HVAC stands for Heating Ventilation and Air Conditioning. Its purpose in a vehicle is to clean, cool, heat, regulate, ventilate and dehumidify the air entering the cabin, depending on the inputs of the operator as well as electronic sensors. Different systems will use different ways of controlling air flow into the cabin but they all work on the same basic principles.
The first thing of note is the recirculation flap, it determines whether air for the HVAC system is taken from outside the cabin or recirculated air from the cabin. Outside air has to pass through a cabin air filter before making its way into the rest of the HVAC system. Outside air is usually taken from under the wiper cowl. Next is the blower motor, this determines how hard to blow air into the cabin. After that air passes through the A/C evaporator. Many people are surprised to find out that all air entering the cabin goes through the A/C evaporator core. Keep in mind the evaporator core does not absorb heat/cool the air unless the compressor is activated, therefore the evaporator does not need to be bypassed when the A/C is not in use. From there the air comes to the blend door. The blend door is typically the most active door. It determines how much air passes through the heater core. The heater core has engine coolant passing through it, so it is hot any time the engine is hot. After the air has been set to the correct temperature, all that is left is to send it to the correct vents. The mode doors are in charge of this and are the last decision that needs to be made by the HVAC system.
Pieces of a Basic HVAC System
Cabin Air Filter
These filters make sure only clean air enters the HVAC system. (as well as the occupants lungs) They are a quick sell at lube shops, this is because they are usually very easy to replace. If you don’t know when the last time it was changed on your vehicle, just change it yourself. These can get disgusting but there are enough DIYs on the web that you shouldn’t need to pay someone else to change it.
Heater Blower/Blower Motor
The blower motor provides the air movement in the HVAC system. These electronic motors can have an almost infinite number of fan speeds on a system that uses PWM. (pulse width modulation) These usually pull air in through the centre and use centrifugal force to fling it outwards with only one direction for air to go.
The heater core is what warms the air up when you select a warm temperature. It is basically a mini radiator. It has coolant running through it and transfers heat from the engine coolant to the air in the HVAC system. If these fail the entire dash may need to come out to replace it. You may notice your windows fog up with the heat on, your carpets are wet or your nice beige mats are suddenly pink or green. Changing a heater core can get very pricey, but on some cars they can be done by simply reaching under the dash and removing a few screws.
Any air entering the HVAC system passes through the evaporator core. This is what cools the air when the A/C is activated. Some systems will activate the A/C to dehumidify the air, even though it will pass the air through the heater core soon after to warm the air. For more information on evaporators and A/C please feel free to visit our A/C page.
Air can hold more moisture when it is warm than when it is cold, so when air is cooled by the evaporator, the air must give up some of its moisture. This moisture condenses and builds up on the evaporator. The evaporator drain is in place to allow that condensation to drain out onto the road. If this drain becomes clogged, you may notice a sloshing water sound coming from the dash when you take a courner or accelerate/decelerate.
There is usually only 2 mode doors on a simple HVAC system to control where the heated/cooled air ends up. Typical systems have floor vents, upper vents and a defrost vent. They can also enable multiple vents at the same time. On some cable systems you can “cheat” by putting the mode knob between settings.
The blend door is not much different than the mode doors except it controls how much air passes through the heater core. The more air that passes through the heater core, the warmer the air will be at the vents. Assuming the engine is also warm.
In this setting the operator has demanded full heat. 100% of air flow is directed through the heater core by the blend door to deliver the most heat possible to the main upper vents.
In this setting the operator has selected full cold but has left the A/C off by either selecting economy mode or by not selecting A/C on. 100% of the air flow is directed around the heater core to allow filtered air at ambient temperature to enter the cabin through the main upper vents. In some cases A/C is not required to provide adequate cooling. Without the A/C compressor running the vehicle will get noticeably better fuel economy.
In this setting the operator has selected max A/C. The A/C compressor engages and the evaporator begins to absorb heat. To get the most cooling power, the HVAC system takes air from the cabin that has already been cooled once and cools it again instead of trying to cool hot ambient air.
Mode doors direct heated/cooled/dehumidified/ambient air to the vents selected by the operator. In this configuration the operator has selected full heat to the defrost and the floor vents.
