Understanding Your Car's Heating System

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A car's heating system is essential to the driver's safety and comfort. This article describes how this system works.

Submitted: January 03, 2016

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Submitted: January 03, 2016

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Few automotive systems have remained as unchanged as the heating system in a vehicle equipped with a water-cooled, internal combustion engine. The reason for its staying power is its simplicity and ability to adapt to the environment already present in this widely used platform.

As the term implies the “internal combustion” engine is a heat generator. Heat and internal parts that need lubrication are not good bedfellows. Ever since gasoline engines became the power plant of choice, the automotive industry has solved this dilemma with a water-based cooling system. Simply stated a mixture of water and coolant/antifreeze is supplied to temperature sensitive parts to keep them operating optimally.

Available heat source

The convenient by-product of this infrastructure is the ability to heat the passenger’s cabin with the heat generated by the hot water/coolant mixture. Hoses are routed from the engine block, through the firewall and into the cabin to a miniature radiator which is called a heater core. The core is enclosed in a box against which a fan called a blower motor is mounted.

The heated fluid is supplied continuously to the heater core but warm air is not allowed to come into the cabin until directed to do so. When you adjust the controls on the dashboard and call for heat, a door is opened allowing the heated air into the cabin. The blend door opens to a degree proportional to where you set the temperature adjustment. The fan speed of the blower motor is controlled similarly.

Dependable system

This system works pretty well and is no more susceptible to typical engineering flaws than other systems or parts. The switches or motors, as they are called, which power the blend doors sometimes fail, and occasionally an obstruction will inhibit the operation of the door. At one time, heater cores were made of copper and brass and would regularly fail, but that is rare since that material has been replaced with aluminum.

Lack of heat

Some symptoms of failure are of course lack of heat as well as coolant spraying on the windshield or leaking onto the passenger’s floor. There can be many causes of the first complaint ranging from a thermostat that is stuck open to a plugged up heater core. When this part is restricted the heated coolant travels around the tubes of core and its ability to radiate heat is lost. When this happens the core is quickly cooled down by the blower motor since no fresh and hot coolant is being supplied to the core.

Leakage is critical

Since the heating system like the cooling system operates in a closed loop, leaks are a sign of failure. If your windshield is sprayed with an oily film when using the defrosters, it could be that your heater core has begun to leak. Likewise, if the passenger’s floor dampens with coolant something related to the heater (core, hoses or clamps) has failed. Both of these conditions should be considered critical, since if they are ignored engine overheating could occur.

A comfortable interior space is a priority, especially in the winter. Knowing how the system works and the possible reasons why it may be underperforming can be the first step to getting them resolved efficiently and economically.


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