Car heating system: how it works (2023)

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March 15, 2020

How the heating system works in a car with an internal combustion engine: The heating system is connected to the engine cooling system and uses heat from the engine to warm up the cabin.

Car heating system: how it works (1)Car heating system diagram.

(Video) How the Heater Inside the Car Works

There is a heat exchanger called a heater core built inside the vehicle HVAC system (see the diagram). The heater core is a small 'radiator' that is connected to the engine cooling system.

The cooling system of an engine is filled with liquid coolant that circulates between the engine and the main vehicle radiator. The part called a water pump creates the flow of coolant within the system. The thermostat valve blocks the circulation through the main radiator when the engine is cold; it opens when the engine reaches the specified temperature. See the photo of the water pump and thermostat. Read more about the water pump.

As the engine is warming up, hot coolant starts flowing through the heat exchanger (heater core).

Car heating system: how it works (2) Air flow in the HVAC system of a car.

(Video) How the AC and Heat Work in your Car

The blower motor of the HVAC system creates the air flow. The air flows through the A/C condencer and then through the heater core; see the illustration. Various ducts and doors inside the HVAC system distribute the warm air to the vents. The door that controls the mix of warm and cold air is referred to as a the temperature blend door. Doors that direct the air flow to different vents are called mode doors. Most cars have electric door actuators that opend and close air doors as needed. Read more: Blend door actuator: how it works, when to replace?

If the air blowing from the vents is not hot enough, the first thing to check is the coolant level in the overflow bottle (see the owner's manual for your car).

Car heating system: how it works (3) Normally, the coolant level should be between Min and Max marks in the coolant expansion bottle.

If it's low, the coolant must be topped up and the cooling system must be checked for possible leaks.

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If you want to top up the coolant yourself, don't open the cooling system cap when the engine is fully warmed up, as the coolant is very hot and can spray out under pressure. Check the precautions and instructions on how to do it in the owner's manual. Dealership mechanics typically top up the coolant in the overflow bottle during regular oil changes.

The coolant density also needs to be measured.

Car heating system: how it works (4) Checking coolant density.

If the density is not within specs or the coolant appears to be dirty in the expansion bottle, your mechanic may recommend flushing the cooling system and the heater core, as the heater core could be partially clogged up. This problem is not uncommon in older high-mileage cars. Flushing a cooling system in a car costs from $140 to $250.

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A thermostat that is stuck open can also cause long warm up times, however, in most cars, the Check Engine light comes on if the thermostat is stuck open. Another symptom of a stuck-open thermostat is when the engine temperature drops below normal when driving on a highway.

In some cars, if the temperature blend door inside the HVAC system is not functioning correctly, it can also cause a lack of heat from the vents. Often this problem is caused by a bad blend door actuator or, in case of manual HVAC system, worn parts of the cable/link mechanism. In a system with manual heater controls, it may feel like the hot/cold knob is jamming and not moving all the way to "Hot".

(Video) How Car Cooling System Works

If there is a lack of air flow from the vents even at full blower motor speed, the cabin filter must be checked. A dirty cabin filter can restrict the air flow in the HVAC system. Normally, when the temperature is set to "Hot" and the engine is fully warmeed up and running, the air from the vents should be very warm regardless of the vehicle speed. If the air from the vents is hot when driving but becomes cold when the vehicle is stopped, suspect a lack of coolant flow through the heater core or air trapped within the engine cooling system.


How does the car heating system work? ›

The warmth from the engine goes from the radiator to the heater core, which basically acts as a heat exchanger. It allows coolant to flow through, and this flow of coolant is regulated by the heater control valve. As the engine's heat is carried by coolant into the heater core, the device starts to get warm.

Why is my car heater not getting hot enough? ›

Faulty heater cores and low or contaminated coolant levels often come hand in hand, and both issues may lead to your engine overheating and your heater not blowing hot air.

Why is my car not heating up fully? ›

Low coolant: Whether caused by a leak or water evaporation, low coolant is the most common source of poor heater output. Thermostat: A stuck-open thermostat prevents the engine (and coolant) from heating up. Heater core: A clogged heater core restricts coolant flow, giving up little or no heat.

What are the main parts of a heater system in a car? ›

There are a few basic components to the heating system; the heater core, blower motor/fan, heater hoses, heater control valve, and the HVAC (heating, ventilation, air conditioning) control panel inside the cabin.

How long should a car heater take to warm up? ›

Most older cars can require several minutes or so to come up to operating temperature and for the heater to start working. However, some newer cars require only a minute or two.

