Efficient Cooling and Heating in a Hot Humid Climate

Claudette Reichel  |  6/25/2008 10:32:35 PM

ENERGY STAR labeled cooling and heating equipment: Cooling and heating equipment with ENERGY STAR labels are more efficient than the government minimum standard, yet cost-effective. Visit www.energystar.gov for more information and current standards, since they change with technology improvements. In the Deep South, a high efficiency heater is less important than a high SEER air conditioner, especially in a well-insulated and tight home.

SEER and EER: Energy Efficiency Ratios show how readily air conditioners and heat pumps convert electricity into cooling. The higher the number, the more energy-efficient the unit. Seasonal Energy Efficiency Ratios (SEER) reflect energy-efficiency over the normal cooling season. As technology advances, cost-effective SEER tends to rise. SEER 10 was the minimum allowed to be produced, but it has been raised to SEER 13. Qualified ENERGY STAR labeled central units are at least SEER 14; geothermal heat pumps are at least EER 14.1. The indoor and outdoor units of a packages system must be matched to achieve the rated SEER.

AFUE: Annual Fuel Utilization Efficiency shows the average annual efficiency at which a gas (or other fuel) furnace operates, usually as a percentage. At this time, the minimum manufactured is AFUE 78. ENERGY STAR qualified gas furnaces have an AFUE of 90 or higher, and provide the added benefit of direct vent and sealed combustion safety (no risk of backdrafting).

HSPF: Heating Season Performance Factor measures the average number of BTU’s (British Thermal Units) of heating delivered for every watt-hour of electricity used by a heat pump. It takes into account the different conditions the heater may experience during the heating season. ENERGY STAR qualified heat pumps have an HSPF of 8.0 or higher.

COP: Coefficient of Performance measures how many units of heating or cooling are delivered for every unit of electricity used in a heat pump or air conditioner. Other ratings are generally used instead of the COP, except for geothermal heat pumps because they operate in more stable conditions underground. A COP of 3.0 is roughly equivalent in performance to an HSPF of 10.2.

Alternatives with special advantage: HVAC equipment alternatives that have additional advantages for southern homeowners include the following:

  • Geothermal heat pumps with integrated water heating can have a substantially lower life cycle cost despite a higher initial cost and are highly resource efficient. Unlike air source heat pumps, they use fluid-filled pipes buried beneath the earth or in a body of water to take advantage of more stable temperatures and the renewable heating and cooling energy stored there. When properly sized and installed, geothermal heat pumps are among the most efficient and comfortable systems available. They can reduce cooling and heating costs compared to an air source heat pump and heat water efficiently. Geothermal systems are especially cost-effective in homes with large cooling and heating loads.
  • A variable capacity air conditioner or heat pump provides added humidity control. It changes operating capacity to fit the cooling load, providing dehumidification in mild temperature seasons. It also may have a high SEER rating. The energy savings and humidity control in a single system can make the 20% to 30% higher initial cost worthwhile.
  • Supplemental dehumidification and ventilation systems offer superior air quality and comfort. Fresh air is essential for indoor air quality, but should be controlled and filtered for optimal benefit. The more humid the climate and energy-efficient the home, the greater the need for dehumidification. Supplemental ENERGY STAR whole house dehumidifiers allow homeowners to choose a relative humidity setting and efficiently control humidity even when cooling is not needed.
  • An integrated gas space and water heating system can be an efficient and economical option in an energy-efficient home that needs little heating capacity. In such a home, the smallest available heating system may be oversized. A high efficiency gas water heater can provide both hot water and space heating by pumping hot water to a heating coil in the air handling unit. A gas tankless water heater offers higher efficiency and lower cost for both space and water heating.

Manual J sizing: Energy-efficient homes require less heating and cooling than standard homes, so smaller capacity HVAC systems should be installed, saving money upfront and throughout the life of the equipment. Oversizing, based on a general rule for standard homes, is common. Properly sized equipment will operate more efficiently; provide better dehumidification and comfort; last longer; and cost less to buy. An oversized air conditioner cools, but will cycle on too briefly to remove much moisture from the air.

Manual J sizing refers to a publication by the Air Conditioning Contractors of America (ACCA) that gives load calculations for residential heating and air conditioning based on many details about the home and its efficiency features. It is required by the IRC and energy code.

Specify in your home building or HVAC replacement contract that you want a copy of your contractor’s Manual J work and keep it in your home records. This will be useful both to ensure that your system is right-sized for your house now and will save work in sizing replacement units after future home improvements.

Sensible Heat Ratio (SHR): The SHR is a measure of the portion of the cooling capacity that reduces indoor temperature (sensible heat removal) in certain conditions. The remaining portion is for dehumidification (latent heat removal). Some high SEER air conditioners may not provide sufficient dehumidification for this climate, so it’s important to check the SHR on the equipment specifications sheet. Look for an SHR of 0.7 or less, the lower the better for dehumidification.

Zoned heating/cooling: Large and multi-story homes typically will benefit from using two or more separate HVAC systems or installing zone control systems with separate thermostats that control dampers and air flow to different areas of the house so they can be conditioned at different temperatures. It can save energy without sacrificing comfort to have separate settings for night and day zones and different levels of multi-story homes.

