SPACE HEATING & COOLING: Air Conditioning

Questions:

What does a SEER rating on air conditioners mean, and how do I compare ratings between units?
SEER stands for seasonal energy-efficiency rating. This rating measures how well an air conditioner uses energy throughout the cooling season.

The SEER is equal to Btus of cooling supplied during the year divided by kilowatt-hours of electricity consumed in a year. The higher the SEER rating, the more efficient the air conditioner will be.

For example, a unit with a cooling capacity of 24,000 BTU that consumes 2,400 kilowatts of electricity would have a SEER of 24,000/2,400, or 10. Units with high SEERs will cost more initially, but the energy savings throughout their lifetime will more than make up for the cost difference.

When comparing SEER ratings of different air conditioners, compare only those with similar capacities (Btu).

Is it cost-effective to buy high-efficiency, air-conditioning units?
Yes, if the unit serves a home or business that air conditions throughout the summer rather than on an intermittent schedule. The additional cost of the higher efficiency units can be justified from the energy savings.

The minimum seasonal energy-efficiency rating (SEER) is 10, but the Department of Energy is considering increasing the minimum to a SEER of 12.

Homeowners and business operators can justify the purchase of air conditioners with a SEER of 13 or 14 in applications where energy costs are high or the cooling season is long. In buildings used less frequently, such as churches and meeting rooms, energy savings usually won't offset the cost of the highest efficiency units.

When buying a new central air conditioner, what should I look for to ensure a high-efficiency unit that will last?
One of the best guides to the efficiency of an air conditioner unit is the seasonal energy efficiency rating (SEER).

The higher the SEER, the more efficient the unit will be. Federal legislation dictates a minimum SEER rating of 10 for central air conditioners sold in the residential marketplace. Air conditioning units are now available with SEER ratings as high as 16.

Long life and ease of service are two other important considerations when purchasing an air conditioner. One recent development in compressor design, the scroll compressor, offers a long, trouble-free life and low noise level. Scroll compressors are also more efficient than conventional compressors. Scroll compressors are often used on units with a SEER of 12 or greater. When receiving bids, be sure to ask if the unit uses a scroll compressor.

Contact at least three air-conditioning service companies in the area to obtain bids for comparison of features, warranties, and efficiency. Be sure to carefully evaluate the proposed size of the units. Purchasing a properly sized unit is critical to achieving good performance

Why is it important to properly size an air conditioner?
Today, it is recognized that accurately sized, or even slightly undersized air-conditioning equipment, will result in greater operating economy and improved comfort because the air conditioner cycles on and off less often. This reduces wear and tear on the compressor, increases efficiency, and improves humidity control.

Determining the proper size for a residential air-conditioning system calls for a cooling load analysis. This procedure takes into account the size of the home, insulation levels, roof color, orientation of windows, shading of windows, tightness of construction, and number of occupants.

However, on extremely hot days, usually less than three percent of a normal cooling season, the indoor temperature may rise or swing upward a few degrees Fahrenheit during the hottest part of the day.

This is a small price to pay for improved performance and comfort during the balance of the cooling season.

Furthermore, comfort can be easily maintained during a designed temperature swing by using a fan to create air movement and delaying activities, such as cooking, that produce internal heat gain until the air conditioner has recovered.

A cooling load analysis of a home can be performed by most heating and air-conditioning contractors or by an independent energy auditor.

What is the status of the refrigerant used in my home air conditioner? Is it being phased out like the refrigerant in my car?
Unless a central home air conditioner is relatively new, the refrigerant used is R22. It is chemically different from the refrigerant used in an auto and has only one-twentieth the impact on stratospheric ozone. Because it is not as harmful to the ozone layer, it is not scheduled for phase-out until 2020.

Some air-conditioner manufacturers are offering equipment filled with refrigerants that pose no harm to the atmosphere. The operating efficiency of these air conditioners is no higher than those filled with R22.

These products may carry a higher price, but the refrigerants will be available after the scheduled 2020 phase-out of R22.

When adding central air conditioning to an older home, what do I need to watch out for?
There are several issues to consider when adding central air conditioning to an existing heating system.

