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AC keeps coming on

Hi

I have a problem which I was hoping someone could help with.

For heating we only ever use the windscreen demister. It’s a kind of 2 for 1 it heats up the car and clears the windscreen.

My wife has been driving the car a lot and I noticed the energy consumption was astronomical.

it turns out when she puts the heating on the ac automatically comes on I changed it and saved the settings but when the car is used again it reverts back to ac coming in.

how do I set the demister so that it doesn’t use ac when the car is switched on after a journey?
 

Adopado

Well-Known Member
Aug 19, 2019
5,827
4,428
Scotland
Hi

I have a problem which I was hoping someone could help with.

For heating we only ever use the windscreen demister. It’s a kind of 2 for 1 it heats up the car and clears the windscreen.

My wife has been driving the car a lot and I noticed the energy consumption was astronomical.

it turns out when she puts the heating on the ac automatically comes on I changed it and saved the settings but when the car is used again it reverts back to ac coming in.

how do I set the demister so that it doesn’t use ac when the car is switched on after a journey?

As has been said, AC takes the moisture out of the air so it's part of demisting. It's a good thing to be running AC anyway .. it won't actually be using power a lot of the time as it will kick in now and again when needed, just like a thermostat does with temperature. The reason it's a good thing (other than for effective demisting) is that it reduces the moisture content in the car generally. One issue that can arise is misting behind the glass in the B pillar cameras. If the AC is on that is very much less likely to happen. Another issue is that the air filters can end up being moist all the time and result in mould growth leading to odours and a need for early replacement.

The AC has never been implicated in "astronomical" consumption overall. It may be detectable but not a major issue even in earlier non-heatpump cars. The only (unlikely) scenario that I can think of relating to AC is that there is some weird interaction of the heat pump/AC software controlled octovalve thingamabob that may occur in the refresh cars, but we've never hard of anything to date!

The reason for high consumption lies somewhere else. Has you wife been attending rally driving classes on the sly?
 
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Hi

I have a problem which I was hoping someone could help with.

For heating we only ever use the windscreen demister. It’s a kind of 2 for 1 it heats up the car and clears the windscreen.

My wife has been driving the car a lot and I noticed the energy consumption was astronomical.

it turns out when she puts the heating on the ac automatically comes on I changed it and saved the settings but when the car is used again it reverts back to ac coming in.

how do I set the demister so that it doesn’t use ac when the car is switched on after a journey?
How do you think the demister "works"?
 

Jason71

Active Member
May 8, 2019
4,037
4,308
Shropshire
not sure generating heat and pointing it straight at a cold piece of glass is the way to go about it tbh if you want efficiency

not sure what you have but in my experience with a non heat pump M3 the most efficient way to heat the car while being comfortable is:
AC on
heated seats on medium/high
cabin temp set to 20
fan on 1-2
heat sent to feet

I have never had a problem with the windscreen being misted up. its always cleared by the time I finish preheating and stays that way
 
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Jason71

Active Member
May 8, 2019
4,037
4,308
Shropshire
To be fair that's how cars that don't have AC have always cleared their windscreens... and it works ... but better if you have AC of course.
if its the sort of cold damp day where serious demisting is needed will the AC actually work anyway? no idea about the Tesla system but in those sorts of conditions car AC often doesn't work properly anyway due to the heat exchanger icing up.
 

Adopado

Well-Known Member
Aug 19, 2019
5,827
4,428
Scotland
if its the sort of cold damp day where serious demisting is needed will the AC actually work anyway? no idea about the Tesla system but in those sorts of conditions car AC often doesn't work properly anyway due to the heat exchanger icing up.

Who knows! It's a mistery ! Get it ... mystery mist..ery ...;) (I think it does ... it certainly clears a screen mega fast compared with our Fiesta with the broken AC!)
 
not sure generating heat and pointing it straight at a cold piece of glass is the way to go about it tbh if you want efficiency

not sure what you have but in my experience with a non heat pump M3 the most efficient way to heat the car while being comfortable is:
AC on
heated seats on medium/high
cabin temp set to 20
fan on 1-2
heat sent to feet

I have never had a problem with the windscreen being misted up. its always cleared by the time I finish preheating and stays that way

Spot on these settings. With preconditioning the car before I leave in the morning even at -5 the car was nice and warm for all my commute and the energy spent is minimal.
 

Adopado

Well-Known Member
Aug 19, 2019
5,827
4,428
Scotland
Spot on these settings. With preconditioning the car before I leave in the morning even at -5 the car was nice and warm for all my commute and the energy spent is minimal.

I use similar settings but I don't even need to use the seat heater (though I can't be bothered taking my coat on and off so I'm normally well wrapped up!)
 
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Hi

I have a problem which I was hoping someone could help with.

For heating we only ever use the windscreen demister. It’s a kind of 2 for 1 it heats up the car and clears the windscreen.

My wife has been driving the car a lot and I noticed the energy consumption was astronomical.

it turns out when she puts the heating on the ac automatically comes on I changed it and saved the settings but when the car is used again it reverts back to ac coming in.

how do I set the demister so that it doesn’t use ac when the car is switched on after a journey?
I think I get what you're meaning... when you press the icon for windscreen demist on the screen you've been using that to heat up the car / demist and then when you turn it off by pressing the icon your automatic temperature-based climate control is remaining switched on?

Might not be this, but could be worth a try: it's maybe down to the driver profile and possibly the easy entry - I had similar with seat heaters randomly switching on every time I got into the car despite being switched off previously! Go through each profile including the easy entry profile and make sure climate is specifically set to off, then save on every profile.
 

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