Just as homes need to be heated in
the winter months, an increasing number of homeowners opt to cool
their abodes during the summer months. Over the last twenty years, the
number of households that use air condition has almost doubled. With
that growth has come a tremendous increase in demand for electricity,
a demand that peaks for all households simultaneously on the hottest
For regions with high humidity during
the summer months (which covers most of the US), refrigeration is the
cooling technology of choice. Refrigeration involves the use of a gas
(refrigerant) to transfer heat from one environment to another. The
diagram below illustrates the mechanics of this cycle.
As pictured above, an air
conditioning system consists of a refrigerant fluid running a copper
tube circuit that contains a compressor to pressurize the refrigerant,
two sets of coils to exchange heat with air, and a control valve to
regulate the pressure drop (not shown). Two fans, each blowing air
across the coils, complete the system.
Inside the refrigeration circuit,
changes to the pressure, temperature, and state of the refrigerant
enable the extraction of heat from a home. At point A in the above
diagram, the refrigerant is a high pressure, low temperature liquid.
As it passes through the control valve (not shown) and the Evaporator
Coils, it absorbs the heat of the air being blown over the coils by
the furnace fan. The air cools as heat is drawn out of it as it passes
through the coils to its way to the various rooms of the house. Once
through the coils, a point B, the refrigerant is a low pressure,
higher temperature vapor. In the compressor, the refrigerant is
compressed, yielding a high pressure, high temperature vapor (point
C). As the refrigerant vapor runs through the condensor coils, the
outside air draws heat from the refrigerant, causing it to condense to
a high pressure, low temperature liquid at point A. From this point
the cycle repeats itself.
The A/C refrigeration cycle is in
motion whenever the fans and the compressor are turning. The
thermostat, which is located in the living area of the house, sends on
and off signals to the system depending on the temperature in the room
and its target or "set" temperature. Whenever the room
temperature rises above the set temperature, the thermostat turns on
the air conditioning. When the room temperature reaches the set point,
it turns the system off.
The unit of capacity for
refrigeration systems is tons. One ton equals 12,000 BTU/hour of
extracted heat. BTU stands for British Thermal Unit, which the amount
of energy it takes to increase one pound of water by one degree
Fahrenheit. Most central residential units range from 2-5 tons. Room
air conditioners are rated directly as BTU's per hour. Today's market
offerings range from 5,000 - 18,000 BTU/hour.
Unlike heating systems, which use a
variety of fuels, all air conditioning systems are powered by
electricity. Since most buildings today have air conditioning, and its
usage is driven by the difference between indoor and outdoor
temperature, utilities' power demand always peaks on the hottest
summer days. For many homeowners, electricity consumed at these peak
times is more expensive than electricity consumed during off-peak
hours. Consequently, raising the efficiency of air conditioning can
produce a sooner than expected payback.
For central air conditioning,
efficiency is measured as SEER (Seasonal Energy-Efficiency Rating).
The higher the rating the higher the efficiency. The ratings are
linear, i.e. a rating of 12 uses half the energy as a rating of 6 for
the same tonnage. As of October 2000, the minimum SEER rating allowed
for new units sold is 9.8. Currently, the highest performing units on
the market have a rating of 18.
Room air conditioner efficiency is
rated in EER (Energy-Efficiency Rating) which is similar to SEER. The
US Federal Government requires that all room air conditioners sold
through retailers have a bright yellow EnergyGuide label, as shown
below. It displays the EER rating for the unit, which can be used by
the shopper as basis of comparison between competing units. For
otherwise comparable models, the one with the higher EER will use less
Source: US DOE
The heart of a refrigeration system
is its compressor. Improvements in compressor technology over the past
two decades have resulted in substantial improvements in air
conditioning efficiency. Compressors come in two basic types:
reciprocating and rotary. Reciprocating compressors have pistons that
move back and forth like a car's engine. Rotaries are similar to jet
engines - they compress air as they spin.
The keys to minimizing the energy
usage of an air conditioning system are to
- Minimize the amount of heat
generated inside the house - Minimize the amount of heat flow into the
house - Create virtual cooling inside the house via air circulation -
Minimize the heat load outside of the house - Optimize the
distribution of cooled air around the house - Keep the system's
equipment in peak operating condition
Minimizing heat generation
In a typical residence, heat is
unintentionally generated from several sources:
Lighting, especially the incandescent
kind, throws off an abundance of heat. (In fact, 80% of the energy
consumed by a light bulb is waste heat) During the cooling season, use
of electric lights should be minimized.
As the dryer pushes hot damp air
outside, it draws in conditioned room air. Consequently, unconditioned
outside air will be drawn into the house to equalize the air pressure.
In addition, as the unit is in operation, the dryer's cabinet heats
up, which creates convection currents in the room.
Heat from the hot water used in the
washing process conducts through and convects off of the dishwasher's
Ranges and ovens employ heating
elements to cook food. As with dryers and dishwashers, conduction and
convection transfers heat from the appliance to the surrounding indoor
Some ranges use a hood to draw away
the warm, humid, and grease-laden air out of the kitchen. As with the
dryers, this air expelled to the outside is eventually replaced with
unconditioned outside air.
Consumer electronics, including TV's,
stereos, VCR's, and computers are all generators of heat. If plugged
into an outlet, their power supplies continually dissipate heat, even
when turned "off".
