Heater hose clamps like pictured can be removed easily with special hose clamp tools.

Summary: In this auto service repair article, we cover how car heaters work. Knowing how the automotive heater system is designed to work, allows the car owner or auto tech to diagnose problems more quickly and easily. Car heaters are designed much differently than home heaters, therefore diagnosing and repairing auto heaters requires different service procedures. Auto service tips and information provided in this car repair article is written by a master automotive technician that has many years in the field. The info. is provided in simple language that is easy to understand by anyone – experienced in auto service or not.

It’s always best to have a basic understanding on how a particular system works, to be able to diagnose problems easier. When a car heater blows cold air, there are several possibilities that should be considered. Read on to to learn more. Many people are unaware that there’s NOT an electric element in a car’s heater system. Heater elements are common in portable heaters used in the home – however automobiles use coolant from the car engine’s cooling system to transfer heat to the passenger compartment, through the use of a heater core. That’s why a car heater doesn’t start blowing warm air until the vehicle reaches operating temperature. The heater core looks similar to a small radiator – it has coils and fins to transfer heat to the air.  The car engine’s coolant is pumped through the heater core while the fan (also called a blower), pushes air through the heater core fins. When air blows through the heater core fins the air is warmed and in turn heats up the passenger compartment.

One of the most common causes of a car heater blowing cold air is from low coolant. As stated before, coolant is what warms the heater core – if the coolant is low, there may not be enough heat transfer from the heater core to the air to heat the passenger compartment. When it ’s cold outside, low coolant may not cause the engine to overheat right away. If it did cause the engine’s temperature gauge to read hot, the driver would be warned and low coolant would be discovered more quickly. The point is, when checking the heater be sure the radiator is full of coolant. Once the coolant is verified to be full, feel the heater hoses that go to the firewall. With the engine at normal operating temperature, BOTH of the heater hoses should be hot to the touch. If only one is hot, this indicates there is a blockage in the heater core or there is air trapped in the heater core preventing proper flow. Using a Lisle coolant funnel is best way to remove air from a car’s cooling system. If the coolant is really brown, has been neglected, or if stop leak has been used at some time in the car’s service history, the heater core could be stopped up. The blockage can sometimes be cleared  by removing the heater hoses and using a garden hose with a sprayer to flush the heater core out. If neither of the hoses are hot to the touch, there could also be a malfunctioning heater control valve, if used on the model you’re working on. Check for presence of a heater control valve by following the heater hoses back to the engine. Sometimes, a vacuum line could have a break causing there to be no vacuum to operate the valve. Also it should be noted that if this is your first winter with this car (and you’re unfamiliar with the car’s repair history), the previous owner could have by-passed the heater core due to a leak. When a heater core leaks, the passenger side carpet will become soaked with coolant. This should not be confused with an AC (condensation) water leak. The labor involved to change a heater core is usually several hours depending on the model. This is the reason some people will loop the heater hoses together with a 5/8″ union – by-passing a leaking heater core instead of repairing it properly.

After the previous steps are taken to insure the heater core is in the loop, the coolant is full and hot water is circulating in and out of the heater core, read the following regarding the HVAC (Heating, Ventilation & Air Conditioning) case. Doors within the heater case are either controlled by electric actuators, cables or vacuum motors. Adjust the heat control to both extremes while listening for movement of the blend door. With a cable operated heater door it’s easiest to hear the door thump when it closes. If the door is not operating, find the door that controls the airflow across the heater core. If there is an electric motor that controls the door, tapping on it can sometimes make it work temporarily for testing purposes. A vacuum operated motor needs vacuum to work, so using a hand-held vacuum pump for testing is usually the easiest way to check operation. If the vacuum motor does not hold vacuum, the diaphragm is leaking requiring replacement. To go deeper into diagnosis of the AC control head’s function and diagnosis, specific vehicle repair diagrams may be needed or help from technician that has experience with the specific model may be most beneficial.

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