If a home has an older heating system with no provisions for central air conditioning, the ductwork may be smaller than what is required for air conditioning. Increase the fan speed to compensate for the ductwork. A larger motor often is required to achieve this higher flow rate.
In extreme cases, it may be necessary to replace the supply ductwork.

The location of the return-air registers also plays a role in comfort. In older homes, there were often no return-air registers installed on the second floor of a two-story home. It is difficult to cool the second story if this is the case. It may be necessary to install return-air ductwork.
Another consideration is the requirement for a floor drain below the furnace level. Air conditioners produce condensate when they operate. This condensate is the consequence of removing moisture from the air.

If a floor drain is not available below the level of the furnace, it is possible to purchase a small condensate pump set. For approximately $60, this set will pump the condensate to a convenient disposal site.

A final consideration is the arrangement of the ductwork at the furnace outlet. The ductwork around the furnace must leave sufficient room for the installation of the cooling coil. When installing central air, it is an excellent time to check the supply and return air ducts for leaks. Inadequate air flow across the cooling coil is the No. 1 cause for poor air-conditioning system performance.

What can I do to reduce summer air-conditioning costs? (Part I)
First, inspect the envelope of the home. The envelope is composed of the roof, ceilings, walls, floors, windows, and doors. Various opportunities exist for improving energy efficiency, such as insulation, radiant barriers, and weatherstripping. Insulation levels as high as R-38 in the attic are appropriate. It is permissible to mix insulation types, such as covering fiberglass with cellulose. Any exposed ductwork in the attic also should be sealed and insulated.

Weatherstripping and caulking reduce both heating and cooling costs. Inspect existing weatherstripping for wear and possible replacement.

In addition to caulking window and door frames, inspect for hidden cracks such as those that exist along foundations, or where exterior wiring or air-conditioning lines may penetrate the wall.
South-facing windows can be a real benefit during the heating season but can add significantly to the cooling load.

It is preferable to block the sunlight before it penetrates the window. Although a drape will delay the instantaneous solar gain, it's more effective to stop the sunlight completely by using exterior shading or reflective blinds.

Deciduous trees provide an excellent means for natural shading in the summer, yet allow exposure of the window in the winter. Removable exterior awnings can provide a similar advantage.

Unventilated attics can reach high temperatures during the summer, contributing considerably to the cooling load in the home.

Attics should be properly ventilated by having sufficient openings along the low side of the attic, such as in soffits as well as openings along the high side of the roof for exhaust.

For ventilation, have at least one square inch of free opening for every square foot of attic space. Openings should be distributed equally between the low and high sides of the attic. Remember that screens and louvers block up to 50 percent of the ventilation area.

Move air in and through the home without relying on an air conditioner. When the outdoor air is cool, yet the home is warm, a whole-house fan, which draws air through open windows and discharges into the attic, may provide all the cooling necessary.

Additional attic ventilation is necessary when using a whole-house fan.
Have one square foot of free opening for every 750 cubic foot per minute (cfm) of air moved by the whole-house fan.

Within the home, portable fans or ceiling fans can provide some cooling relief.

What can I do to reduce summer air-conditioning costs? (Part II)
Household appliances can add considerably to the cooling load in a home. Refrigerator condenser coils should be cleaned at least twice annually.

Inspect the gasket around the refrigerator door to assure that it has not worn and needs to be replaced. The cooking range and clothes dryer should be vented to the outdoors, as should exhaust fans in bathrooms.

Heat loss from a water heater adds both to water-heating costs as well as air-conditioning costs. A water heater that is warm to the touch should be insulated with a water-heater insulating jacket.
Thermostats on water heaters should be turned down to provide hot water at the tap no greater than 140 degrees.

Prepare a furnace for summer by replacing or cleaning the air filter, and lubricating, where possible, any bearings on the blower or motor. Consider extinguishing the pilot light if the furnace is equipped with a pilot. Many new furnaces use an electronic device for igniting the flame whenever the thermostat calls for heat rather than using a standing pilot light.

