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Visions of the scorching heat of summer fill your mind as you walk
through the department store. You're closer than ever to
buying that air conditioning unit you've wanted for years. The
only questions that remain are, should you buy a window unit or
a central air system? And, what size should the unit be to properly
cool your home?
There are advantages to both window and central systems. The window units are lower in cost and can target certain areas of the home but are not as efficient as central air systems and have to be manually installed each season. Their cost is between $200 and $300 for a one-room window unit and $450 and $550 for a large window unit intended to cool several rooms on one level. The life expectancy for the window unit is approximately 10-12 years.
The advantages to the central system include the efficiency and
overall house coverage. The downside is the initial cash outlay.
If you already have an older system in your home and are just replacing the compressor, the central air system will cost between $450 and $600 per ton, including labor. To cool a 2,000-squarefoot, two-story colonial home, you probably need a 3-ton or 36,000 BTU unit. Your cost would be $1,200 - $1,800. If you have never had a unit and need the ductwork installed, you will probably spend between $4,500 and $6,500 for the system, including installation. The life expectancy on a central unit is typically 10-15 years.
The more important question you need to address is the sizing of
the unit or system, especially the central system. You should size
an air conditioning system carefully because it has to dehumidify
as well as cool. A system that is too large will certainly cool.
However, when a unit is too large, it will not remove enough moisture from the air (dehumidify) and will still be uncomfortable because of the moist air. You will have to call on the oversized unit to reduce the temperature three or four degrees more than you would have if you had a properly sized unit. This is not as critical with the window units as with the central systems.
Regarding efficiency, both the window units and central systems
need to be closely reviewed. Check the units for efficiency. On
the newer units, both window and central, you can look at the energy guide stickers for a SEER number. Most window units and central systems will have this, but if they don't, or you would like to know what the efficiency is or what SEER means, you can figure it out by comparing the BTUs (British Thermal Units), or tonnage, of the unit/system to the electrical draw in amperage. (There are some gas units, but they are nearly obsolete in residential applications. They do, though, exist in commercial settings.)
The SEER number is directly related to the efficiency of the unit.
The higher the SEER number, the more efficient the unit. One point
of SEER is approximately 10 percent more efficient than its lower
counterpart. This is important because the initial purchase price
of a 12 SEER unit is approximately $700 or $800 more than a 10 SEER
unit. However, if your cooling costs are $500, you can save 20 percent by using a 12 SEER unit versus a 10 SEER unit. This translates into $100 saved each year.
If a 12 SEER unit costs $700 more than a 10 SEER unit, you will
break even at seven years and double your savings at 14 years. The
more you use, the more you save with the efficient units. Someone
in the South who doesn't buy a 12 SEER unit is probably making a
big mistake. The homeowner in the North may not be making a mistake
at all, due to limited usage of the unit.
SEER numbers range from about eight to 16, but above 12 are still
somewhat experimental. They may be a more prudent buy in a couple
of years, after they are time-tested.
It's a hot day and your air conditioning system is not cooling
you properly. What can you do? Well, to start, take a look at these
recommendations. They may save you a lot of time and money.
Testing your central air system
To test the air conditioning, the outside temperature should be
above 60° Fahrenheit for 24 hours (some manufacturers recommend
65°). If you turn the air conditioning system on when the temperature is below 60°, you may damage the compressor. Turn the fan switch to "auto" and set the thermostat below the room temperature in the ductwork. The fan and condensing unit should come on, unless there is a time delay, which may typically delay its start for two to six minutes.
Let the system run for eight to ten minutes to balance the temperature in the ductwork. Check the temperature at the supply and return registers. The temperature at the supply register should be 14° to 20° Fahrenheit cooler than at the return ducts or ambient room temperature.
If the temperature differential is more than 20°, it indicates
restricted air. This points to: (1) a dirty filter, (2) improper
ductwork, or (3) a fan that is sized wrong, not working properly,
or moving too slowly.
If it is less than 14° differential, the possible causes are: (1)
refrigerant loss, (2) a dirty coil, (3) a laboring compressor, (4)
an oversized fan, or (5) a deficient return air system.
There are two copper lines going from the condensing unit on the
exterior to the air handler on the interior. The smaller line is
the liquid or high pressure line and will be warm, not hot, to the
touch when it is operating properly. The larger line covered with
insulation is the suction or low pressure line and will be cold
to the touch. The suction line should be "sweating" or
condensing, the way a glass of iced tea "sweats." It should not be forming ice.
Two common central air conditioning problems
- The refrigerant in the system may be low from a possible leak.
If the leak can be located easily, the repair and recharging would typically cost less than $200. (A leak in an aluminum coil would be an exception.)
- Compressor failure is a problem which usually requires replacement of the unit. Usually, when a compressor is replaced, the entire condensing unit on the exterior is replaced. The cost is about $500 per ton, or per 12,000 BTUs. A three ton, or 36,000 BTU,
condensing unit may cost about $1,200 to $1,800 installed.
The typical life expectancy of a compressor in a central air conditioning system is eight to 15 years, depending on the geographic location.
Air condition unit efficiencies
When discussing air conditioning efficiencies, the SEER number is
the typical guideline. The higher the SEER number, the more efficient the unit. (e.g., a 12 SEER unit is 20% more efficient than a 10 SEER.) This information can be a considerable help when evaluating the initial cost versus the operational cost of a new system.
A whole-house exhaust fan in the ceiling draws air from the house,
forcing it into the attic and out through the gable vents.
A fan should be used only to draw relatively cooler and/or drier
outside air into the house. If it is operated simultaneously with
air conditioning, it will exhaust the cooled and dehumidified air
from the house and replace it with warm, humid air from the outside.
A valid use might be to purge accumulated super-heated air from
a house that has been closed up , perhaps during a work day or vacation, prior to activating the central cooling system.
In order to operate safely and effectively, the gable vents may
need to be enlarged so that their cumulative free area equals the
area of the fan orifice.
The following guidelines should be followed when using a whole-house fan:
- Use it only when the outside air is not higher in temperature
than the interior space being ventilated.
- Open only the windows where ventilation is presently needed:
the living room during the cooler evening hours, the bedroom at
night, or the kitchen/breakfast area early in the morning.
- Do not use the whole-house exhaust fan in conjunction with a
heating or cooling system or while a fireplace is burning.
Consider building a safety grille around the exposed fan mechanism
in the attic to help prevent accidental injury from rotating blades
and other moving parts.
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