Programmable thermostats automatically adjust the temperature setting on a user selectable schedule. If used properly, a programmed thermostat can save energy by automatically changing the setting to reduce cooling or heating at times when no one is home or when sleeping. Be sure to select a programmable thermostat that is easy to understand and set, and is designed for the particular equipment it will be controlling, especially with heat pumps. They should be centrally located within the house or zone. They should not receive direct sunlight or be near a heat-producing appliance. A recommended location is 4 to 5 feet above the floor in an interior hallway near a return air grille.

Outdoor thermostat on heat pump: Make sure the heat pump selected has an outdoor thermostat. It prevents electrical strip heat from coming on if the outside condition is above a selected temperature, normally where the heat pump can no longer meet the heating load. This allows for nighttime setback without the heat pump thinking it can’t meet the load when the thermostat setting is raised in the morning. If the heat pump thinks it can’t meet the load, it calls for auxiliary heat - which often is inefficient electric strip heat.

Heat pump with gas back-up: A dual-fuel or piggyback heat pump system heats with natural gas or propane (instead of an inefficient electric strip heater) when the outdoor temperature drops low enough to need auxiliary heat. The colder the climate and more inefficient the building, the more cost-effective it is for a high efficiency gas back-up furnace.

Minimized duct losses: Typical leaky duct systems in attics waste about 30% of the cooling and heating of a home. Improving the efficiency of ductwork can be the single most important energy-saving measure for many affordable homes. Leaky ductwork can waste hundreds of dollars each year, cause poor indoor air quality and lead to mold problems. The best way to prevent losses is to locate the air handler and ductwork completely within conditioned space.

Locate in conditioned space. With today’s higher ceiling height trends and some thoughtful design, ductwork can be routed in interior soffits and dropped ceilings below the true insulated and sealed ceiling. It may be installed between floors sealed from the outside; in or within an unvented, semi-conditioned, cathedralized attic (with spray foam insulation under the roof or rigid insulation over the roof decking). Ducts in conditioned spaces still need to be properly sealed and insulated to avoid condensation and provide balanced air flow but can generally be downsized

Or, seal and insulate well. All joints and connections of ducts in unconditioned space should be sealed with UL 181 approved mastic, a thick paste that solidifies, but remains elastic to provide a durable seal, or special foil tape strictly following correct installation instructions. Regular “duct tape” quickly fails and should never be used to seal ductwork. Ample insulation (R-8 recommended) should be installed on the ductwork to reduce heat transfer, but it does not stop air leakage. The return air path should be a sealed duct (preferable) or an airtight plenum. After installation, it’s a good practice to have the duct system tested to measure leakage and balanced with an air flow hood. Expect less than 5% leakage.

Or, use a ductless HVAC sytem. In ductless (mini-split) systems, there is usually only one outdoor unit serving multiple indoor units (each containing a refrigerant coil and blower). Refrigerant is piped from the outdoor unit through small-diameter insulated lines directly to individual rooms or zones. Cooled air is blown into the room by a fan in the individual evaporator units. The term "mini" is used to describe the small indoor units located in each room or zone. Because they do not rely on leaky ductwork, mini-split systems can boost energy efficiency. The ability to control each zone separately can also contribute to energy efficiency.

Manual D and balanced airflow. Manual D refers to another publication by the ACCA for designing duct systems. Although it is seldom used in practice, its use can improve both system performance and comfort. Actual airflow for each duct run should be measured and balanced to comply with Manual D specifications. Multiple air returns or air transfer grilles are recommended for each bedroom. (See Healthy/Ventilation section)

Duct layout. An efficient duct layout reduces losses, air resistance and costs. There should be one or more central supply trunks with multiple take-offs. An “octopus” layout with all duct runs originating at the air handler is generally inefficient because it usually results in more duct surface area. (In addition, in a well-insulated home with high performance windows, it is not necessary for ductwork to extend to the exterior walls.) This can save both labor and material costs (as well as save energy), offsetting the higher cost of better windows.

Other Efficient HVAC Equipment (see Healthy/IAQ for Ventilation): Select ventilation and dehumidification equipment that are both effective and energy efficient.

ENERGY STAR dehumidifier: A dehumidifier uses energy, but an efficient dehumidifier in an energy-efficient home can increase comfort when cooling is not needed and even reduce the amount of air conditioning. When an air conditioner does not provide enough dehumidification during mild weather, people often lower the thermostat setting to a lower than needed temperature in an attempt to reduce humidity. Using an ENERGY STAR dehumidifier is a more energy-efficient and comfortable solution. A portable standard capacity model can be located (with a water drain line) in a central area, in a closet with louvered doors, or possibly in the return air plenum (if allowed) to provide extra dehumidification as needed. “Whole House” dehumidifiers offer higher energy efficiency and capacity for optimal dehumidification. They are typically installed in the attic and deliver air through the central air supply ducts.

ENERGY STAR ceiling fans: ENERGY STAR ceiling fans are more energy efficient than standard ceiling fans. Ceiling fans help cool by increasing air movement and thus aiding the body’s evaporative cooling system. The thermostat may be raised a few degrees when ceiling fans are used, but fans should be turned off when the room is not occupied.

ENERGY STAR quiet exhaust fans: Kitchens and bathrooms should have energy-efficient exhaust fans ducted to the exterior (not into attic) to remove excess moisture that is produced in these rooms. Low noise exhaust fans are more likely to be used when needed. The level of noise for a fan is rated by sones. High quality models are often below 0.5 sones. A timer or humidity sensor prevents exhaust fans from being left on longer than needed.

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