Home Cooling |
| -------------------------------------------------------------------------------- HOME COOLING Understanding how your house is affected by different types of heat gain will help you take steps to reduce heat without using mechanical cooling. Heat is absorbed from the sun’s rays, which in summer are almost directly overhead (see Figure 1). Heat also comes from warm, moist outdoor air entering the house through tiny cracks around windows and doors, numerous other small openings, and the foundation and other porous materials. Finally, heat is generated inside the home by people, appliances, lighting, cooking, and bathing. Reducing solar heat gain. Strategic planting of deciduous trees significantly reduces heat gain from the sun’s rays. Give first priority to planting shade trees due west of west-facing windows; planting shade trees east of east-facing windows is second priority. Also, installing awnings, sunscreens, or overhangs can reduce heat gain by as much as 90 percent while still letting in light. Keeping shades and curtains closed on the sunny side of the house also reduces heat gain. Reducing infiltration. A good way to understand cooling is to think about how your house works in winter. The same measures that keep the cold out and the heat in during a January cold snap do the reverse in an August heat wave. Weatherizing measures to reduce air flow in and out of the house are fundamental conservation measures in any season. These include insulating, caulking, and weatherstripping. Keeping windows closed during the day (and opening at night to take advantage of cool breezes) also reduces heat gain, as well as leaving storm windows or plastic coverings on windows that do not need to be opened (the extra insulation helps keep the heat outside). Reducing indoor heat generation. Some easy cooling steps that cost little or nothing and bring immediate results are to: Schedule heat-producing tasks like baking and vacuuming during the cooler evening or morning hours. Use covered electric frypans, microwave ovens, or similar small appliances, rather than the oven, for cooking. Go easy on hot water — it produces both heat and humidity. Use kitchen and bathroom exhaust fans when cooking or bathing to remove unwanted moisture quickly. (See sidebar, Combustion Air) Reduce the use of artificial lighting (especially incandescent) because lights produce heat. Avoid using the dry cycle on your automatic dishwasher; allow dishes to air-dry instead. Make sure your clothes dryer is vented to the outside. Increase natural ventilation in the attic by opening attic windows or louvers. Drink plenty of cool liquids — they really do help keep you cool. All of the above measures to reduce heat — from the sun’s rays, air infiltration, or indoor activities — can be applied even if you have air conditioning, since they will cut operating costs. Portable window fan. For about $30 you can create cross ventilation with a portable window fan, making this an excellent and inexpensive cooling method, especially during the night or after a rain storm when the outside air is cooler than indoors. Place the fan in a window on the coolest side of the house, using it to pull cool air in and push hot air out through open windows on the warm side of the house. When used correctly, window fans can cool several rooms at once. One caution: using a fan to push air out of the house can create negative air pressure in the house, leading to backdrafting. (See sidebar, Combustion Air) A ceiling fan is quieter than a portable fan and is safely out of the reach of children. When used along with air conditioning, ceiling fans circulate the cooled air effectively, making the space feel cooler. This may allow you to set your thermostat a few degrees higher, thereby saving energy and money without sacrificing comfort. The table below will help you choose the correct size ceiling fan. Fans should have multiple speed settings so that air flow can be reduced at lower temperatures. A whole-house fan is no longer recommended. Cutting a hole in your ceiling to install the fan often creates another “attic bypass†or source of heat loss in the winter, since it is very hard to seal around the fan. Whole-house fans also may create negative pressure in your house which can cause harmful backdrafting of appliances. (See sidebar, Combustion Air) Attic fan. Using an attic fan (which is not the same as a whole house fan) to cool your house is of doubtful value. The fact is, there should not be any active relationship between the attic air and the air in your living space. It is true that adding an attic fan will help pump hot air out of the attic, lowering the attic temperature and perhaps reducing heat in the top floor of the home; however, because most homes have some attic bypasses (openings through which air from the living space leaks up into the attic), the fan will pull cool air through the insulation, which could lead to backdrafting. (See sidebar, Combustion Air) Although attic fans are not generally recommended, attic vents are recommended. They help keep the attic cooler above the insulation and reduce moisture in the winter. If your attic has a vapor barrier, one square foot of outside ventilation should be provided for each 300 square feet of attic area. If there is no vapor barrier and the roof has less than a three-foot rise from eave to peak, one square foot of ventilation is needed for each 150 square feet of attic floor space. Room air conditioners Those who want more cooling than the above measures provide will look to air conditioning. One option is to purchase a room air conditioner to fit in a window or wall. Sizing. Oversizing is the most common mistake made by shoppers, who think that “bigger is better.