Question about 1998 Chevrolet Blazer
It sounds like you either have bad controls or stuck dampers in your vents. The stuck dampers are usually caused by plant debris getting in to the air system.
Posted on Jun 07, 2010
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Posted on Jan 02, 2017
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Keep this principle in mind: Heat, flows INTO cold... When you hold an ice cube in your hand, your hand gets cold because the heat from your hand is flowing INTO the clod ice cube, NOT because the cold is flowing into your hand.
On this particular vehicle (like many modern vehicles), hot water from the engine flows through the heater core at all times, even when you have the A/C on or when you have nothing (A/C, heat or vent) on. The system uses a door that closes down over the air opening to the heater core to keep you from feeling warm/hot air when it is not desired. The reason(s) you feel that the vent air is warm even when the outside temperature is somewhat cool is/could be caused by the following:
First, although as mentioned above, the heater core is restricted from receiving air when you are not commanding heat by the control panel, the heater core ultimately ends up as warm as the engine temperature. This is generally somewhere around 200 degrees F. Because all the air coming out of your vent(s) must pass over the door that restricts air over the heater core, and because the heater core gets so hot, some of that heat is picked up by air passing over the hot door even though it is closed.
Second, the door that restricts airflow from passing over the heater core has a seal around it. After a period of time this rubber (or usually felt) seal deteriorates, allowing some air to actually pass over the heater core. Since the core is hot all the time, some of the air gets heat loaded, producing warmed air.
Third, the door is operated by (in the case of your vehicle) an electric motor which responds gradually/proportionately to the setting you select on the temperature dial. If the motor gets out of calibration (can happen from time to time), if the control head has a 'glitch' and sends an incorrect signal (chronically, usually), or if the electric motor (called an actuator and contains its own little electronic 'brain') becomes faulty, the door may not be closing completely. This symptom can also be caused by a problem with the door itself, where the door gets stuck due to warping of the plastic case, a foreign object preventing it from closing (usually a pen or toy dropped into a vent or defrost opening), or a broken or cracked door hinge.
Last, the vent intake opening, where the 'fresh' air comes into the car on the 'vent' setting, is located just below the windshield on the passenger side. This opening receives its air directly off of the sun-baked, engine-heated hood and because of this the air is never as cool as outside air even on a cooler night.
I hope this answers your question. Many domestic vehicle manufacturers have begun installing heater control valves on their vehicles once again as they nearly all used to have. Note that the fix for most everything I have mentioned here requires removal of the dash of the vehicle or replacement of the control head and therefore is in many cases not worth the investment if it is not an extreme problem (for example, unless the system is stuck on full heat in the middle of summer. Additionally, remember that, if you so desire, manual shutoff valves can be placed in the heater hose lines (please consult a qualified mechanic as water from a vehicle can and will scald and burn you!), allowing you to manually shut off the flow of water into the heater core, thus reducing some of the heat (Readers note that certain foreign vehicles REQUIRE water flow through the heater core at all times for engine cooling and are not candidates for this procedure!).
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