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The Basics of Reducing Air-conditioning Operating Costs

Making an air conditioning system operate more efficiently can save as much as 30% or even 50% on total heating, ventilation, and air conditioning (HVAC) energy costs. Just getting acquainted with the specialized language used by air conditioning professionals will go a long way toward making the right decisions when it comes time to improve or replace equipment. This bulletin explains "need to know" information about air conditioning systems in a way that can save money. Bolded terms are listed in the glossary at the end.

MAINTAINING A "COMFORT ZONE"

Lowering the indoor humidity is the key to creating a "comfort zone." Many environmentally conscious business and homeowners have reduced humidity levels, allowing them to raise thermostats slightly while maintaining comfort. In general, the lower the humidity, the more comfortable people feel. By keeping humidity low, it is possible to raise the thermostat setting and still be comfortable. Most people feel just as comfortable at 78 degrees Fahrenheit -- if the relative humidity is 40% -- as they would at 72 degrees with a 70% relative humidity. A healthy indoor environment maintains a relative humidity of no more than 60%, a comfortable level that also prevents the growth of most mold and mildew. Lowering indoor humidity levels allows the thermostat setting to be raised by as much as 4 degrees without a noticeable change in comfort.

Indoor humidity below 40% is difficult to achieve with conventional air conditioning equipment. With humidity below 40%, people may experience dry eyes (especially contact lens wearers) and dry skin. Besides comfort, the most important part of the heat/humidity balance is that you can achieve energy savings of 3% to 15% for each degree the thermostat is raised. That means that reducing high indoor humidity can mean significant dollar savings. Relative humidity can be measured with an inexpensive meter, usually available where thermometers are sold.

Other factors affecting comfort are clothing, activity level, air movement, and radiant temperature. Of course, dressing in light clothing will increase comfort. Areas of high activity such as an exercise room will require a lower thermostat setting for occupants to feel comfortable. Increasing air movement with fans also increases comfort. Also, reducing radiant temperature by limiting the use of ovens and other high temperature appliances, and using blinds, reflective window film, or solar screening also will increase comfort.

VENTILATION

A major concern is keeping outside air infiltration to a minimum while avoiding "sick building syndrome." Sick building syndrome can occur from mold or bacteria growth in a building when humidity regularly exceeds 60%. It also can occur from off gassing of carpeting, copy machines, etc. when there is too little ventilation. The best way to keep indoor humidity low is to keep humid air from leaking into a building. It takes over twice as much energy to dehumidify outside air as it does to cool it, in hot & humid climates such as the Southeastern U.S. and the Caribbean. Over ventilation and/or leakage of outside air can add from 20%-40% or more to the cost of air conditioning.

It is generally agreed that one-third to one-half of a building's air volume should be changed every hour, or that 15 to 20 cubic feet per minute (CFM) of outdoor air per occupant should be brought in through the air conditioning system. In buildings that have been tightly sealed to avoid air leakage, the most effective way of reducing the cooling and dehumidification load on the air conditioning system is through an air-to-air recovery unit also called a fresh air ventilator, or energy recovery ventilator. This brings in fresh outside air and transfers the heat and humidity of this outdoor air to the cool, dry indoor air that is being replaced.

In a typical home, more than 30% of air leakage comes from electrical wall outlets openings and exterior windows and doors. Another 25% of air leakage comes from beneath the home's sole plate, the framing lumber that is put down on the house slab as a base for the walls. Fireplaces, sliding glass doors, and clothes dryer vents also allow hot humid air inside. Caulking and weather-stripping are the most effective ways of reducing air leakage into a house. The best time for caulking the sole plate is when the house is built. The cost of the caulking is quite low and the resulting savings are significant.

AIR CONDITIONING EQUIPMENT

Air conditioners use energy to take heat from a cooler area (indoors) and move it to a warmer area (outdoors). This works against the natural gradient, therefore it requires an energy input. The basic split system air conditioning unit is comprised of an outside unit that expels heat to the outdoors, an indoor unit and a system of duct work that cools the air and brings it into the building, and a thermostat that allows you to control the temperature. Refer to the Air Conditioning Components diagram above. A package system air conditioning unit has all components in one case, usually on the roof.

The compressor and condenser are located outdoors in the condensing unit, along with a fan. The compressor pumps refrigerant through the system. The fan blows air over the condenser coils, transferring heat to the outside air. The evaporator and condensate drain pan are located in the indoors in the air handling unit, along with a blower. The blower moves air over the cold evaporator coils, transferring cool air to the space. The drain pan collects condensed humidity which is piped outdoors. The indoor and outdoor units are connected with refrigerant lines. A "package system" air conditioning unit has all components in one case, usually on the roof.

