A ToolBase TechNote
Dramatic improvements in the energy efficient design of homes have been made over the past 50 years. Homes have evolved from being built to "breathe" with virtually no insulation and leaky, single pane windows to being the well-insulated, tightly-constructed homes of today. Along with the increase in insulation and air sealing measures (and, often, mechanical ventilation) comes a need for careful design of HVAC systems. Efficiency measures have made such an impact on HVAC design requirements that traditional simplified estimating methods (such as tons per square feet) can actually be harmful to home performance. Careful design can prevent under- or oversized equipment, which can shorten equipment life or create uncomfortable conditions in a home.
Careful design is especially important with cooling systems-which often have traditionally been sized by number of outlets or tons per square feet. Oversized cooling systems can result in poor dehumidification-although the system lowers the temperature quickly, it does not run long enough to dehumidify the home. Experts agree that abnormally high indoor humidity conditions contribute to poor air quality and may create conditions for mold growth. This is especially true in the humid regions of the U.S.
All New Homes Require Careful HVAC Design
There are several important design principles that are essential for a home designed to "push the envelope" of energy efficiency. However, these principles are nearly identical for any home, regardless of design or energy efficiency standard. To understand what is needed for a super energy efficient home, first consider what is important for all homes. Instead of relying on tradition, an HVAC contractor should calculate the heating and cooling loads for each room using an approved method, such as the Air Conditioning Contractor of America's (ACCA) Manual J. A load calculation determines the amount of both sensible1 and latent cooling needed on the hottest summer day (design day). The calculations are based upon the amount of solar gain through windows, heat transmitted through walls, and heat exchanged through air leaking into the house.
To assist the designer, the builder should communicate clearly the R-values and type of material used in walls and ceilings and the specifications of windows (usually provided by the manufacturer or the NFRC label on the window). Overhangs and house orientation (North, South, etc.) are also important. Describing materials used in a new home in general terms such as; "R-19 wall insulation with low-e glass" windows may not provide adequate information for an accurate load calculation.
The HVAC designer then uses the Manual J load calculations to determine sensible and latent loads, when added together equals, total capacity (e.g., a 3-ton system). The ratio of sensible to total load is called the sensible heat ratio. Dehumidification is accomplished by the latent capacity of the equipment. In new, well-insulated homes, latent load tends to make up a larger portion of total capacity than it does in older homes. Since manufacturer's equipment is designed for a predetermined sensible heat ratio, designers are often faced with difficulty finding equipment that meets their design calculations. Using only overall capacity (tons) when comparing HVAC equipment may lead to a bad equipment selection decision. Today's savvy designer will select equipment that matches or slightly exceeds the design latent load and will allow slight undersizing of the sensible capacity. In this manner, good dehumidification can take place and provide sufficient cooling comfort.
After load calculations and equipment selection, the designer should focus on duct design. The best duct system will be sealed with approved sealing materials (typically mastic or aluminum tape) to limit air leakage, duct length and the number of bends will be minimized, and ducts will be installed within the conditioned envelope of the home. Return grille sizes and locations are important along with correct type and number of supply diffusers to ensure proper air circulation. Do not ask a mechanical contractor to reduce the number of supply outlets or returns: this is a recipe for comfort problems.
It is important to give homeowners information about the proper maintenance and operation of their air conditioning equipment for optimal performance. Information should include a schedule for servicing and cleaning. Cleaning is especially important for high efficiency equipment (12 SEER and above) because the coil fins are more tightly spaced than in standard equipment and will therefore collect airborne dust more rapidly. A dirty indoor coil reduces efficiency--in some cases dramatically. To help with keeping equipment clean and at top efficiency, homeowners should use a high quality air filter.
Combining High Efficiency Equipment and Exceeding Energy Code Requirements
As energy prices continue to rise, more homes are being built beyond energy code requirements. Proper HVAC design is important for homes built simply to code; it is paramount for a home that exceeds energy code requirements. A very efficient home, often because of interior latent loads (e.g., showering, cooking, cleaning, etc.) and/or mechanical ventilation, can require dehumidification when there is no need for air conditioning. If a designer cannot find standard equipment to match the sensible heat ratio, there are technologies that will assure comfort including equipment upgrades, control systems, dehumidifiers, and air exchange systems. When designing systems for very efficient homes, the focus should be on the total system including proper equipment selection, air tight and well-designed ducts, air distribution that circulates air properly, equipment efficiency (rated by SEER), and controls.
Manufacturers have produced equipment for efficient homes for years; this is good news, because the type of equipment needed is proven technology and readily available. Technologies for efficiency include variable speed air handlers, two speed compressors, and programmable thermostat/humidistat controls. These technologies work together to squeeze the maximum amount of moisture from the air, often with lower energy cost. For example, variable speed air handlers have the ability to move air at reduced speeds over the air conditioner coils, thereby increasing moisture removal.
Some programmable thermostats will start the compressor prior to the air handler fan operation, thus allowing the coils to start dehumidifying early in the cooling cycle. Two speed compressors allow the air conditioner to operate at higher efficiency and lower energy costs during most of the cooling days. The higher speed (which operates at lower efficiency) is used only during the hottest days. Since air conditioners are sized for design days, which are relatively rare, a two-speed compressor optimizes performance for both hot and mild summer days and eliminates problems associated with using oversized equipment.
Other technologies are available that will reduce humidity levels in very energy-efficient homes during periods when air conditioning is not needed, such as energy recovery ventilators (ERV), and dehumidifiers. ERVs are similar to heat recovery ventilators (HRVs) which preheat incoming ventilation air with outgoing conditioned air. ERVs take the efficiency a step higher by recovering latent and sensible energy from the airstream. Dehumidification equipment is available for incorporation into the overall HVAC system and can be controlled by a programmable thermostat/humidistat. A dehumidifier that incorporates fresh air exchange is less expensive than an ERV system to install, but does not offer energy recovery.
Commercial cooling systems often have desiccant systems to dehumidify the conditioning air. Desiccants are materials that attract moisture and can be dried or regenerated by adding heat supplied by natural gas, waste heat, or the sun. Currently, there are no mass produced commercially-available desiccant systems for residential applications, but efforts are underway to develop this technology. The promise for desiccant systems is that they are more economical to operate than conventional dehumidifiers.
Conclusion
Moisture control is a critical issue relating to air conditioning design. For any new home, a properly designed HVAC system is important for proper moisture control. The tighter and more energy efficient the house, the more important it is to avoid oversizing and provide good latent heat and moisture removal. Find a contractor who follows guidelines set by the associations relating to mechanical system design (listed below). Finally, make sure the homebuyer understands the HVAC system and the need for regular service.
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