†In fact, buying too large a unit is not only expensive, it can increase discomfort by not removing enough humidity from the air, leaving you feeling cold and clammy. An air conditioner’s primary tasks are to cool and dehumidify, but a typical unit is much more efficient at cooling. Since the major control in an air conditioner is a thermostat, not a humidistat, the unit comes on and shuts off in response to air temperature, regardless of humidity level. A system that is too large often achieves the desired temperature before the humidity is adequately removed. If a system is too small it may dehumidify well, but not cool the air sufficiently. In a properly sized unit, the operating cycle should be long enough to remove heat and humidity. Air conditioners are sized according to Btus of heat removed per hour, or in “tons†of refrigeration, with one ton equaling 12,000 Btus per hour. The load on the unit determines the size you need. The load has two parts: the energy it takes to cool the air (sensible load) and the energy required to dehumidify (latent load). Together, the sensible and latent loads total 100 percent of the air conditioner’s load. Your first step is to carefully determine the size of the area to be cooled, making sure to close off unused rooms or areas where cooling isn’t necessary. As a rule of thumb, based on size alone, an air conditioner should have 10–15 Btus for each square foot of living space. Keep in mind, however, that this formula is simply a rule of thumb. A number of factors, such as the amount of shade around your house, window area, amount of insulation, or the size of your family means that you may have to go one size larger or smaller. Efficiency. All new room air conditioners are required by law to carry an Energy Guide label showing the energy efficiency ratio, or EER. This bright yellow label lists the EER and compares it to the EER of models with similar features. The higher the number, the more efficient the appliance. The Energy Information Center recommends purchasing a model with an EER of 11 or higher and an Energy Star labeled appliance. The Energy Guide label also lists the average yearly operating cost, based on average electric rates. Wiring requirements. Since air conditioners consume large amounts of electricity, they may require too heavy a load for some circuits. Make sure the unit you buy will not cause an overload. In some cases you may need a special circuit with a separate fuse. Newer homes built to newer building codes have at least 100 ampere, 220 volt service, but many older homes have only 30 ampere, 110 volt service. Be sure to ask an electrician about the adequacy of your home wiring. Installation. Always follow the manufacturer’s instructions, since each unit has its own specific installation requirements. Location is also a key factor in operating efficiency. Ideally, window units should be placed in the middle of the area to be cooled and on the north or shady side of the house. Do not obstruct the free flow of air around the unit. Operating tips for maximum efficiency include: • Use a table or wall thermometer as a guide in selecting the air conditioner setting (room air conditioners don’t come with thermostats calibrated by degrees). Keeping the indoor temperature within 17 to 20 degrees of the outdoor temperature on extremely hot days is important. You will be cool and at the same time save. Central cooling Central air conditioners are designed to cool, dehumidify, and filter the air throughout your house. Generally, these systems are more expensive and use more energy than room air conditioners, but the costs can be minimized by following the same guidelines as for room units: buy the right size, choose a high efficiency unit, locate the unit properly, and maintain it well. Sizing. Central air equipment is usually purchased through a heating/air conditioning contractor who is responsible for calculating what size unit you need. Ask your contractor if he or she is using the sizing guidelines established by the Air Conditioning Contractors of America or the American Society of Heating, Refrigeration and Air Conditioning Engineering, Inc. (ASHRAE). Make sure the following factors are considered in the sizing calculations: Size of area to be cooled.Amount of insulation in the attic and walls. Tightness of the home: very leaky, average, or very tight.Window area, particularly on the south, west, and east sides of the house. The larger the window area, the greater the solar heat gain. Also Figure 2. A ductless system provides quiet room cooling with wall mounted units and an outdoor condenser. consider whether the windows have double panes, low-e coatings, and other features that enhance efficiency.Orientation of the house. A sunny exposure requires a bigger unit, especially if the windows are not shaded. Amount of shade. Trees, fences, shrubs, and other landscaping features, as well as window shades, may reduce heat gain. Family size and lifestyle. Larger families release more heat energy into the house from body heat and activities such as showers and cooking. Also, make sure your house has the proper wiring. Central air conditioning requires a separate circuit and is operated on 220 volts. You should obtain bids from at least three contractors, not only to compare costs, but also to make sure that you are getting the right size unit. The size estimations should be fairly close; if they are not, find out why. Efficiency. After you determine the proper size, compare efficiency ratings of different models with the same capacity. Even though the more efficient unit may be higher