Controlling the Doors
Cable style is by far the most simple as well as the most reliable system. They are usually found on older base model vehicles but some newer midrange manufacturers still use this system for its reliability. Cables are attached to your heater control knobs, which open and close the mode and blend doors to achieve the air flow desired. The blower motor, A/C compressor engagement and recirculation flap may still be electronically controlled. The easy way to tell if you have this setup is to see if you can turn your mode knob (the one that controls which vent gets airflow) all the way around. If you can’t make a 360° turn with that knob and you can hear the doors moving instantly with the engine off, it is most likely a cable system. Cable systems are generally the most reliable but they are not the most convenient. It is possible that every time you get in your vehicle, you will have to adjust the heater controls in some way. You may also have to change heater settings while driving if the outside air temperature changes. These systems usually do not have any sensors other than the sensors normally found in all A/C systems.
Vacuum actuated systems use engine vacuum to open and close the blend, mode and recirculate doors. A vacuum hose from the engine intake manifold comes through the firewall and into the cabin. Engine vacuum is redirected to the appropriate actuator to control the doors and airflow, based on the operators request. This system may still require you to make corrections to the air temperature or it may have an auto feature that will maintain a set cabin temperature. You may still need to turn the A/C on or off manually. The way to tell if you have this system is to move the mode door control knob (unless controls are fully electronic) with the engine off and listen for the doors moving. If your heater controls have knobs and you can’t hear the doors moving with the engine off, you most likely have a vacuum actuated system. One of the serious downsides to this system is the complex vacuum routing under the dash. Over time the vacuum lines can leak and cause poor idle or other drive-ability problems because now the engine is pulling in unmetered air. It can be tricky to find these leaks. If you set your heater controls to a certain setting and the engine starts to stumble, you are not going crazy! Most of the time you don’t get a nice hint like that and you end up checking the entire vacuum system.
These systems are fully electronic, and almost always have a automatic temperature regulating feature. Simply put, a stepper motor is an electronically controlled motor that can move to a fine tuned degree of rotation and hold. Some people who have this system set the controls to a preferred temperature and never touch their heater controls again. They use a series of sensors to decide at what temperature, and how hard to blow the air into the cabin. These systems usually use a PWM (pulse width modulation) variable speed heater blower to better control air flow. The mode doors can still be left up to the operator even in auto mode. The electronic stepper motors may be attached to the doors themselves or be connected by a cable or lever. One problem with this system in the past was the placement of the cabin air temperature sensor. If the sensor is near the drink holder, and you happen to buy a hot coffee, the system would blast full cold in the middle of winter. (A/C should be disabled) Of coarse the opposite would also be true, buy a refreshing cold beverage in the middle of summer heat, the system would roast you with as much heat as it can give you. This is no longer a problem on modern systems, modern systems use multiple cabin air temperature sensors and place them away from anything that may skew readings.
Cabin Air Temperature Sensors
Cabin air temperature sensors watch for any changes in cabin temperature that would require action from the HVAC system. (eg: if someone opened a door to get in or out) Like I mentioned in the previous section, if something causes these sensors too read warmer or cooler than the actual cabin temperature, it could cause your HVAC system to blast you with full hot or full cold.
Vent/Outlet Temperature Sensors
These sensors are located in the vents, just before the air enters the main cabin. These sensors tell the HVAC system what temperature as well as what vent air is coming out. This allows the system to find the exact location of the blend door to provide the exact temperature required. They also act as a diagnostic device for the system, if air is coming out of the wrong vent or if the system can’t get the desired temperature, it will set a fault code. These codes may be part of the OBD system or they may need to be extracted from the HVAC controls themselves.
Duel Zone/Quad Zone
By basically splitting the mode door in 2, and having two separate plenums, the driver can have his/her side of the vehicle at their preferred temperature and the passenger can have his/her side set to a temperature slightly different. For quad zone climate control, the manufacturer needs to run coolant lines, A/C lines, a heater core and an evaporator to the rear of the vehicle. The rear has its own blend doors to control vent temperature. These systems are always electronic stepper motor style.