Leaving AC On While Car Is Parked And Engine Is Running?

If the cooling system of your car engine is working well and you take good care of it, you can keep the air conditioner on while your car is parked and the engine is running too. But leaving the AC on in a parked car with the engine running can drain the battery. It uses more fuel and costs more money. It also puts more pollution into the air from the engine running at idle. Also, if the engine runs out of gas and stops while the AC fan is still on, it will use up all the power in the battery. You might need someone to jump start your car to get the engine going again.

Leaving the air conditioner on while your car is parked and the engine is running at idle can be tricky. It can feel very uncomfortable to get into a hot car when you’re already warm outside. Or you may be waiting in a hot car for someone.

But leaving the AC running while your car sits there idling can waste fuel. It can also affect how long your car’s engine lasts.

In this article, we’ll talk about whether it can damage your car’s AC and engine to leave the AC on while the engine idles.

Some Key Insights for You
  • Leaving car AC on while parked engine running is ok for 2 hours max in modern cars, 30 mins for old cars.
  • Car can overheat when AC is on and it’s parked because not enough airflow to cool radiator. Check for problems with water pump, radiator fan, thermostat.
  • AC puts extra load on the engine so burns more fuel when parked idling. No mileage covered, hurts MPG.
  • The battery drains faster because the alternator not charging with sufficient power at idle. AC blower keeps running even if the engine stops.
  • Can be unsafe if exhaust fumes get trapped inside with AC on. Crack window open for ventilation.
  • Modern cars have sensors to adjust fan speed, airflow direction, etc to avoid suffocation.

How Long Can I Keep My Car Ac On While My Engine Is Running But Car Is Parked?

You can park your car with air conditioner on while your engine is running for up to 2 hours if your car has a modern climate control system. For old cars with manual AC, you can park your car with ah AC on and engine running for up to 30 minutes.

The climate control system is well-equipped with several sensors that intelligently control the fan speed and engagement of the AC compressor clutch so that they do not overload the engine.

Also, modern cars also have a remote start feature that can start the engine from a key fob or a mobile phone application. With remote start, the climate control system/AC also runs.

By default, remote start in cars only works for 5 to 10 minutes, and it can be extended up to a maximum of 35 minutes. So, when the remote start stops, AC also stops. After that, you have to start the engine manually.

The ambient temperature outside will also play a role. If it is hot outside, the car AC will have to work harder to cool the car and will eventually consume more fuel if left running for too long.

Can Car Overheat If AC Runs While the Car Is Parked?

The engine can get too hot if the air conditioner (AC) runs while the car is parked. When parked, the engine runs but does not move fast. So there is no airflow through the radiator.

The radiator needs airflow to cool the engine. At normal speeds, air moves through it without needing a fan. But stopped or slow, there is no airflow. So the engine can overheat with the AC on.

Three parts are key for engine cooling:

  • Water pump
  • Radiator fan
  • Thermostat

The water pump circulates the coolant to the radiator. It connects to the engine through a serpentine belt. As you drive faster, the water pump goes faster to keep the right temp.

If the water pump is damaged, it won’t pump enough coolant when idling. The impeller inside can wear out. Or the pulley and belt can break. Then it cannot cool an idling engine.

The radiator fan must spin faster at idle since air moves slower through the radiator. Check if dirt blocks the airflow in front. A clogged radiator won’t cool an idling engine. Watch the temperature gauge when idling with AC on. Overheating causes faster wear on engine parts.

How Does Leaving The Car AC Affect Fuel Economy?

The air conditioner (AC) compressor runs using power from the engine’s crankshaft. So when you leave the AC on with the car parked, the compressor puts extra strain on the idling engine.

As a result, the engine has to work harder to spin that AC compressor and maintain a steady idle speed. This extra effort causes more fuel consumption.

You see, the AC compressor connects to the engine via a belt that goes to the crankshaft. So when turned on, the AC makes the engine work against some resistance from that spinning compressor.

More resistance means the engine has to generate more power to overcome it. And more power requires more fuel.

Furthermore, a car’s fuel economy is measured in miles per gallon (MPG). When parked with the engine idling, a car burns fuel without covering any distance. This brings down the overall MPG rating.

If your AC compressor is short cycling, you can read my guide on AC clutch engages intermittently.

How Can Battery Drain Faster When A Car AC Is On While It Is Parked?

The air conditioning (AC) takes a lot of energy to run. When your car engine is on, the alternator makes enough power to keep charging the battery, even with the AC running.

But when parked with the engine off, the alternator stops, so the battery has to supply all the energy for the AC. With no charging, the battery will drain out faster than normal.

The AC compressor is powered by the engine through the crankshaft pulley. So if your engine runs out of gas and stops, the compressor will stop too. But the AC blower fan keeps running off the battery. With maximum drain and no charging, your battery will lose charge quickly if you leave the AC on in a parked car.

Is It Safe To Sit In A Running Car With The Air Conditioner On?

Resting in a running car on a hot day can be pleasant with the cool air from the air conditioner. But there are a few risks to think about for safety.

With closed windows and doors, the exhaust gases containing carbon dioxide and carbon monoxide have nowhere to go. Breathing these gases for too long causes headaches, dizziness, nausea and even suffocation

Luckily, new cars have sensors to measure things like humidity, airflow, and body heat. The sensors tell the air conditioner to adjust the fan speed, air recirculation and vent direction automatically for comfort and safety.

If your car is older and missing those helpful sensors, take precautions. Leaving the window(s) cracked open allows dangerous gases to escape, keeping the air inside fresh.

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