This will probably not result in a significant reduction in gas costs; however, the pilot does contribute a small amount of heat to the home that then must be removed by the air conditioner. Contrary to some earlier information, extinguishing the pilot light will not shorten the life of the furnace. Be sure that if the furnace is equipped with a central humidifier that it is turned off, drained, and cleaned.

An air conditioner needs adequate air flow through the condenser for the unit to operate at maximum efficiency. Plantings and fencing should be no closer than three feet to the condensing unit.

The condensing unit should be cleaned annually by carefully removing any debris from the fins of the condenser.

Consider hiring an air-conditioning service contractor to clean the condenser thoroughly, particularly if it has not received maintenance in the last two or three years.

Service contractors will use a variety of cleaning solutions to remove any buildup on the condenser fins as well as straighten any fins which may have been damaged, lubricate any exposed bearings, and check for appropriate refrigerant levels in the air conditioner.

Taking advantage of these and other opportunities should help to reduce cooling costs this summer.

My home has a whole-house fan and central air conditioner. How can I use them both for the most economical cooling?
A whole-house fan provides cooling by moving a large volume of air into and through the house, typically exhausting it through the attic.

Use the fan when the outdoor air temperature is at or below the desired indoor temperature, usually during the late evening and night hours. Use the fan at night to help create more comfortable sleeping conditions without air conditioning.

Central air conditioning cools by lowering the temperature and humidity of indoor air. When it is hot and humid outside, the house is closed up and the indoor air is conditioned. Central air conditioning and a whole-house fan should never operate at the same time.

The best strategy may be to use the whole-house fan extensively during the late spring and early fall when the demand for cooling is rarely large and extended heat waves are unlikely. Also, use the fan in the summer when temperatures and humidity are at or below normal.

When outdoor temperatures and humidity rise to uncomfortable levels, close up the house and switch to air conditioning.

In deciding what will be most comfortable, follow the weather patterns and use one system or the other for a few days. In general, avoid using both systems every day, especially during very humid weather.

The whole-house fan may significantly raise the moisture level inside a house by bringing in outdoor air. Switching to air conditioning the next day may drop the temperature, but might be less comfortable.

The most economical approach to minimize the effects of the sun's heat on a home is to use fans and the whole house fan as a first choice, and switch to air conditioning when the heat and humidity become oppressive.

Can ceiling fans effectively reduce air-conditioning costs?
Any type of fan can be effective in reducing air-conditioning costs if the air movement helps occupants feel comfortable and results in increasing the thermostat temperature setting. If the air conditioning thermostat setting is not increased, there are no savings.

The cooling effect of moving air can compensate for as much as a four-degree rise in temperature.

Keep in mind, that during the heating season, the air movement caused by the fan will still have the same cooling effect.

How can I keep my home cooler in the summer without air conditioning?
The simplest, least expensive method to keep a home cool is shading walls, windows, and the roof.

Interior shades are inexpensive and easy to install. Use pull-down or Venetian blinds in addition to regular window coverings. Window coverings should be light colored (white or beige).
There are several ways to keep a home cool without overusing the air conditioner. Of these options, install shades first. Compare utility bills before and after the installation of shades. If satisfied with the savings, stop there, but if savings are not significant, look into other options. One option to consider is exterior awnings. They are more expensive than interior shades, but would be a great way to shade south windows.

Natural shading is another way to block heat gain in summer. For example, plant broad-leafed trees on the south and west sides of the home. They shade a home in summer months and will let in sunlight during winter months when they have shed their leaves.

Certain steps will help keep a home warm in winter and will help cool it during the summer. Insulated walls and roof reduce heat gain, just as they lower heat loss in winter. As a general rule, ceiling insulation should have an R-value of 35 to 45, and walls from 19 to 27. A light-colored roof also decreases heat gain.

Use the above suggestions, coupled with circulating fans inside the home, and utility bills will be less than if air conditioning was the only cooling source.

Is it better to leave the fan running continuously with the air conditioner or to place it in the automatic position?
It is more efficient to leave the thermostat in the automatic position.
The fan consumes only one-tenth the energy of the compressor, but when it runs continuously, the fan can cost up to $30 a month.

