Central Air Conditioning

Overview
Why Central Air Conditioning
How Air Conditioning Works
Considerations (Efficiency Ratings, Capacity, Refrigerant, etc.)
What Next 

Overview

Why Central Air Conditioning?

Air conditioning provides two great comforts:  it lowers the temperature of your inside air and it lowers the humidity.  But, besides comfort, there are some other advantages.  With lower humidity you can eliminate the dampness in your basement and cooler areas of the house.  On humid summer days, warm air condenses on cooler walls and pipes and other surfaces in the basement.  This leads to a damp, musty condition that can be unhealthy as well as unpleasant.  The other alternative is a dehumidifier, but it uses electricity, requires maintenance, is limited to a small area, and is noisy.  Besides, it doesn't cool the place down.

Central Air conditions the air throughout your home, not just in one room.  Even if you only want to cool one or two rooms, you may want to control humidity everywhere.  Room air conditioners aren't nearly as effective in lowering the overall humidity in the home.  They do help lower the humidity in the room they service, but that isn't usually the room that gets musty.

How Air Conditioning Works

Air conditioning works by removing heat from the inside air and releasing it to the outside air.  The key components are refrigerant, tubing, coils, and a compressor/pump.  The compressor performs two functions: it moves the refrigerant through the refrigerant lines and it compresses it.  Cold, compressed liquid refrigerant is slowly passed through coils inside the house.  The warm air inside the house is passed over the coils and the heat is transferred to the refrigerant causing it to warm up and expand into a gas. It evaporates; therefore, this coil is called the evaporator coil.  The refrigerant moves through the lines to coils outside the house where a fan and the compressor force the heat out of the refrigerant into the outside air. As the refrigerant cools, it condenses into a liquid; therefore, this coil is called the condenser coil or just the condenser..  The cooling of the air inside the house also results in the lowering of its humidity.  As warm, humid air is cooled, its ability to retain moisture decreases and the water condenses. 

Type of Air Conditioners

This process is essentially the same whether we're talking about central air conditioning, a window mount (or through-the-wall), or a ductless split system.  The differences are in where the components are located and how the air is passed over the coils.  In central air, the warm indoor air is passed over the evaporator coil  in the ductwork and the cooled air is circulated throughout the house.  The fan and compressor, the noisy parts, are located outside the house with the condenser coil..  This separation of the evaporator and condenser coils is called a "split system."  There is another type of split system called "ductless" air conditioning.  Ductless air conditioning is used in houses or buildings in which there is no ductwork to the areas you want to cool.  One part, the part that contains the evaporator coil, is set inside.  The other part, which includes the condenser coil and the noisy parts (the fan and compressor), is located outside.  This system is limited to cooling the area in which it is located, however, because it draws in the warm air around it and blows cool air back into the room, much like a traditional window mount or through-the-wall system.  And, finally, there's that window mount system, in which all the parts are contained in one small package.  It's disadvantages are noise, the drip of condensate, and its limitation to a relatively small cooling area.

The Science behind it

The refrigerant, like most substances, expands when it is heated and contracts when it is cooled.  The refrigerant is specially selected for its ability to transfer heat.  It is especially well-suited to absorbing heat and expanding and then giving off this heat when exposed to cooler air and compression.  Historically, freon has been the refrigerant of choice.  However, for environmental reasons,  Freon is being phased out as a refrigerant and being replaced primarily by R410A, which appears under the trademarks of Puron™ and Environ™.  It is now against the law to vent freon to the atmosphere, but leaks and accidents do happen.  See more under Refrigerant below.

Considerations in Central Air Conditioning

Efficiency

Central Air Conditioners all have SEER ratings (Seasonal Energy Efficiency Ratings).  These are numbers ranging from 10 to about 17.  The higher the number, the higher the efficiency.  You can think of the numbers as units of cooling you get for every dollar you spend on energy.  The minimum efficiency allowed is now 10.  That's quite an improvement than the typical 6 to 8 rating of the units produced before 1992.  A unit with a SEER rating of 12 to 14 or so is generally considered a high efficiency unit.  Even more efficient units are available that make sense for southern climates, where they run a lot more.  

The math is a little tricky, but here's a chart that compares their relative efficiency.  In this example, we'll compare a model with a 10 SEER to more efficient models.  We'll assume a 3-ton (36,000BTUh) unit.

What you should care about is energy operating cost.  For our comparisons, this will depend on the SEER rating, your climate, the cost of electricity in your area.  In reality, it will also depend on the characteristics of your house, the comfort level you demand, and your living habits.  If you like to leave the windows open while the A/C is running or you're constantly going in and out the doors.....well, you know what we mean.  Also, the SEER rating doesn't take into account all the electrical usage of the system.  The electrical usage of the blower motor, for instance, is not considered in SEER ratings because it is a part of the air handling system shared by the furnace.

Let's say you live in a house that requires a 36,000 BTU (3 ton) air conditioning unit and you live someplace where your air conditioning will run about 1000 hours per year -- Kansas City, Missouri, for example.  Here are your approximate costs for units with various SEER ratings at various costs per kilowatt hour.

(For a quick comparison, you'd probably run the A/C about half as much in Madison, Wisconsin and twice as much on the Gulf Coast of Alabama.)

1000 Hours per year (Kansas City, Missouri, for example)

For a final way of looking at it efficiency and cost, let's compare SEER ratings over 10 years..  Let's say you use 1000 hours per year, as in the example above.  And let's say the first year you pay 10 cents per kilowatt and the cost of electricity rises 5% per year.