PREVENTIVE MAINTENANCE

Monitoring the indoor humidity, checking thermostats and setting air registers so they cool each space or rooms comfortably are some of the tasks many owners do themselves. Checking thermostats helps maintain the setting you want and assures that thermostats have not been tampered with. Adjusting air duct supply registers can be as simple as moving a lever that allows more air into a warm room and less air into a cooler room without having to change the thermostat. Diffusers should be adjusted so that cold air is not directed towards return grilles, thermostats, or exhaust fans.

Use a thermometer to measure the temperature of the air at the supply register periodically and let that measurement serve as an indicator of the production and efficiency of your system. A typical supply air temperature should be no higher than about 55oF in a 74oF, 50% relative humidity (rh) room. If the room is warmer than 74oF, the supply air will be correspondingly warmer. If the supply air is too warm, humidity will rise. If the supply air is much colder than 55oF, the supply register may "sweat" and drip water.

Preventative maintenance on your air conditioning equipment should be performed every three months and should include

Changing the air filter in an air conditioning system is one of the simplest and most important maintenance activities. Clean filters make the indoor air fresher and keep dust out of fans, motors and coils. High efficiency pleated filters keep the coil and ductwork cleaner and filter more dust from the air. You can change air filters yourself; check them every month and replace at least every three months. An HVAC technician can give your system a yearly checkup that should include:

It is important to be aware of the type of service being performed on your air conditioning equipment and when. Ask if the procedures listed in the checklist above were performed and if anything unusual was found. Keep a record of service visit dates and what was done.

THERMOSTATS

Thermostats come in two basic varieties: the standard mechanical thermostat and the electronic programmable types. Programmable thermostats will raise the thermostat setting when a space is unoccupied -- saving money -- and then bring the temperature down to a more comfortable setting when occupants arrive. With any type of thermostat, use the "auto-fan" setting to save energy and reduce relative humidity unless the air conditioning system supplies fresh outdoor air. Commercial thermostats can be set to close off outdoor air intakes when the building is unoccupied, helping to reduce humidity.

Basic programmable models cost little more than standard thermostats. More elaborate versions measure radiant temperature, can anticipate set-point changes, and can maintain the set-point to within one-half of one degree. Other features include lockable set point limits, which do not allow the temperature to be set outside a specified range and an occupancy sensor that detects the presence of people and adjusts the temperature up when no one is in the space.

DUCT SYSTEMS

The duct system is often overlooked because it is out of view. This is unfortunate because duct leakage is one of the biggest energy wasters. It is not unusual for 10% or more of the expensive cold air in the ducts to leak into an attic or crawlspace. In addition, there can be significant heat gain if the ducts are not insulated. Duct joints should be carefully inspected for leaks. Leaks should be repaired with mastic or professional grade aluminum foil tape. Duct tape does not last on ducts in attics and is not allowed for repair by most building codes. Ducts should be wrapped with at least R-6 (2.2 inches thick) insulation.

EFFICIENCY RATINGS

Climate controlling equipment is rated according to the energy efficiency ratio (EER), or the seasonal energy efficiency ratio (SEER). The higher the EER or SEER numbers, the more efficient the air conditioner will be and the less it will cost to operate. For window air conditioners, the typical new unit carries an EER rating of about 10, but the most efficient units on the market have EER ratings of 14 to 16. The benefit of higher EER units is that the additional purchase expense will pay for itself in electric savings. A simple, easy-to-use formula for calculating how long it will take for more efficient equipment to pay for itself is below.

The SEER is equal to the total number of British thermal units (Btu's) of cooling divided by the total number of kilowatt-hours used during the cooling season. (SEER= Btu's/kWh) By most building codes, the minimum allowable SEER is 10.0. Units are now available with SEER ratings of 16 or higher.

A close look at SEER ratings is one way to judge air-conditioning equipment when it considering replacement. For example, a new air conditioner with a SEER of 13 will use about 23% less energy than a unit with a SEER rating of 10. A new SEER 16 unit will use 19% less than a SEER 13, and 50% less than an old SEER 8 air conditioner. An older, inefficient unit often can be replaced with a new unit that will pay for itself in only a few years. If maintenance costs for the old unit are high, or if utility rebate programs will help pay for the cost of new equipment, a new unit can pay for itself very quickly.

Comparisons between air conditioning equipment are easy to make. To compare SEER's, use the following formula:  % Savings = New SEER - Old SEER

New SEER

Helpful information for comparison between air conditioning equipment and systems is printed right on the EnergyGuide label. Look for energy use ranges that are printed just beneath the energy efficiency ratio.