This amount can be reduced by cycling the fan only when it's needed.
Additionally, the air conditioner will dehumidify the only air when the compressor is running. However, if the fan remains on after the compressor cycles off, some moisture on the coil will re-evaporate. This moisture must be removed during the next compressor cycle, which increases the energy consumption.

If air distribution is poor within the home or business and hot spots or very cold areas result, the fan can be run to even out the temperatures.

However, the fan should be set to the auto position when the building is unoccupied. Even better, shut the air conditioner off or raise the thermostat setting when leaving the building.

Will I save energy by turning off my air conditioner when I leave home, or am I better off just letting it run?
If gone for four hours or more, more energy will be saved by turning off the air conditioner or turning up the thermostat.

During the day, keep windows shut and close curtains or blinds on any windows that will be exposed to sunlight.

The thermal mass of the house will probably keep the indoor temperature well below the outdoor temperature, and the house should cool quickly when the air conditioner is restarted. Use a programmable thermostat or timer to turn on the air conditioner 30 to 45 minutes before the expected arrival home. If the home is still warm upon arrival, turn on a fan to create air movement.

Moving air can make the air feel about four degrees cooler than it really is.

Can I plant bushes to hide the outside of my air conditioner?
When landscaping around an outside condensing unit, remember that the air conditioner must reject all the heat from a home.

Although it is possible to plant bushes near the condenser, leave room for adequate air circulation. Without good air circulation, the temperature near the condensing unit will rise. The higher temperature will reduce the capacity of the air conditioner, causing it to work harder and provide less cooling. This could also kill the shrubbery.

If the shrubs will not form a continuous wall around the unit, plant them so that, when they mature, there will be three feet of clearance. If the shrubs will be continuous, then allow five feet of clearance.

Are there any simple checks I can perform to see if my air conditioner is operating properly?
Check a few items that should indicate if the air conditioner has problems.
First, check the two lines connected to the outside of the air conditioner. The larger on -- the suction line -- should be cool to the touch. It should not be so cold, however, that frost develops.

The smaller line -- the high pressure line -- should be warm, but not hot. It should be 20 to 30 degrees warmer than the outside temperature. In extreme cases, it will be hot to the touch, so be cautious. If this is the case, call a service technician.

Some air conditioners are equipped with a sight glass in the high-pressure line (the small line). The glass should be clear, with no bubbles visible, while the system is running. Cloudy liquid in the sight glass may indicate contamination of the system.

One final check is to measure the temperature of the air as it leaves the register. It should be 15-20 degrees cooler than the room temperature.

If the building is warm, humid, or if the ductwork is not insulated, then there may be smaller temperature differences.

These guides are not intended to eliminate the need for an annual check by a qualified service person. If problems are suspected, call for help from someone familiar with air conditioners.

Would I be better off using several window air conditioners?
Using multiple window air conditioners has both advantages and disadvantages. A distinct advantage of window units is that they can operate individually. This flexibility allows cooling only the occupied room rather than the entire house.

A central system is more convenient to operate when cooling the entire home continually, and possibly at a lower cost of operation.

If sound level is a consideration in the home, keep in mind that window units are typically noisier than central air conditioning.

In terms of efficiency, top-of-the-line central units are generally more efficient than window units.
Look at the seasonal energy-efficiency ratio (SEER) when selecting units -- the higher the SEER, the higher the efficiency under similar conditions.

Since there are positives and negatives about window units and central systems, consider personal needs and preferences before choosing a system.

What is a ton of air conditioning?
A ton is the measure of the cooling capacity of an air-conditioning unit. It is an indication of the rate that the unit removes heat from a building.

One ton of air conditioning removes 12,000 British thermal units (Btu) of heat an hour. The term was derived from the time when ice was used for refrigeration. One ton of air-conditioner cooling capacity removes the same amount of heat required to melt 2,000 pounds, or one ton, of ice in 24 hours.

Typically, residential central air conditioners will range in capacity from one and one-half to four tons. Window units will many times be rated in Btu per hour. For example, a 6,000 Btu/hr. window unit would have the capacity of one-half ton.

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