What temperature should my car heater be? ›

Check the temperature with a thermometer inserted into the driver's and passenger's registers. Although many vehicles deliver a toasty 135° to 155°, 115° to 120° is acceptable. If the air temperature is okay but airflow is weak, the problem is the blower or a dashboard ductwork problem.

How can I improve the heat in my car? ›

How to Get the Most Out of Your Car's Heater
  1. Get the Car Moving. ...
  2. Don't Crank the Heat to 'High' ...
  3. Keep the A/C On. ...
  4. Don't Use the Recirculation Mode. ...
  5. Crack a Window. ...
  6. Help Backseat Passengers. ...
  7. Keep It as Warm as You Like.
Nov 27, 2021

How can I make my car heater stronger? ›

Keep the heater on low or medium settings until you've been driving for a few minutes. Use a high setting with a lower heat. Warming up the whole car is more effective than blasting hot air in one area. If you keep a lower heat setting with a higher fan setting, you'll spread warm air all over the car!

How can I make my car heater heat up faster? ›


Yes, that may seem backward but that's how a cars heating/cooling system works. Then, after driving for a bit, turn it to hot and turn on the fan. It'll get hotter faster this way, than if you were to get in and turn it to hot with fans on full.

Why does my car take forever to heat up? ›

Inspect Your Thermostat

If your coolant is at a good level, but you still feel that your vehicle is taking too long to warm up, there could be an issue with your thermostat. Your vehicle's thermostat is a valve located between the engine and radiator.

Why is my car blowing out cold air when the heat is on? ›

Low Coolant Level

The coolant (usually a mixture of water and antifreeze) in your engine doesn't just work to keep the engine from overheating: It's also the source of heat supporting the heating system. If there isn't enough coolant in your system, the heater core will blow cold air into your car.

What controls the heater in a car? ›

The blower motor blows air through the heater core and to the passenger compartment inside of your automobile. The heater control valve is the device that controls the flow of the hot engine coolant through the heater core. It is usually located in one of the heater hoses to regulate this flow.

What are two signs that the heater core has failed? ›

First, the heater core could be malfunctioning and, as such, the defrosters aren't working. Second, the core could be blowing fog/smoke into your vehicle's cabin. Both are bad signs and need to be checked into right away.

What are the 3 parts of a heating system? ›

Your home heating system consists of three key parts: the heat source, the air circulation system, and the thermostat. Heat source – The heat source is most often a furnace or heat pump.

How many times should I warm up my car? ›

The practice of warming up your vehicle in these cold weather conditions varies from person to person. To some, 20 minutes seems excessive. To others, five minutes isn't enough. But, according to experts, for a newer model vehicle with the proper winter oil, 30 seconds may be all you need.

Does revving your engine make your car heat up faster? ›

Revving your engine will not make it warm up any faster, but it will subject your engine to increased stress before the oil has had the chance to circulate properly. You should also avoid accelerating too quickly until the engine has fully warmed up.

How long do you let your car heat up in the winter? ›

Even the U.S. Department of Energy notes that guidance from most car manufacturers says your vehicle is ready to drive after just 30 seconds of warming up.

Is 72 degrees good for heater? ›

If someone is at home in the daytime, 72° F (22° C) is a good start, but aim for 68° F (20° C). If everyone is away from home in the daytime, or you're asleep at night, we feel 66° F (19° C) to 62° F (17° C) is best.

What is the most efficient heater temperature? ›

Keep the internal temperature of your heating set to between 18°C and 20°C. Every degree you increase your heating can add up to 10% on your energy use. Set your thermostat to warm your house for times you need it. Turn it off overnight and when you're away from home.

How can you tell if a heater control valve is clogged? ›

Below are the signs of a bad heater control valve:
  1. No heat coming out.
  2. Heat always on and cannot be turned down.
  3. Heat output changes with no changes in the heat setting.
  4. Temperature gauge reads hot.
  5. Low coolant level.
  6. Coolant leak.

How do you unclog a heater core? ›

To clean the blockage, you need to flush water or air through the heater core. By doing so, the grime and dirt that's causing the clog will come out through the inlet hose. Now, insert a water hose into the outlet hose. If you have access to an air compressor, you can use that to push out the coolant and grime.

What causes cold air to come out of vents? ›

Clogged air filters could be causing cold air to blow out of your vents. A dirty air filter can block airflow over your furnace's heat exchanger, which can cause it to overheat.