PROPER SYSTEM DESIGN

Air conditioning equipment is available in many different sizes for different types of buildings. The term "size" refers to the combined cooling and dehumidification capacities. A cooling load analysis gives the required size of the AC unit based on the largest expected heat gain. To get the most efficient air conditioning system size, you need to know the largest expected heat gain of the space you intend to cool. The air conditioning unit should be sized to meet this load. Depending on what units are available, it is better to slightly undersize. Over sizing, which is common, results in lower operating efficiency due to short cycling, inadequate dehumidification. Here are some advantages of having a properly sized unit installed:

Heat gain is the amount of heat the home will admit in one hour when the outdoor temperature is 92-95įF, or whatever the design temperature is for the location. This heat passes through the walls, roof, windows, and doors; and is in addition to heat generated by equipment and the occupants. Cooling load and equipment size are expressed as Btuh or Tons (based on the heat absorbed by one ton of ice as it melts over 24 hours). One ton equals 12,000 BTUH.

Use the form How To Properly Size The AC Unit, at the end of this bulletin, to roughly estimate the size of the air conditioning equipment needed. Note the number of occupants, the level of insulation, and the type and number of windows when using the form. For example, you may have tinted or coated windows that block out solar heat. In humid climates, humidity accounts for 30%-50% of the cooling load, so it is important to have an accurate load analysis that considers the humidity or latent heat load separately and in detail.

The Sensible Heat Ratio rating, or SHR, indicates how much of the unit's capacity is dedicated to dehumidification. The lower the SHR, the more dehumidification the unit will provide. To determine the SHR needed, the cooling load analysis should include a calculation of what percentage of the cooling load is humidity load. This percentage corresponds to the SHR. For example, if the humidity load is 40%, of the total cooling load, the AC unitís SHR should be 1.0-0.40=0.60 or lower. If the SHR of an AC unit is too high, high humidity will result.

SUMMARY

Air conditioning is a major expense for businesses and homes. Following the suggestions described in this bulletin can get your system operating efficiently can save as much as 30% to 50% on your total heating and cooling bill.

Keeping the humidity low means you can raise the temperature of the thermostat and still feel comfortable while saving electricity. Sealing your home or building by caulking and weather-stripping will help stop air infiltration and reduce humidity.

To assist you in selecting an efficient air conditioning unit, compare EnergyGuide labels and look for the one with the highest SEER/EER. Also, estimate payback period to see which unit will return your investment the soonest.

To save money in both the long run and short run, have a properly sized unit installed. Make sure you pick a licensed, conscientious, and bonded contractor to do a cooling load analysis and pick an air conditioning unit that is closely matched to the cooling load, particularly the SHR and dehumidification (latent cooling) ratings.

Regular maintenance is needed to keep system efficiency high. Preventive maintenance should include making sure all parts of your system are clean, lubricated, and sealed properly and should include a yearly checkup by an HVAC technician.

Programmable thermostats allow you to adjust the settings for periods of occupancy, thereby saving electricity when rooms are empty or at night when the outdoor temperature is cooler.

Sealing ductwork can prevent leakage of cold air into non-conditioned space.

GLOSSARY OF AC TERMS

British Thermal Unit- (BTU) A unit of heat energy. The energy required to increase the temperature of one pound of water by one degree Fahrenheit (F).

condenser - component of an air conditioner (refrigeration cycle) that rejects heat to either the outside air (air-cooled), or a water medium such as a cooling tower (water cooled).

cooling capacity - a rate of heat removal used to describe the amount of heat an air conditioning unit removes per hour. Typical units are BTUH.

cooling season - The months when you run the air conditioner to keep comfortable. In Florida, this is typically April or May through September or October.

cooling load - the amount of heat gain in a space from both outside and inside sources, such as warm, outside air, solar radiation, occupants and cooking.

diffuser - takes the cooled air from the condenser and moves it to spaces that need cooling.

Energy Efficiency Ratio - (EER) is the amount of cooling capacity divided by the number of electrical watts required to make that cooling capacity. More cooling for less input provides a greater EER, and therefore, greater efficiency.

Evaporator Coil - This is where the refrigerant absorbs heat from the air, cooling and dehumidifying the air.

Expansion Value - This is where the refrigerant's temperature is lowered.

Fans - They blow the air cooled by the evaporator coils throughout the living space.

Fresh Air Ventilator - An air-to-air recovery system.

heat exchangers- Air is heated by gas or oil burning then the heated air passes over the air flowing through the air handler.

HVAC - Abbreviation for Heating, Ventilating, and Air Conditioning.

latent heat - This is the energy which produces a change in phase without causing a temperature change.

occupancy sensor - A motion detector device used to determine whether an area is occupied.

payback period - A financial measure that indicates the time it takes for an investment to save an amount of money equal to the cost of investment. This is calculated by dividing the initial cost by the projected first year's savings.

radiant temperature -The flow of energy across an open space via electromagnetic waves such as light.

relative humidity - The ratio of partial pressure of water vapor to the saturation pressure at the same temperature.

