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

If the cooling system of your engine is not compromised and is well-maintained, you can keep your car AC on indefinitely while it is parked and the engine is running. But, leaving the AC on while the car is parked and the engine is running can be a drain on a car battery, fuel and money. It can also have an adverse effect on the environment due to the additional emissions created by the engine running idle. Moreover, leaving the AC fan on when the engine runs out of fuel and is turned off will drain all the power of your battery. You may need to jumpstart your car to restart the engine.

Leaving the air conditioner on while your car is parked and the engine is running at idle can be a tricky situation. On the one hand, it can be quite uncomfortable to get into a hot car when you’re already warm outside or waiting in a hot car for someone.

On the other hand, leaving the AC on while your car is idling can be a waste of fuel and can affect the longevity of your vehicle’s engine.

In this article, we will discuss whether leaving the AC on while the engine is idle can be harmful to your car’s AC and engine or not.

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?

A car can overheat if AC runs while the car is parked and the engine is idling. When the engine is idling, there is not enough airflow through the radiator. As a result, the coolant is not cooled down to a lower temperature, due to which the car overheats when AC runs while the engine is idling.

Moving at a certain speed lets air pass through the radiator without a fan. But at slow speeds or stops, there isn’t enough air to cool the engine. So, there are chances of engine overheating when a car is stationary and the AC is on.

In an engine cooling system, the following components play an important role:

  • Water pump
  • Radiator fan
  • Thermostat

The water pump is responsible for circulating the coolant through the radiator. The water pump is run by a serpentine belt that gets power from the engine’s crankshaft. As you drive the vehicle and increase the speed, the speed of the water pump also increases to maintain the engine’s operating temperature.

If the water pump has problems like a damaged impeller or the pulley/belt is damaged, it will not be able to produce sufficient coolant flow while the engine is idling. A water pump can become bad due to corrosion on its impeller or damaged bearings.

The radiator fan has to run at a higher speed while the engine is idling because of lesser airflow over the radiator when a car is parked.

In that case, you should check if there is dirt and debris on the front of the radiator that is blocking the airflow. If the radiator is gummed up, it will not allow sufficient air to flow through it when the engine is idling.

So, you should check the temperature gauge while you’re sitting in a car and the AC is on as the overheating of the engine can lead to accelerated wear and tear of its components.

How Does Leaving The Car AC Affect Fuel Economy?

If the AC is left on while the car is parked, the compressor puts extra load on the engine the engine has to work even harder to spin the compressor and maintain the RPMs, which results in additional fuel consumption.

The AC compressor is run by the AC clutch that gets power from the engine’s crankshaft through the belt.

So, when the AC is on, it puts extra strain on the engine. As a result, the engine has to produce additional power to overcome that resistance. As a result, more fuel is consumed.

Furthermore, the fuel economy of a car is measured in terms of MPG (miles per gallon). If the car is not in motion, i.e. engine is idling, it will burn fuel without covering any distance which will decrease the MPG of your car.

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

When a car is parked with the air conditioning (AC) unit on, the battery can drain faster due to the additional electrical load it experiences.

The AC unit takes a significant amount of energy to operate, and when the car is running, the alternator is able to provide enough energy to the battery to keep it charged.

When the car is parked, the alternator is not running with sufficient speed to produce enough electric power to charge the battery. Furthermore, if your engine runs out of fuel, it will stop running, due to which the AC compressor will stop as it gets power from the engine’s crankshaft.

But the AC blower will keep running as it gets power from the battery. Since the alternator also stops when the engine stops, the battery will no longer be charged. So, leaving the AC on while the car is parked will cause the battery to drain out faster.

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

Sitting or sleeping in a running car with the air conditioner on can be a safe, convenient, and comfortable way to cool off on a hot day, but there are some potential safety risks that should be considered.

A running car with windows and doors closed can trap the exhaust fumes, i.e. carbon dioxide and carbon monoxide inside, which can cause suffocation, dizziness, nausea, or headaches if you sit in a running car with air conditioner on.

However, modern cars are equipped with multiple sensors in the air conditioning system that can adjust the fan speed and direction of airflow from the vents.

Moreover, in some cars, sensors can also measure humidity inside the car’s cabin and human body temperature so that it can control air recirculation.

If the car is older and does not have these features, then it is important to take a few extra precautions. In that case, it would be suitable for you to leave the window(s) cracked down a little.

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