Lighting, as well as appliances for
clothes drying, dishwashing, and cooking, are all considered essential
elements of modern living. To minimize their impact on the cooling
load, they should be used at times of the day when the heat load on
the house is the lowest, typically in the evening or very early
morning. For homeowners on a "demand" schedule, a secondary
benefit is the lower off-hour cost of electricity to run these
In the summer months, cooking of food
can be done outdoors with a gas grill. In addition to keeping the
kitchen cooler, cooking outdoors with gas may be cheaper than using
the electric range in your kitchen.
[table - electricity vs. gas cost]
Consumer electronics are as bad at
generating heat and as they are at wasting electricity. When the TV or
stereo or PC is "off" its power supply dissipates
electricity as heat as long as it is plugged into a wall outlet.
[table of typical drainage currents]
Besides remembering to turn off all
devices when they are not necessary, they should also be unplugged
from the wall socket. If you have a home entertainment center or home
office, power can easily be disconnected through the use of a switched
power strip. When the area is not in use, which is typically more than
12 hours per day, the power strip should be turned off. Its switch
physically opens the circuit, making current flow, and power
consumption, impossible. Typical power consumed in the on and off
modes in a 24 hour period is summarized below:
When purchasing new electronics,
always look for the Energy Star label. This certification signifies
that the product consumes at least 20% less energy than the maximum
allowed by the US Department of Energy. This label is becoming more
common in the marketplace as consumers seek goods that are greener and
cheaper to operate.
[link to Energy Star]
Minimize Heat Flow
Minimizing heat flow into the house
is best accomplished by minimizing heat buildup in the attic and
cost-effective insulation. See the Insulation section for further
When you are standing in the hot
summer sun and no wind is present, you quickly become uncomfortable.
Not enough temperature differential exists between your body and the
outside air for the waste heat from your body to transfer effectively.
Your body reacts to this situation through perspiration, its own means
of cooling itself.
If you are standing in the hot sun,
but a breeze is present, the heat feels more bearable. As the wind
passes over you skin, your perspiration evaporates, which makes you
feel cooler. Actually, a gentle airflow can lower the sensed or
"virtual" temperature by 6F.
In your home, virtual cooling can be
achieved through the use of whole house fans and ceiling fans.
The whole house fan draws air through
open windows in the house and forces it out through the attic. It
creates two benefits - (1) the gentle breeze induced throughout the
house lowers the virtual temperature and (2) hot air driven out of the
attic reduces the amount of heat conduction into the living space
below. For outside temperatures up to 83F, a whole house fan can be a
reasonable substitute for air conditioning. Using one will enable you
to delay usage of your air conditioner at the start of the hot season
and to turn it off earlier towards the end of the season.
[Diagram of fan action]
When the outside air temperature
rises above 83F, only air conditioning can maintain a pleasant indoor
temperature. However, by using ceiling fans in occupied rooms, you can
raise the set temperature on the cooling thermostat a few degrees
higher and still maintain comfort. Air conditioning energy savings
will be realized with even slightly higher thermostat settings
Minimize the Heat Load Outside
Tall trees can block the sun's rays
from hitting the roof and sides of a house. With proper positioning
the cooling load could be reduced as much as 40%. Ideally, in a
heating dominated climate, tall evergreen trees flank the north and
western sides of a house, while deciduous trees shade the east side .
In the wintertime, evergreens oriented on the northern and western
sides of the house block the prevailing winter winds, which accelerate
heat loss from the exterior walls. The low-in-the-sky sun would be
able to radiate on the east and south sides of the house, helping to
heat the home in the winter.
In the summer, the east side
deciduous trees would block the sun's morning rays that would add to
the home's cooling load. Because the sun is high overhead during the
summer months, it will not shine directly on the south side of the
house. Appropriately sized roof overhangs can block any direct
sunlight on the south face during the entire cooling season.
In cooling dominated climates, i.e.
the sunbelt states, it may be beneficial to add deciduous or evergreen
trees to the southern side of the house. In the fall and spring
months, with the sun descending or ascending in the southern sky, the
heat load induced by the sun on the south face of the house may add
more to the air conditioning bill than it saves on the heating bill.
Blocking the path of the sun's rays
to the outside unit of an air conditioner, will allow the air
conditioner to operate more efficiently. Locating the unit of the
north side of the house helps. Planting trees to the east and west of
the unit will provide shade for all but a few hours per day. Care must
be taken to not block the unit's air flow out of the top.
Optimizing Cooled Air Distribution
Optimizing the distribution of cooled
air involves the location of ductwork and use of vents. Every room
should have a supply air duct and a return air duct. If a room is part
of the desired conditioned space, both ducts should be open.
Otherwise, the supply duct should be partly shut in the winter and
full closed in the summer. If the supply duct is completely closed in
the winter, the temperature may drop to a level that would cause pipe
Every duct should be unobstructed to
ensure full air flow.
If cooling ductwork is routed through
unconditioned space, it must be insulated to minimize heat losses.
Air Conditioning Maintenance
A checklist of servicing items for
air conditioning items is shown below:
- Check refrigerant level
- Clean and flush coils, drain pan,
and drainage system
- Vacuum the inside of the blower
- Check drive belt, if any -
Lubricate, if bearings are not permanent seal design
- Inspect wiring connections and
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