Does a radiator flush clean the heater core? ›

After they will drain the system once more. Then they will wash out the area near the water pump, where there are a lot of little seals that, if cracked, can cause leaks and add even more debris to your cooling system. After another drain, your mechanic will wash out the heater core.

What happens when heater core is clogged? ›

While your vehicle's heating system needs coolant to warm the interior, the engine needs coolant to keep it from overheating. If there is a clogged heater core, it will keep the coolant from flowing through the engine, causing it to overheat.

How do you know if your thermostat is bad in your car? ›

  1. Overheating And Overcooling. Overheating is the most common symptom of a failing thermostat. ...
  2. Coolant Leaking. If you have experienced problems with overheating or notice liquid dripping under your car, it is a sure sign of a coolant leak. ...
  3. Strange Sounds And Temperature Changes. ...
  4. Heater Problems.

Does thermostat affect heater in car? ›

Car Heater Thermostat is Faulty

A damaged component or malfunction can get a thermostat stuck open or closed. That can cause issues with your heater working but also damage your engine's cooling system. A broken thermostat makes you uncomfortable during a trip – but it can also overheat and ruin an engine.

What controls a heating system? ›

Heating controls is a broad term covering timers, thermostats and plumbing and electronic components, which help manage when the heating should be on and what temperature your rooms should be.

Does the thermostat control the heater? ›

As the main control of your home's temperature, your thermostat allows you to communicate with your heating and cooling system.

How do you diagnose a heating problem? ›

What are signs of an engine overheating?
  1. Steam (often looking like smoke) coming from the front of the vehicle under the hood.
  2. The engine temperature gauge in the dashboard or driver console spiking to “H” or moving into the red area of the gauge.

How do you diagnose a heater problem? ›

Heater Not Working? 7 Troubleshooting Tips:
  1. Check that your thermostat is set to “heat.” ...
  2. Change the filter. ...
  3. Make sure the gas is on. ...
  4. Clear the chimney exhaust flue. ...
  5. Clean away leaves and debris from exhaust vents. ...
  6. Flush out the drain lines. ...
  7. Check for blocked ducts restricting airflow.
Nov 16, 2016

What are 2 types of heating systems? ›

5 Different Types of Heating Systems
  • Forced Air Systems. A forced air heating system is the most common option that is found in a residential home. ...
  • Electric Systems. ...
  • Geothermal Systems. ...
  • Radiant Heat Systems. ...
  • Steam Radiant Heat Systems.
Jan 8, 2015

What are the 3 processes that needs heat? ›

Principles of Heat Transfer

Heat is transferred to and from objects -- such as you and your home -- through three processes: conduction, radiation, and convection.

What are the four types of heating systems? ›

Four Types of Heating Systems
  • warm-air heating system;
  • hydronic heating system;
  • steam heating system; or.
  • electric heating system.

Do you turn on AC for heater in car? ›

Therefore, the AC should always be on in most cars (even in the winter). Most cars are actually DESIGNED for the A/C and heater to run together. When you run the A/C and heater simultaneously the A/C compressor is energized and the evaporator coil (under the dash) removes moisture from the air.

Do you burn more gas when the heat's on in your car? ›

So we can verify, YES, using your car heater is hurting your gas mileage but not by too much.

Why is my car blowing cold air when the heat is on? ›

Low Coolant Level

The coolant (usually a mixture of water and antifreeze) in your engine doesn't just work to keep the engine from overheating: It's also the source of heat supporting the heating system. If there isn't enough coolant in your system, the heater core will blow cold air into your car.

Does a car heating system use fuel? ›

Car heating is free

Though many people don't know it, turning on the heat does not increase the car's consumption of petrol. Heat is obtained from the energy generated by the engine itself, and so it is enough to have it turned on to provide heat.

What controls car heater? ›

The blower motor blows air through the heater core and to the passenger compartment inside of your automobile. The heater control valve is the device that controls the flow of the hot engine coolant through the heater core. It is usually located in one of the heater hoses to regulate this flow.

Does heat run better with AC car? ›

Running the air conditioner in a car during the winter months has many different benefits. Those who do so will be able to more efficiently control the temperature and moisture in the car. Plus, they can also protect the cooling system and possibly avoid high repair costs.

How can I make my car heat hotter? ›

The heater core is a place to start, make sure it's up to temp and free flowing. Then make sure the transfer doors and it linkage are actually directing hot air correctly. A partially blocked heater core is very common after years of operation. Try to change your thermostat with a hotter running one.


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2. How a Car's HVAC System Works
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6. Why Doesn't My Car Have Heat?
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