HVAC systems are as critical to your home as your roof for making your home safe and comfortable. Keeping it running efficiently and effectively takes a little know-how and at least being familiar with how the system runs. Discover everything you need to know about the critical components of your HVAC system and what they do.


The thermostat is the one component most people are familiar enough with to at least set their desired temperature. To have the thermostat work properly, make sure that it’s installed on an interior wall that doesn’t receive direct sunlight and isn’t directly above a supply vent. Also, consider that you’ll get the most from it when you use programs to reduce the demand on your system when you’re not home.

Circulating Fan

Every central air system has a single air handler that’s responsible for moving air through the system. It moves this air through a single circulating fan that runs for both air conditioning and heating. To continue operating effectively, it needs periodic cleaning and balancing, which is a common part of routine HVAC maintenance.

Air Filter

Every system also has an air filter to help keep airborne contaminants out and prevent airflow restrictions. To continue working well, it’ll need periodic replacing for central systems or cleaning for mini-split systems. For central systems, plan to check the filter every month with the intent of finding how quickly it clogs. Chances are you won’t have to replace it every month, but you want to prevent it from running long periods where it isn’t passing sufficient air.

Air Ducts and Vents

Air ducts direct air through your system and home, with vents at the ends of those ducts. Supply ducts send conditioned air throughout your home, while return ducts bring air into the system for it to condition. Your vents need at least 2 inches cleared around them to allow conditioned air to circulate evenly around your home.

Evaporator & Condensing Coils

An air conditioner or heat pump only works because of the coils inside and outside your home. Standard air conditioners have an evaporator coil inside your home, which absorbs heat from the circulating air into the refrigerant. The outside unit has a condensing coil, which transfers heat to the air outside. With heat pumps, the mode determines which coil acts as the evaporator or condensing coil.


Heat pumps and air conditioners both require a compressor to make them work properly. When the refrigerant moves to the condensing coil, it passes through the compressor first. This does exactly what it says, compressing the refrigerant, which increases the pressure, thereby raising the temperature of the refrigerant, allowing it to transfer more heat to the air moving over the condensing coil. To work properly, the compressor needs the proper amount of refrigerant in the system.

Expansion Valve

Like a compressor helps the refrigerant to transfer heat, the expansion valve helps it to absorb heat. Refrigerant gets colder when it expands, reducing its pressure. The expansion valve restricts the amount of refrigerant going to the evaporator coil, allowing the refrigerant to expand, which reduces the pressure and lowers the temperature.

Reversing Valve

The reversing valve is a special component in heat pumps that allows them to offer both cooling and heating functions. The valve itself is a slide that moves when the reversing solenoid is activated. In the heating mode, the valve directs the high-pressure refrigerant to the coil inside your home. In the cooling mode, it moves to direct the high-pressure refrigerant to the outside coil. The heat pump will also change the orientation of the valve when it’s in defrost mode, moving high-pressure refrigerant through the outside coil to prevent it from icing when the coils get too cold.

Condensing Fan

With both air conditioners and heat pumps, the entire process relies on air moving over the coils. Inside, the circulating fan draws air into the system and over the coil. The outside unit relies on air movement just as much, which is controlled by a fan. In air conditioners, this fan is called the condensing fan or condensing unit fan.

For the fan to draw air effectively through the outside coils, there must be enough clearance around the unit. The industry standard recommendation is to keep at least a 2-foot clearance around the unit, including above it.

Ductless Mini-Split Air Handler

If you have a ductless mini-split heat pump, you won’t have a central air handler. Rather, you have several wall-mounted units around your home, which are smaller air handlers. Each one is complete with a thermostat, refrigerant coil, and circulating fan.

Gas Burner and Ignitor

Forced air gas furnaces have several important components, starting with the ignitor and gas burner. Depending on your model, you may have a hot surface ignitor, an electric spark ignitor, an intermittent pilot light, or a standing pilot light. It’s important to know which of these your unit has so that you know what you’re looking for if there’s a problem with the unit lighting.

The ignitor lights the gas burners, which produce heat for your system. Both the ignitor and burners need periodic cleaning, testing, and adjusting. This is commonly included with routine professional maintenance.

Heat Exchanger

The heat exchanger is a series of tubes that conduct the hot exhaust to the flue pipe. The circulating air flows over the heat exchanger and absorbs the heat from the metal tubes. The biggest concern with heat exchangers is a crack that allows carbon monoxide to leak into your home.

Inducer Fan

Some furnace models have a special fan that helps with the ignition process called the inducer fan. This is usually located near the top of the unit where the flue connects to the heat exchanger. The purpose is to draw air through the combustion chamber and the heat exchanger before there’s enough heat to create a natural draft. Not all furnaces have an inducer fan, but they are especially prevalent in high-efficiency furnaces.

High Limit Switch

All HVAC equipment comes complete with various safety measures, especially gas furnaces. Aside from excessive gas buildup in the system, one of the most serious risks is a cracked heat exchanger, which is often caused by an overheated system. The high-limit switch, also called the high-temperature limit switch or the furnace plenum thermostat controls the heat in the combustion chamber. This particular switch cuts the gas supply to the furnace when the temperature of the burn chamber exceeds the safe operating range. It can also become faulty, causing the furnace to short cycle, which is why it’s tested during routine furnace maintenance.

People around San Diego have turned to Friar's Heating and Air to keep their HVAC systems running reliably since 1989. Our expert technicians provide heating and air conditioning installation, repair, and maintenance combined with indoor air quality and solar energy solutions. Call to schedule your HVAC repair or replacement consultation with one of our trusted technicians today.

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