Central Heating Ventilation and Air-Conditioning
(HVAC) Systems

First things first. What does HVAC mean?

While the Energy Center usually tries to avoid the use of acronyms, HVAC is in common use in the heating and cooling industry. It stands for "heating, ventilation and air conditioning," three functions often combined into one system in today's modern homes and buildings. Warmed or cooled or dehumidified air flows through a series of tubes - called ducts - to be distributed to all the rooms of your house. A central HVAC system is the most quiet and convenient way to cool an entire home.

Unless you live in an amazingly temperate climate, the HVAC system in your home uses more energy and drains more energy dollars than any other system in your home. Typically, 44 percent of your utility bill goes for heating and cooling.

Like many other appliances, HVAC systems have improved in energy efficiency over the years. As a result, you can save money and increase your comfort by properly maintaining and upgrading your HVAC equipment.

Another development is the whole house approach to heating and cooling. Coupled with an energy efficient furnace, heat pump or air-conditioner, the whole house approach can have a great impact on your energy bills. By combining proper equipment maintenance and upgrades with appropriate insulation, weatherization and thermostat settings - properly regulated with a programmable thermostat, of course - you may be able to cut your energy bills in half.

All major appliances including gas furnaces, boilers, air conditioners and heat pumps sold in California meet the Title-24 energy efficiency standards. If you are thinking about purchasing a new central furnace, check the ENERGY STAR® database, which uses information supplied by the California Energy Commission. It displays information on most energy efficient appliances in a consumer-friendly, easy-to-use fashion.

Central Air Conditioning

Central air conditioning units are usually matched with a gas or oil furnace to provide heat through the same set of ducts.

There are also central HVAC units called heat pumps that combine both the heating and cooling functions. If you heat your home with electricity, a heat pump system is the most efficient unit to use in moderate climates. It can provide up to three times more heating than the equivalent amount of electrical energy it consumes. A heat pump can trim the amount of electricity you use for heating as much as 30 percent to 40 percent.

Even though air conditioners and heat pumps require the use of some different components, they both operate on the same basic principles.

How They Work

Heat pumps and most central air conditioners are called "split systems" because there is an outdoor unit (called a condenser) and an indoor unit (an evaporator coil). The job of the heat pump or air conditioner is to transport heat from one of these units to the other. In the summer, for example, the system extracts heat from indoor air and transfers it outside, leaving cooled indoor air to be recirculated through your ducts by a fan.

A substance called a refrigerant carries the heat from one area to another. Basically, here's how it works:

The compressor in your outdoor unit will change the gaseous refrigerant into a high temperature, high-pressure gas. As that gas flows through the outdoor coil, it loses heat. That makes the refrigerant condense into a high temperature, high pressure liquid that flows through copper tubing into the evaporator coil located in your fan coil unit or attached to your furnace.

At that point, the liquid refrigerant is allowed to expand, turning the liquid refrigerant into a low temperature, low pressure gas. The gas then absorbs heat from the air circulating in your home's ductwork, leaving it full of cooler air to be distributed throughout the house. Meanwhile, the low temperature, low pressure refrigerant gas returns to the compressor to begin the cycle all over again.

While your air conditioner or heat pump cools the air, it also dehumidifies it. That's because warm air passing over the indoor evaporator coil cannot hold as much moisture as it carried at a higher temperature, before it was cooled. The extra moisture condenses on the outside of the coils and is carried away through a drain. The process is similar to what happens on a hot, humid day, when condensed moisture beads up on the outside of a glass of cold lemonade.

The same process works in reverse in a heat pump during the winter. The heat pump takes heat out of the outside air - or out of the ground, if you have a geothermal heat pump - and it moves that heat inside, where it is transferred from the evaporator coil to the air circulating through your home.

That's not a typographical error, by the way- the heat pump moves heat from outside to warm your home, even on a cold day. That's because "cold" is a relative term. Air as cold as 30 degrees still contains a great deal of heat - the temperature at which air no longer carries any heat is well below -200 degrees Fahrenheit. A heat pump's heat exchanger can squeeze heat out of cold air, then transfer that heat into your home with the help of a fan which circulates the warm air through your ducts.

Heat pumps are often installed with back-up electric resistance heat or a furnace to handle heating requirements when more heat is needed than the heat pump can efficiently extract from the air.

Types of Systems

We've just described what is called a "split system" - the condensing unit is placed outside the house, and the evaporator coil is inside.

There is another configuration called a "packaged" air conditioner that combines the condensing unit and the evaporator coil into one outdoor unit. Which type you should choose depends on your home's location and construction.

Rating a Unit's Efficiency

The efficiency of central air conditioning systems is rated by a Seasonal Energy Efficiency Ratio (SEER). SEER ratings typically range from 13 to 23, with the highest numbers indicating the most efficient units that offer the most energy savings year after year. Fortunately, great strides have been made in the last 10 years to increase the efficiency of new air conditioners and heat pumps.

The typical SEER rating of air conditioners manufactured before 1992 is about 6.0. In 1992, the federal government established the minimum cooling efficiency standard for units installed in new homes at 10. The minimum SEER value changed again in 2006 to a SEER of 13. The SEER rating is usually shown on a yellow and black EnergyGuide sticker attached to the outside unit of the air conditioner.

Central air conditioners that are in the top 25 percent of efficient models may carry the Energy Star® label. To qualify, they must have a minimum SEER efficiency level of 14.5. Additionally, Energy Star® models must also have a minimum Energy Efficiency Ratio (EER) of at least 12 for split systems, and of at least 11.0 for single-package models. Air conditioners that bear the Energy Star® label may be twice as efficient as some existing systems.

Heat pumps also have heating efficiency ratings, indicated as a Heating Seasonal Performance Factor (HSPF). In general, the higher the HSPF rating, the less electricity the unit will use to do its job.

In 1992, the federal government established the minimum heating efficiency standard for new heat pumps at 6.8 HSPF. (Most heat pumps manufactured before 1992 had HSPF ratings below 5.) The minimum HSPF standard changed again in 2006 to an HSPF of 7.7. High-efficiency central air-conditioning heat pumps can also qualify as Energy Star® models.

Higher efficiency units usually cost more to purchase initially, but save money in the long run on operating costs.

Sound Levels

Few people think about how loud an air conditioner or heat pump will be - at least until the unit is installed and running in their back yard. With some units, the noise created by the condensing unit outside can even interfere with indoor peace and quiet. That's why you should compare the sound levels produced by different models when you are shopping for a new unit.

The sound level of outdoor units is measured in bels (a term similar to decibels). The rating scale goes from 0 - the rating for a barely perceptible sound - to 13 - the threshold of pain. Most air conditioners and heat pumps operate in the range of 8 to 9 bels, although some are quiet enough to rate as 6.8 bels. (While that may not sound like a wide range, consider this: the noise output at 9 bels is 10 times louder than 8 bels. That means one 9-bel air conditioner is as loud as 10 units rated at 8 bels!)