Diagnosisng Low Heat Output Problems

The automobile has the ability to heat the passenger compartment quickly and efficiently. Superheated water coming out of the engine block is diverted to the heater core, which is able to transfer its heat to air blown through the fins and tubes that make it look similar to a radiator.

When everything is working correctly the automotive heating system is capable of supplying ample warmth, even in the coldest environments. When problems develop and driver complaints are registered of low heat output, issues can be found in two separate areas. This article will cover the operation of both and provide tips for diagnosing poor performance.

Control Panel Operation

Ultimately the request for heat is input at the climate control panel. Older types of vehicles use a manual slide and cable to control the desired air temp, whereas most modern cars will have an automatic or digital type that operates similar to household systems where a desired number in degrees is selected.

Digital control systems can be very complicated on newer automobiles. As an example, the climate module will monitor many different sources of input. Some of these include a coolant temperature sensor, outside and inside air temp sensors plus some sophisticated units have sun load sensors to compensate for the greenhouse effect on sunny days.

Whether the systems are cutting edge digital or manually operated cables from a slide bar, behind the scenes the same goals are reached, which is to output the desired air temperature. It does this by moving a flap controlling airflow across the heater core or the air conditioning evaporator. It can achieve very precise results by blending air across both heat exchanging devices. This can be compared to the hot and cold sides of a faucet used together to get warm water instead of scolding hot.

How the Heater Blend Door Works

On manual slide or rotating dial panels the left side is blue for cold and the right side is red indicating hot. As the lever is moved from blue towards the red a cable pulls the blend door to direct more air across the heater core. With an automatic climate system the cable is replaced by an electric motor. As the driver raises the temperature the motor positions the blend flap so warmer air is the result.

Car Problems that Cause Low Heat

Sometimes when cars aren’t supplying heat it has nothing to do with the controls or the blend door. Cooling system issues can also cause lack of heat. First thing to check is the fluid level in the coolant recovery tank. Low systems can’t circulate the proper amount of hot water to the heater core. A leak would be a common cause of this.

A defective water control valve can also be responsible for heat complaints. These are either manual or vacuum controlled and often mounted near the heater core in-line with the inlet supply hose. It controls the amount of superheated antifreeze that enters the core. If the valve is stuck in the open position, it can cause summertime complaints of poor air-conditioning performance.

An automotive thermostat can cause poor heater output. If a thermostat is stuck in the open position the engine will never reach the desired range of above 200 degrees F. In this situation plenty of antifreeze is making it to the core, it’s just not hot enough to make a perceivable difference to the passengers.

Diagnosing Low Heat Problems

Taking a path of logical diagnosis, most professional technicians will check antifreeze level and problems with the cooling system because malfunctions are quickly identified in this more basic system. If the engine is hot when diagnosis begins simply feeling the input hose to the heater core can give them an idea if the car is producing enough hot water.

After all checks okay with the primary system they’ll turn their attention to diagnosing problems inside the car. Since the control panel moves the blend door position with a cable or motors, mechanics will locate the flap and then operate the temperature control all the way from the cold side to the hot and inspect the movement of the temperature door. This can provide insight on whether the motors, cables and ultimately the control panel are doing their job.

When a driver is complaining of low heat output, diagnosis can take you down two completely separate paths. Problems with the cooling system can be the root cause and just as likely as malfunctions with the heater control panel or the systems they operate. Things like the blend door motors and water control valves can be manually or automatically operated as an output from the climate control system. As with all automotive diagnosis following a logical path brings you to the solution efficiently.