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Understanding HVAC Systems

Smart consumers know that understanding HVAC systems is the key to making good decisions about choosing a system or knowing when to repair one versus having it replaced.  In this guide we provide a brief overview of how heating and air conditioning work.  After reading it, you’ll be able to discuss your situation more intelligently and confidently with an HVAC contractor.  And we believe you will make more cost-effective decisions that will give you the level of energy-efficiency and home comfort you desire.

We’ll begin by discussing the major components you’ll need to know about for understanding HVAC systems.  The term HVAC outlines our approach: Heating, Ventilation and Air Condition.  We’ll look at heat sources, air conditioning sources and the role of ductwork in how HVAC systems work.

Air Conditioning

Here is an overview of how central air conditioning works to cool your home.  The condensing unit is the large cabinet that sits outside your home.  It contains a compressor, a condensing coil, a fan and other supporting parts.  Inside your home, in the furnace or air handler, you’ll find the evaporator coil.  A copper refrigerant line carries liquid refrigerant into your home and is attached to the evaporator coil. There, the refrigerant rapidly expands into a gas, absorbing heat in the process.  The result is that the coil gets very cold.

The hot gas refrigerant leaves your home via a second copper line attached to the condensing coil where the refrigerant is condensed back into a liquid, shedding heat in the process.  The condensing coil is similar to a radiator.  Heat transfers through it very efficiently and is dispersed by the condensing unit’s fan.

Back inside your home, the furnace’s blower motor starts to run, passing warm air from the house over the cold evaporator coil, cooling it significantly as the refrigerant absorbs heat out of it.  The cooled air is forced through the supply ducts into your home.  This cycle continues until your home is cooled to the level indicated on the thermostat.


Heating Components

The 3 most common heating components in use today in residential settings are the gas or oil furnace, the electric furnace, and the heat pump.

1. Gas or Oil Furnace: These heating components have certainly stood the test of time.  They are affordable, efficient and clean-burning.  Here is how they heat your home.  When the thermostat calls for heat, a draft inducer motor starts.  It is smaller than the blower motor.  Its purpose is to ensure that the furnace vent is open so that combustion gases may escape.

If the vent is open, the fuel valve will open so that oil, natural gas or propane can flow into the burner.  Oil burners and gas burners function slightly differently, but the effect is the same.  The fuel is ignited and begins to heat the air in the combustion chamber.  A fresh-air supply feeds the flame.  The combustion gases never mix with air being distributed into your home.  They pass through a heavy-gauge metal component known as a heat exchanger.  The purpose is for heat to transfer through the metal and heat the air that is being cycled through the ductwork.  The combustion gases are vented through the flue after passing through the heat exchanger. High-efficiency furnaces contain a second heat exchanger that captures even more heat before it is vented.

Once the flame is burning, the furnace’s blower motor starts.  It pushes heated air through the supply ducts into rooms or zones of your home.  At the same time, it draws unheated air into the system via the cold-air returns.  This cycle continues until the temperature in your home reaches the level set on the thermostat.


2. Electric Furnaces: These work similarly to a gas or oil furnace but they use electrically powered heating coils instead of gas or oil.  This simplifies their installation and means that no fuel lines or fuel storage tanks are required. The heating coils heat air that is dispersed by a blower through the ductwork.


3. Heat Pump: To understand how a heat pump warms your home, first read the section above on air conditioning.  Heat pumps contain a reversing valve that changes the directional flow of refrigerant.  This means that heat is captured outside your home and brought in by the refrigerant to be released inside the indoor coil and distributed through the ductwork.  Heat pumps are effective in capturing outdoor heat down to about the freezing mark, and even a bit lower.

Heat pumps most often use an air handler to distribute the heated air.  However, many heat pumps are dual fuel compatible, meaning they can be paired with a furnace instead.  That gives the homeowner the choice to switch to furnace heat when the temperature gets too low to make heat pumps effective.


Split Systems and Packaged Units

The term “split system” is used with central air conditioner and heat pump systems when the condensing unit is installed outside your home and the furnace or air handler is installed inside.

One type of split system we haven’t mentioned is known as a mini split or ductless system.  It is a form of air conditioning system that serves a single room or zone within a home or building.  A small condensing unit is installed outside.  A single evaporator unit or mini air handler is mounted in the room or zone, usually high on a wall or on the ceiling.  They do not use any ductwork.  They cool and distribute air by drawing it into the unit, cooling it and sending it back out.


The term “packaged unit” is used when all the major components of the HVAC system are packaged together and installed outside your home in one large cabinet.  There are gas packaged units that use a gas furnace for heating, heat pump packaged units in which a heat pump does double duty, and air conditioner packaged units that supply cool air only.  One duct connects to the home and supplies treated air (heated or cooled) and a second duct brings untreated air to the system.



Understanding HVAC systems is a matter of knowing how they produce treated air.  Our goal is to help you be a knowledgeable consumer who can take control of your heating and air conditioning decisions rather than feeling like you have no choice but to leave them in the hands of a contractor.  Educated consumers will more often make decisions that are in the best long-term interest!

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