Seasonal Energy Efficiency Ratio - (SEER) This is similar to the EER except this number takes into account the improved efficiency of an air conditioning unit when the outside temperature decreases. SEER is the preferred ratio because it is more inclusive. SEER of 14 is excellent.

Sensible heat - energy that produces a change in temperature only, with no change in phase.

ESTIMATING THE COOLING LOAD

This simplified worksheet, adapted from one published by the Association of Home Appliance Manufacturers and Consumer Reports, will help you understand how a cooling load is calculated. An engineering or contractorís calculation MUST be performed before any equipment is purchased or any work is done. It should be much more detailed and more accurate than this simplified worksheet, which is presented here for educational purposes only.

Here, heat gain is the amount of heat a building will admit in one hour when the outdoor temperature is 93o. A cooling load analysis gives the required size of the air conditioning unit based on the largest expected heat gain of the house or building.

Use this simplified 10-step calculation to roughly estimate your cooling load. Round each number to its nearest whole number before entering it into the box, then multiply by the factor in bold. Use the factors in parenthesis if the air conditioner will be used only at night, otherwise use the number in bold. Also if an archway over 4 feet in width connects a room to another or by a door that is open permanently, consider these rooms as one and make the appropriate measurements in these rooms.

1. Heat through windows: Calculate the area (height x width) of each window. Take the measurements in inches, then divide by 144 to determine the square footage. Write down the area of each window for use in step 6. Add the areas, then enter that figure in the appropriate box below and multiply by the factor given.

For Single glass ________ x 14=________

total window area sq. ft.

 

For double glass or glass block _______________ x 7 _________

total window area sq. ft.

2. Walls: Measure the length of all walls, in feet. (height assumed to be 8 feet.) Consider walls shaded by adjacent buildings as facing north. Write the lengths in the appropriate boxes below and multiply by the factors given.

Uninsulated frame construction or masonry up to 8 inches thick

Outside, facing north, shaded, or interior walls _____________ x 30= ____________

total wall length, ft.

Outside, facing other directions ___________ x 60 (30)=____________

total wall length, ft.

Insulated frame construction or masonry more than 8 inches thick

Outside, facing north, shaded, or interior walls _________ x 20=_________

total wall length, ft

Outside, facing other directions _________ 30 (20)=_________

total wall length, ft.

3. Ceiling. Determine the ceiling area (length x width), in square feet. Enter that figure in the box thatís appropriate for

your house. Multiply by the factor given.

Uninsulated, no space above __ ______ x 19 (5)=________

ceiling area sq. ft.

Uninsulated, attic above _________ x 12 (7)=________

ceiling area sq. ft.

Insulated, no space above _________ x 8 (3)=________ ceiling area sq. ft.

Insulated, attic above _________ x 5 (4)=________

ceiling area sq. ft.

Occupied space above _________ x 3=________ ceiling area sq. ft

4. Door and arches: Note: If the room has an archway more than 4 feet wide or a permanently open door, skip this step and go on to step 5. Otherwise, enter the width of the door or archway and multiply by the factor shown.

_________ x 300 (200)=_______ total width, ft.

5. Floor: Note: If the floor is on ground or over a basement, skip this step and move on to step 6. Otherwise, determine the floor area (length x width) in feet and multiply by the factor shown. The floor area is usually the same as the ceiling area.

_________ x 3=________ floor area, sq. ft

6. Sun through windows: Note If the air conditioner will be used only at night, or if all windows in the room face north, skip this step and go on to step 7. Using the measurements you made for step 1, enter the total window area for each wall in the appropriate box and multiply by the adjustment factor that best describes how the windows are shaded.

Wall orientation Adjustment Factors

ALL BUT NORTH no tint or shades shades awnings

_________ x 85 or x 40 or x25 = ________ area, sq. ft.

7. Heat from people: Calculate the heat contributed by people in the room. In the box below, enter the number of people who normally use the room (use a minimum of 2). Multiply the figure by the factor given.

______________ x 600=_______ people in room (min. 2)

8. Heat from Appliances: Add up the wattage of all lights and appliances in the room, not including the air conditioner itself. Multiply the figure by the factor given.

________ x 3=_________ total wattage

9. Total Cooling Load: Add the figures from steps 1 through 8. Enter the sum on the line at the right.

SUM 1 THRU 8= ____________

TOTAL

This number is the approximate total cooling load in BTUH your air conditioner would need to handle to work effectively. Typically, 30-40% of this capacity should be dedicated to dehumidification.

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