We have, at some point, had to sit and wait in a parked car for hours. Switching off the engine is a sensible thing to do as it will save on fuel. However, if you are waiting for long, the car will soon get very hot under the merciless summer sun – or turn frigid if it is winter.
Many people consider running the air conditioning with the engine turned off. Is it possible?
The AC Can’t Run With the Engine Off.
No. You can’t run your AC with the engine off. Your air conditioner compressor runs off a serpentine belt. This belt spins other accessories like the alternator and gets its power from the engine’s crankshaft. If the engine is off, the crankshaft isn’t turning. The serpentine belt won’t be turning, and your AC compressor won’t run.
The AC compressor moves the refrigerant around and compresses it leading to the heat exchange process. This provides the cool air for the AC blower to circulate in the cabin.
The Case of Hybrids
Hybrid vehicles bring a different proposition to the table. Most have AC systems that can run entirely off the battery pack – as long as it has power. However, the vehicle must be in the ‘ON’/Ready position.
Technically, the vehicle is on. The hybrid system has just decided to turn the internal combustion engine off and run the AC and other accessories off battery power.
Once the battery is depleted, the engine will turn on automatically and run for a while before turning off again once the battery pack has some charge. The cycle will repeat until you power down the vehicle.
The vehicle has to be ON, not in the Accessories position, for this to work. So technically, you can’t run the AC on your hybrid (or electric) vehicle without turning it on.
You are Confusing the AC Blower With the AC Motor
Some people will argue that they’ve repeatedly run the AC with the car turned off. In reality, all you feel is the AC blower noise and a breeze from the AC vents.
The air coming in will be the same temperature as the outside air (or the inside air if set to recirculate). You have just turned on the AC blower, not the entire AC system.
Some vehicles let you run the AC blower with the ignition set to the ON position or even the Accessories position.
This is possible since the AC blower is a simple DC fan that runs off the power on your starter battery. It will keep on running as long as there is some juice in the starter battery.
- You will experience some air circulation and normal AC noise. However, the air won’t be any cooler than the ambient air temperature. The breeze will still give you a cooling effect
- Running the AC blower for long can deplete your starter battery
- The AC button/Notification on your AC control panel/menu won’t light up because, technically, the AC is still off
Running AC while the car is parked is only possible if the engine is idling. Otherwise, you will just be running the AC blower.
Can You Turn on AC Without Starting a Car?
No. You can’t turn on the AC without starting a car. The only thing you can turn on without igniting the vehicle is the AC blower – if your vehicle allows it.
You can only turn the AC blower on if the vehicle allows you to go to the ON position without cranking the engine or can run the blower in ACC mode.
Most push-start vehicles take charge of the engine cranking, making it hard to go to the ON position without turning on the engine. This is to do on Key operated cars since you can just release the key before the engine turns over.
Is it Bad to Have the AC on When the Car is Off?
It isn’t possible to leave the AC on with the car off. The only thing you can leave on is the AC blower.
That aside, it’s always a good process to turn off the AC (not the AC blower) two or three minutes before turning off the engine. This allows the AC blower to circulate normal dry air through the vents, reducing the possibility of getting a musty smell after parking for a while.
Also, with the AC off, the AC compressor won’t instantly put a load on the engine next time you turn on the car. This will make cold starts faster and give your engine the chance to warm up before you turn on the AC.
How Long Can I run AC With the Engine Off?
Again, you are thinking of the AC blower since you can run the AC with the engine off. With that out of the way, you can run the AC blower for as long as there is juice in the starter battery.
This will vary depending on how much power your blower draws at the selected speed and how big your starter battery is.
It could be between 30 minutes on a small and worn-out battery and hours on big new batteries. The only problem is that you can deplete the battery leaving you no juice to run the starter motor and the engine.
Does Sitting in Your Car With the AC On Kill the Battery?
If you are sitting in the car with the engine on to run the AC, this will not kill the battery since the alternator will run and power the AC blower.
However, if the car is off, the AC will be automatically off as the compressor won’t be running. The only way you can kill the battery is if you turn on the AC blower and run it for long enough to deplete the charge on your battery.
How Long Can My Car be Parked With the Air Conditioner on and the Engine Idling?
Theoretically, you can run your AC with the engine idling and car parked for as long as you have fuel – or until something else in the car fails.
A running engine provides the perfect conditions to give the electrical system the charge it needs to run the AC blower and the serpentine belt the power it needs to run the AC blower.
The only issue is that every component in your vehicle, from the engine to the alternator, AC compressor, and AC blower, has a definite run time in hours.
Run them for long enough on idle, and they will start to fail even if your car has very low mileage on the odometer.
The Case of Police Cruisers
A good example of vehicles that perhaps idle for long with the AC and other electronics running is a police cruiser.
Consider an officer sitting by the road using a speed gun or at a parking bay staking out a suspect. They can’t run all the electronics and AC with the engine off. So, they leave the engine idling.
This has led to the common knowledge that buying a police cruiser or vehicle previously used like that gives you more worn-out hardware than what you would get on a similar but normally used vehicle with the same reading on the odometer.
ProTip: Police vehicles are always enhanced for the extra strain. They go for models with the most resilient engines and beefed-up components like alternators and AC compressors.
How to Get AC When Parked
If you must sit in your vehicle when it is parked, consider the following alternatives that can keep you cool without either idling your vehicle or running just the AC blower and risking draining your starter battery.
Park in the Shade and Roll Down the Windows
When waiting in the car, the most sensible thing is to find some shade and roll down the windows.
The shade will get direct sunlight off you, while the open windows will encourage a natural breeze keeping the air as close to ambient outdoor temperature as possible.
This will negate the need to run the AC or AC blower unless it is one of those painfully hot summer days.
Install Secondary AC – If It’s a Van
If you own a van or an RV, installing a secondary RV AC system that runs off a secondary battery bank is more practical.
These fans are often roof mounted and are a great solution for people who camp in their vehicles or spend hours sitting in them.
The AC system can heat or cool, giving you better control over the prevailing temperatures in the vehicle. Moreover, if you deplete the power supply, you will still have your starter battery functional and can drive around to recharge or plug to shore power to keep the party going.
Insulate the Car Better and Add Some Tint
Finally, aftermarket insulation will improve your vehicle’s interior’s resisting outside temperature changes. This will reduce how often you have to cool down the cabin actively.
A simpler insulation technique is adding some legal tint to your windows. The tint reduces how much sunlight gets into your vehicle hence direct control over how much heat the car absorbs.
If you don’t want to tint, go for window shades or blinds that you can draw up when parked and put away when moving.
Bottom Line
Cars are metal boxes. They get very hot or very cold, depending on outdoor temperatures. To counter this, they rely on the cool breeze when driving down the road or the sheer power of the AC to keep temperatures within an acceptable range.
Sitting in such a box when parked isn’t that efficient. It would be best to park and lock the car, sit at a cafe, or go window shopping when waiting for long periods. Sitting in a parked car should always be a last resort.