If you look at the 10 and 12 SEER models, you can see the following:  

Even without accounting for inflation, a 12 SEER will save you $600 over 10 years ($3600 vs $3000).  If you assume a 5% increase in electricity, the savings is $745.

Capacity

It's easy to confuse efficiency with capacity, but after reading this, you won’t do that. Right? They aren’t the same. I can have a 24,000 BTU A/C unit with a 10 SEER and a 24,000 BTU unit with a 13 SEER and they will both cool my house equally well. The only difference is in operating cost. You can get any given capacity air conditioner in a whole range of efficiencies. Speaking of capacity, the industry uses a term called a "ton" and 1 ton is equal to 12,000 BTU. Typical capacities for residential structures are 2 to 5 ton or 24,000 to 60,000 BTU. In case you were wondering, it takes 12,000 BTU to melt a ton of ice; hence the name.

So what's the right capacity for you? You’re not going to decide this alone, so we’re not even going to try to make you an expert here. You will either need to rely on your HVAC contractor or go to the library and do a lot of reading to be able to go solo on this decision. One thing that is helpful for you to know is that there isn’t necessarily one single right capacity for a house. Don't be surprised to see two recommendations differing by a half-ton in a smaller house or a full ton in a larger house.

The sizing of an air conditioner is a balancing act between cooling and dehumidifying the house effectively. You want your air conditioner to control both the temperature and the humidity of your home, but the only time it can control humidity is when it's running.  Therefore, you want your air conditioner to be big enough to cool your house effectively, but not so big that it doesn't run long enough to control the humidity.

Think of a hot, humid day when you want the air conditioning on.  If your air conditioner is too large, it will cool the house quickly, then shut off.  It won't run long enough to control the humidity.  You'll get the cold, damp basement effect.

Then think of a VERY hot, humid day.  If your A/C is on the small side, it will control the humidity better because it will be running all the time, but it will struggle to keep the temperature down.

Lately, an extra environmental issue has become prominent. You now have a choice and a decision to make regarding which refrigerant to go with.  Freon has long been the refrigerant of choice. It has been found, however, that the release of freon into the atmosphere is contributing to the thinning of  the ozone layer in the atmosphere.  This ozone layer, among other things, filters out ultra-violet radiation from sunlight.  The thinning of the ozone layer is believed to increase the amount of UV radiation reaching the earth, which in turn creates an increased risk of skin cancer and numerous other environmental effects.

The major manufacturers are coming out with air conditioners that do not use refrigerants harmful to the earth’s ozone layer if they escape. The most popular new refrigerant is R410A, which is known by various trade names including Puron™ and Environ™. 

There is a federal mandate stating that after 2015, all units manufactured must use the new refrigerants and that, after 2020, the old refrigerants can no longer be produced. Since the life expectancy of a central air conditioner is around 20 years, the most compelling reason to go with the new refrigerants is to protect the environment. The most compelling reason to go with the old refrigerant is the tried and true nature of the technology.  Individuals obviously need to make their own call on this.  It should be noted that air conditioners do not "consume" refrigerant as they operate (just like a refrigerator does not).  Therefore, the type of refrigerant an air conditioner uses is only an issue if there is a leak or a major repair is needed.

One reassuring aside to what seems like a difficult battle to protect the environment is this: it was once general practice to vent all refrigerant to the atmosphere when working on air conditioners or at the end of their useful life. Since that time, it has become illegal to vent these refrigerants. They are now reclaimed and either reused or properly destroyed. This new practice of reclaiming will dramatically reduce the amount released to the environment. Today, the only way freon will be released in this country is by a leak in the system, an accident, or willful violation of the law.

Operating Cost

The efficiency of the unit as measured by SEER ratings (see above) is obviously of primary importance in determining operating cost for a central air conditioning unit.  But there are other considerations as well. Some of them may have already been determined by a choice of furnaces.  The furnace and the air conditioner, for instance, share the same air handler -- that is, the part of the system that forces the air through the house.  The most significant piece here in terms of electrical usage is the blower motor.  Obviously the efficiency of your blower motor will have an effect on overall operating cost.  Most blower motors these days use different speeds for the heating and cooling cycles.  You'll want to pay attention to the efficiency of the motor at both speeds. Also, some systems have controls that operate the blower motor at different speeds depending on how great the heating or cooling need is.  However, unless you're installing a furnace with your air conditioner or are going to have the blower motor replaced, you probably won't have much control over this aspect of operating cost.  

Maintenance costs are another piece of operating cost, so reliability is an issue.  Routine maintenance probably won't vary much from brand to brand, but could be affected by features in the system.

So what features should you look for besides efficiency, capacity, refrigerant, and operating cost? The obvious ones -- initial cost, warranty, sound ratings, quality of construction, and manufacturer reputation. The better manufacturers today have a product for each price range to accommodate different consumer priorities. Warranties and sound ratings tend to parallel the cost of the product. The wiser manufacturers don’t lessen the quality of the product components to get to the lower price bracket. Rather they simply deduct features.

What Next

If you think you're ready, you can go ahead and check out Model Information, where you'll find our recommendations and information on specific model lines.  Be warned, however, that it can be a little confusing without some guidance.  There are models to cover a wide variety of applications and it can take time to determine which ones are appropriate for your needs.  

You may just want to contact us before you go any further.  Fill out a form, send us an email, or give us a call.   We'll collect some information, get you some pricing information, and steer you back toward some appropriate model choices.