The Heater in My Car is Not Hot - How Does The Heater Work and What Could be Wrong?


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Reader Question: The heater in my car is not working. The mechanic told me my heater core is leaking and needs to be replaced. The price he quoted me is not in my budget right now, do I have another option? Bill

Bill I feel for you, the heater in your car (like the air conditioner) is a wonderful comfort item, and it can be very costly to repair if and when it fails. Before we get into repairing it, let me explain briefly what the heater core does.

The heater core works in conjunction with your engine's cooling system. The function of the cooling system is to remove heat from the engine, and it does this for the most part by sending the heated anti-freeze to the radiator located in the front of the car. The position of the radiator allows outside air to blow across the radiator thus cooling the anti-freeze.

The anti-freeze is then sent back to the engine. Hot anti-freeze is circulated throughout the cooling system by the radiator and heater hoses. Think of the heater core as a small radiator located inside the dashboard of your vehicle. Anti-freeze is constantly being circulated throughout the engine, radiator, and yes even the heater core regardless of whether you have the heater dashboard switch on.

When the heater is turned on by the driver, a diverter door opens to the heater core area in the dashboard. A small fan (commonly referred to as the a/c fan or blower) blows air across the hot heater core into the duct work of the dashboard and into the interior of your car.

When the heater core leaks (it leaks anti-freeze of course) it will usually leak inside the car on the passenger floorboard under the carpet. If your heater isn't working properly, or if you smell a sweet odor, investigate the passenger-side floorboard for signs of anti-freeze leakage. A leaking heater core may also cause a greasy film on the inside of the windows.

So what causes the heater core to leak in the first place? Usually the main culprit is rust build up caused from lack of cooling system flushes. Anti-freeze acts as a lubricant and rust inhibitor as well as a temperature controlling substance. Anti-freeze should be flushed and replaced periodically to keep the cooling system in good working order.

Rusty anti-freeze is usually a sign of a coolant leak somewhere in the cooling system that has allowed air to enter the system. It may be a leaky radiator hose, water pump, heater core, radiator, etc. Rust build up can be just as damaging to the radiator and other internal engine parts.

How do you stop the anti-freeze from leaking onto the floor board? Obviously you can replace the leaking heater core itself, which is the correct fix and is what your mechanic recommended. You can also try a can of radiator stop leak additive to see if it will patch the hole (this might be an acceptable temporary repair), or you can cut off the flow of anti-freeze to the heater core all together.

There are two heater hoses that are attached to the heater core from under the hood. These hoses can be cut and blocked off with a hose clamp, or a small hose splice can be inserted between the two hoses to create a loop thus avoiding the heater core all together. This works great to temporarily repair a leaking heater in the summer time.

Another cause of a heater that is not leaking but is not heating properly can be a faulty thermostat. The thermostat is calibrated to keep the anti-freeze inside the engine at a constant temperature. A faulty thermostat might not allow the engine temperature to get hot enough to heat the anti-freeze.

Thermostats generally do not need periodic replacement unless there is an under- or over-heating situation. When replacing the thermostat, make sure to install the proper heat range recommended by the manufacturer. Proper engine temperature plays a vital role in fuel economy and overall running condition of the engine, as well as regulating cooling system temperature.

Low anti-freeze levels or poor circulation of anti-freeze throughout the cooling system will hamper heater performance as well. If there is not enough hot anti-freeze to circulate and deliver to the heater core, heater efficiency will be greatly reduced. If you suspect a problem with your heater, first check the anti-freeze level in the radiator and make sure the fluid is in good shape and of proper color (either green or orange/red if you're using one of the new extended life products on the market).

To check anti-freeze color, dip some out and look at it in a glass container. (We use an anti-freeze hydrometer, which is basically an expensive turkey baster. ) When you are looking at the anti-freeze in the radiator, you can only see the top surface color, and it will usually look okay even if it's not.

If low or contaminated fluid is not the problem, feel the two heater hoses going from the engine to the heater to make sure they are hot (the engine needs to be at normal operating temperature). If the heater hoses are not hot, inspect the radiator hoses for internal cooling system blockage.

With the engine at normal operating temperature, the upper radiator hose should be very hot, and the lower radiator hose should be just slightly less hot. If there is a substantial temperature variation, a blockage or restriction is probably present. A faulty thermostat and a radiator or heater core that is full of calcium deposits or rust build up are a few causes of cooling system restrictions.

Learn more about Car Repair and see the images associated with this article about Car Heater Not Hot Fix your car yourself with this Online Auto Repair Manual


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