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Introduction to HVAC Systems

Every homeowner should have a brief introduction to HVAC systems because they are an integral part of any home.  They represent a significant investment too, and information is needed to make cost-effective decisions about your home’s heating and air conditioning system.  In this guide, an introduction to HVAC systems, we will cover basic terminology, list the most common types of HVAC systems, and look at keys to the cost of a system.


Getting familiar with the terminology is an important part of an introduction to HVAC systems. We’ve listed the terms logically rather than in alphabetical order.

HVAC: This stands for Heating, Ventilation and Air Conditioning.

Condensing unit: The main component in a central air conditioner or heat pump.  It contains a compressor that pumps refrigerant, a condensing coil and a fan, along with supporting parts.

Refrigerant: A substance with the capacity to expand and absorb a significant amount of heat and then rapidly condense and shed that heat.  Refrigerant cools a home by absorbing and removing heat, and heats a home by absorbing heat outside and releasing it inside.  Refrigerant is used in both central air conditioners and in heat pump systems.

Heat pump: A condensing unit is usually associated with central air conditioning because it uses refrigerant to pull heat out of the home, thereby cooling the home.  A heat pump contains a condensing unit that also heats the home.  The flow of refrigerant is reversed.  Heat is captured outside and released by the refrigerant indoors.

Air handler: An HVAC component installed inside a building that pushes treated (heated or cooled) through the supply ducts and pulls untreated air into the system through the return ducts.  It does not produce heat.

Efficiency: This term refers to how efficiently the HVAC equipment uses its energy source to heat or cool air.  For condensing units, the efficiency is measured in SEER (Seasonal Energy Efficiency Rating) for air conditioning and HSPF (Heating Seasonal Performance Factor) for heating. In both cases, the higher the number is, the more efficient the unit is.  For gas and oil furnaces, efficiency is measured by AFUE (Annualized Fuel Use Efficiency), and is given in a percentage.  For example, in an 80% AFUE furnace, 80% of the heat created goes into the home and 20% is wasted in the venting process.  Today’s most efficient furnaces are more than 97% efficient.

Capacity: This is sometimes referred to as the “size” of a unit and refers to how much heating or air conditioned air a unit is capable of producing.  Condensing units are measured by tons, a reference to the cooling capacity of one ton of ice.  Residential condensing units are produced with capacities from 1.5 tons to 5.0 tons.  Their heating capacity is measure in Btuh, or British thermal units per hour.  One ton of cooling capacity is equal to approximately 12,000 Btuh.  Furnaces are measured by Btuh.


Types of HVAC Systems

This introduction to HVAC systems wouldn’t be complete without a discussion of the types of HVAC systems available.

Split system: An HVAC system in which a condensing unit or heat pump is installed outside the home and a furnace or air handler is installed inside.

Mini split system: An air conditioning system in which a small condensing unit is installed outside and up to 4 small evaporator units/air handlers are installed in rooms or zones.  These are also called ductless systems because they disperse air through the evaporator unit rather than through the building’s ductwork.  They are popular installations where no duct work exists.

Packaged Unit: Unlike a split system, both the heating and air conditioning components are housed in one large cabinet, installed outside the home or building.  There are several types of packaged units.  Gas packaged units, also called gas/electric packaged units or simply gas packs, use a furnace to heat and a condensing unit to cool.  Heat pump packaged units use a condensing unit to both heat and cool.  Air conditioner packaged units use a condensing unit to cool but have no source of heat.  They are used in residential settings where an alternate form of heating is used, and in commercial buildings in warm climates.


Keys to Cost

We’ll conclude this introduction to HVAC systems with a brief discussion of the major factors that affect cost.  In order of importance, they are:

1. Efficiency:  High-efficiency HVAC systems cost more for the equipment, but they use less energy and so produce reduced utility bills.  You’ll need to decide if purchasing high-efficiency equipment makes sense in your situation based on your area’s climate and how long you intend to stay in the home.

2. Capacity: The greater the volume of air that can be treated, the more the unit will cost, all else being equal.

3. Performance features: Certain features optimize the homeowner’s ability to precisely control indoor climate.  These include a 2-stage compressor in a condensing unit, multi-stage gas valve in a furnace, and a variable-speed blower in an air handler or gas furnace.  They are attractive to those who want very even temperatures with little fluctuation, but they also allow better humidity control in all seasons and better air filtration or purification.  This is primarily because they operate on low most of the time, and so they run more consistently.  Air circulates more continuously to achieve these desirable results.



While this introduction to HVAC systems isn’t exhaustive, it will give you the information you need to understand the basics of today’s heating and air conditioning systems.  The more information you have, the more likely you are to make decisions you’ll be satisfied with in the years to come.

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