priced, it is usually the best buy because high efficiency units cost less to operate. Central air conditioners are rated according to their Seasonal Energy Efficiency Ratio, or SEER. The Energy Guide label lists the SEER and compares it to SEERs of similar models; it does not, however, list an annual estimated operating cost. The Energy Information Center recommends buying a central air system with a SEER of 13 or higher and an Energy Star labeled appliance. Installation. The installer should always follow the manufacturer’s instructions. Location is also important. If possible, place the condenser on the north or shady side of the house. Always make sure to allow for plenty of air circulation around the condenser. Operation and maintenance. To improve efficiency and keep equipment running smoothly, follow these guidelines: Replace or clean filters monthly. Remove dirt and dust from outside coils and fins.Keep all return and supply registers clean and unobstructed by furniture and draperies. Install a setback thermostat that will turn the air conditioner on one hour before you come home each day. Have a qualified service person tune-up the system at the same time as the furnace tune-up is done. Types of central air systems Described below are the major types of central cooling systems used in this area. They provide the same benefits, but differ in configuration. The traditional type, generally costing around $1,600, uses the same ducts and registers as a forced air heating system. The ductless systems range in cost from $2,500 to $5,000 installed and are basically a combination of room and central cooling, offering an alternative for homes without ducts and registers. A third type, heat pumps, costs $2,000 or more and both cools and heats the home through a two-way heat transfer system using air, water, or ground. Ground-source heat pumps use the ground as the heat source/heat sink, depending on the season, and operate with great efficiency. Although the purchase price is high, these pumps may be cost effective in many installations. Traditional, or forced air system, is the most common in our area and the likely choice if you have a forced air heating system. The condenser unit is outside, while the evaporator (the cooling component) is installed in the furnace. The same duct work used for heating is used to distribute cool air throughout the home. (Home owners should be aware that leaking air ducts affect efficient distribution of cooled air, as well as pose a potential backdrafting problem. See sidebar on Combustion Air). Ductless systems are appropriate for homes with hot water or steam heat or those divided into separate areas (see Figure 2.) This type of system distributes cool air to selected portions of the home without the need for ductwork. Instead, small diameter tubes (5/8 to 7/8 inch) run between the outdoor condenser and the indoor wall-mounted unit, forming a closed loop system. The cooling component(s) is mounted on the wall of one, two, Combustion Air (Con’t) Central air conditioning, forced-air systems, also present a potential backdrafting problem. Leaks in return air ducts have been identified as a cause of backdrafting year-round, since they add to air depressurization in the basement. In addition, they lead to inefficient and ineffective cooling. This is how it works: When the air conditioner runs, cool air leaks out of supply ducts and sinks to the lowest level of the house. In addition, leaks in return ducts draw cool air back to the central air conditioner coil before the cool air ever reaches the upper floors. The result of both supply and return duct leaks is a cold basement and difficulty in keeping the rest of the house —especially the upper floor — sufficiently cool. This, in turn, leads to the air conditioner being run longer, resulting finally in higher energy costs. For more information on combustion air problems and leaking air ducts, call the Energy Information Center and ask for the publications Combustion and Makeup Air and Duct Sealing. The CFC Dilemma CFCs (chlorinated fluorocarbons), a commonly used refrigerant, have been identified as a major factor in depleting the earth’s protective ozone layer. By international agreement, CFCs are being completely phased out. Alternative refrigerants comparable to CFCs in safety, cost, and efficiency, but without their harmful effects, are available in some models. Ask your dealer for details on the air conditioner you are considering. or three rooms and resembles a room air conditioner, but is much quieter. The condenser is installed outdoors, like that of any central air conditioner. Some ductless systems will support multiple terminals having a total cooling capacity equal to traditional forced air systems. An advantage of the ductless system is the easy ability to “zone†or cool only those areas in the home you are using. Other systems require additional investment for the ability to zone. A high velocity system is another option, especially attractive to those who want central air conditioning but do not have a forced air distribution system. The high velocity system uses smaller ductwork, sometimes flexible insulated duct. Main trunks may be only six inches in diameter, with delivery ducts possibly only three inches in diameter. The ducts are supplied with air from an air handler using a higher velocity fan. The air conditioning cooling coil is installed within the air handler. The smaller flexible duct allows for smaller openings in walls and ceilings. Proper sizing of cooling equipment is always important, but it is critical with high velocity equipment. Heat pumps in general can be used successfully in Minnesota — with certain