]

If you don't have the solar sensor, then how do your automatic headlights work? (Or do you not have those either?) Regardless, you aren't missing anything, as they don't work properly anyway.

There is a light sensor, but I do not believe it is a solar radiation sensor for the HVAC. I'm drawing this conclusion from the fact that my previous cars have had separate devices for the automatic headlights and the HVAC system. My car is very early and the whole area of the windshield up by the rear view mirror is completely different than on newer cars.

 

[vdiv]

The humidity level may have a role to play of how hot/cold you feel and it may be different at the higher altitude. Also the solar radiation would be stronger.

It is possible that with less air passing through it the condenser would be less efficient at dissipating heat so yes, the AC would have to work harder.

 

[Barry]

The humidity level may have a role to play of how hot/cold you feel and it may be different at the higher altitude. Also the solar radiation would be stronger.

It is possible that with less air passing through it the condenser would be less efficient at dissipating heat so yes, the AC would have to work harder.

Humidity is similar in both locations, usually in the 10-20% range. No question solar radiation is stronger at the higher elevation.

 

[Ingineer]

In what sense? Mine work pretty well.

The only areas I can comment on where they're not perfect are:
1. They're a little sensitive (like when I'm going through an underpass, they sometimes turn on).
2. If I had to manually turn them on, occasionally I'd do it a little earlier than they do it on their own (i.e. I'd turn them on before it gets as dark as it does), but with DRL's I just leave it on full auto.

Sorry Max, I meant the solar sensor as it pertains to HVAC.

- - - Updated - - -

There is a light sensor, but I do not believe it is a solar radiation sensor for the HVAC. I'm drawing this conclusion from the fact that my previous cars have had separate devices for the automatic headlights and the HVAC system. My car is very early and the whole area of the windshield up by the rear view mirror is completely different than on newer cars.

Ok, well I can't speak for your early car, but it definitely seems like mine is affected by the sun (sort of inversely) as it pertains to the delta of set temp vs. actual result.

 

[mknox]

Ok, well I can't speak for your early car, but it definitely seems like mine is affected by the sun (sort of inversely) as it pertains to the delta of set temp vs. actual result.

On a sort of related note, all of my previous cars have had the light sensor (for auto headlights and such) mounted on the top of the dash and looked like a little dome or bubble. That seemed a lot more effective than Tesla's idea of mounting it on the windshield. From late fall to early spring, the sun here is lower in the sky, and depending on the direction I'm driving, the sensor on the glass gets shadowed and my headlights come on, dash goes in to night mode etc. even in broad daylight.

 

[president_ltd]

Usually I keep my AC set on 67. I just spent a few days in the mountains of Colorado (9300+ ft ASL) and found cooling to be insufficient at 67. Pushed it down a degree to 66 and it was fine. On my way home, coming down the big hill from the Eisenhower tunnel, I started to feel cooler at about 8000 ft ASL and pushed it back up to 67.

Just curious about the physics behind this. Lower air pressure = less efficiency?

Into a 2nd page of the thread any noone has actually responded to on anything physics related. heh.

Short answer is yes: lower air pressure (higher altitude) = lower convective heat transfer capability.
Put another way, the air is thinner and has less ability to carry heat. (in the context here, 'heat' can mean 'cool' too.)

At 9300ft above sea level, the effective level of HVAC is de-rated by around 17%.