conditions. A heat pump looks and operates like a central air conditioner, but it can be used for heating as well as cooling. During the heating season, the heat pump extracts heat from the outside air (or from a water supply or the ground) and brings the heat into the house where it is distributed through a duct system. In the summer, the heat pump cools the house by moving heat from the inside to the outside air (or water or ground) using a basic closed loop refrigeration cycle. In Minnesota, air source heat pumps are used for whole house cooling, but must be supplemented by another heating system during the winter. Currently, the most effective type of heat pump for our climate is the ground-source system which uses the earth as its heat source in the winter and as a heat sink in the summer. Ten Common Questio “Super Saver Switch†and similar programs have become popular among residential electric customers using air conditioning. These programs offer customers substantial discounts for allowing the utility to control their air conditioners during periods of high demand. The utility cycles the condenser off and on in a manner that allows for comfort and at the same time alleviates pressure on the utility’s generation capacity. Contact your utility about the availability of such a program. One caution is needed for homes in which the central air conditioner uses the same duct system as the heating system. For those who do not have a sealed combustion furnace, the combustion air supply for the furnace is often brought into the home through the cold air return line. When the condenser is shut off by the utility, the furnace fan, which brings in the combustion air supply, continues to run. As a result, warm moist air is drawn in through the combustion air supply, increasing temperature and humidity in the home. Consumers with this arrangement should consider changing the combustion air supply system to one which brings the air to the furnace area. The Home Energy Guide, Combustion Air, describes how this can be done. Energy Information Center Twin Cities: 651-296-5175 TTY: 651-297-3067 Statewide toll free: 1-800-657-3710 E-mail: energy.info@state.mn.us This information will be made available, upon request, in alternative formats such as large print, Braille, cassette tape, CD-ROM. This publication was produced with funds from a U.S. Department of Energy State Energy Program grant. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author and do not necessarily reflect the views of the Department of Energy..Minnesota Home Energy Guides This guide is one in a series of publications designed to help Minnesotans save energy in their homes. Copies of the titles listed below are available by calling or contacting the Minnesota Department of Commerce. CD-ROM contains all of the Home Energy Guides as well as several other publications of interest to homeowners, builders and contractors. Appliances advises consumers on what to look for in energy efficient appliances and includes information on efficient operation and maintenance of refrigerators, freezers, washers, dryers, dishwashers, cooktops, ovens, and home office equipment. Attic Bypasses explains how to find those “hidden air passageways†and fix them to prevent costly heat loss and damage to roofs, ceilings, walls, and insulation. Basement Insulation discusses the pros and cons of interior vs. exterior insulation and provides detailed how-to instructions. Caulking and Weatherstripping describes how to identify sources of air leaks, lists various types of caulk and weatherstripping, and provides illustrated how-to-apply instructions. Combustion & Makeup Air describes the causes of dangerous combustion air problems and tells how to install an outside combustion makeup air supply. It also tells how to test your home for combustion air problems. Energy Saving Landscapes describes how to use trees and shrubs for long-term energy savings, and lists trees appropriate for energy-savings. Home Cooling tells you how to cool without air conditioning, and provides information on buying and operating energy efficient air conditioners. Home Heating describes proper maintenance techniques and helps you become an educated shopper if you are buying a new heating system. Home Insulation helps the homeowner evaluate the benefit of added insulation, providing information on buying and installing insulation. Home Lighting looks at new technologies for residential lighting, identifying four basic strategies and providing examples for putting them into practice. Home Moisture describes symptoms of moisture problems, lists common indoor and outdoor causes, and discusses preventive and corrective measures. Indoor Ventilation describes the types of home mechanical ventilation systems that are available, the amount of ventilation air needed, and how best to operate and maintain the system. Low Cost/No Cost addresses the often overlooked energy saving tips for all areas of your home. New Homes discusses a wide range of options for increasing energy efficiency beyond the normal building code requirements. Subjects covered include insulation, ventilation, air-vapor controls, heating and cooling, windows, doors, and appliances. Water Heaters helps you determine whether to buy a new water heater or improve the old one. It explains the efficiency of different types of water heaters and provides installation tips. Windows and Doors helps you decide whether to replace or repair windows or doors and gives a good summary of energy efficient replacement options. Wood Heat offers advice on purchasing and installing a wood stove, with special emphasis on safety. |
Author Minnesota Home Energy Guide
