Things to Consider When Selecting Your Environmental Control System
Home owners/buyers should be aware that humidity control (or moisture removal) is only a secondary benefit of standard air conditioning (i.e. moisture is only removed when the thermostat is calling for cooling). Although currently accepted HVAC load estimating methodologies attempt to maximize the moisture removal capabilities of standard air conditioning, it does not ensure satisfactory results, particularly in warm humid climates. Humidity control can be improved with controls, the use of variable speed air handlers and the use of multi-stage compressor systems. Additionally, whole-house dehumidification systems can be installed to precisely control humidity independent of the air conditioning system. Those with mold allergies, asthma or other respiratory concerns should be particularly aware of the benefits of interior humidity control.
The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) recommends that residential structures be provided with active mechanical ventilation. Guidelines are set forth in ASHRAE Standard 62-2 2003 and are based on the size of the home and number of bedrooms. Although not currently required by code, mechanical fresh air ventilation is particularly important in today’s tighter homes to purge contaminants created by occupants, pets, cooking, etc. and is one of the three key elements for improved indoor air quality (humidity control, fresh air ventilation, filtration).
Major advances have been made in the area of filtration, both media-type and electronic. Combined with humidity control and fresh air ventilation, high efficiency filtration is the third key element to improved indoor air quality. Those with allergies, asthma or other respiratory concerns should be particularly aware of the benefits of high efficiency filtration.
High efficiency filtration can be counterproductive, however, if the duct system is not taken into account. For example, if a homeowner installs a pleated filter (readily available and marketed to improve air quality) at the return filter grille, but the duct system is in poor condition, air quality can, and usually does, go down. Reason: The air handler has to work much harder to pull air through a pleated filter – since air will take the path of least resistance, air is readily pulled into the duct system through leaks. If the duct system is located in an unconditioned attic, or worse, a crawl space, the amount of “bad” air being introduced into the house is increased.
Ultraviolet (UV) Lights
UV technology offers an additional defense against microbial contamination by irradiating the evaporator coil located within the air handling portion of an air conditioner. Constant irradiation of the evaporator coil significantly limits the propagation of microbial activity on the wet coil and surrounding cabinet resulting in a cleaner air handler and overall improved indoor air quality. Additional benefits are realized as the air is irradiated as it passes by the UV lamps.
Controls for your interior environment control system can range from standard thermostats to programmable IAQ controls. IAQ controls can be effective in improving humidity levels even with standard air conditioning, but are particularly effective when combined with multi-stage compressor systems and/or whole house ventilating dehumidifiers.
Spray Foam Insulation
In addition to keeping your home energy efficient, various insulating strategies can directly impact your ability to aggressively maintain indoor air quality. One of the most significant benefits of sprayed-on foam insulation is in the attic. By foaming the roof line in the attic, the “chimney” or “stack effect” commonly associated with ventilated attics is greatly reduced. By eliminating the stack effect, infiltration of warm, humid, unfiltered air into the home is significantly reduced. Spray foam insulation in the walls further reduces infiltration and provides for overall improved energy efficiency and comfort. Caution must be used, however, to ensure that adequate ventilation air is provided. Accordingly, many new or retrofit foam application also involve the installation of a fresh air make-up system.
Bathroom Exhaust Fans
Bathroom exhaust fans are only effective at removing moisture if they are used. One of the biggest reasons given for why bathroom fans are not used is noise. In-line bathroom exhaust fans have drastically reduced the noise associated with bathroom fans and are, therefore, more likely to be used. Timers or other delay controls can be used to further maximize the removal of moisture and odors.
The minimum efficiency for air conditioning systems is 13 SEER, however, higher efficiency systems are available. In addition to energy savings, higher efficiency equipment tends to be quieter and offer more options for improving indoor air quality such as improved humidity control.
Air Conditioning Systems
Constant Speed Air Handler / Single Stage Compressor
Traditional (and the most commonly installed) residential and light commercial air conditioning consists of a single speed blower and single stage compressor. The system is typically designed to satisfy the interior design temperature on a design day.
Variable Speed Air Handler / Single Stage Compressor
The addition of a variable speed air handler can significantly improve the performance of traditional air conditioning. Although the compressor is still single stage, part load performance (outside temperatures less than the design day) is improved since the air handler will run at a lower speed on its initial attempt to satisfy the thermostat and ramp up only if necessary. Accordingly, the fan running at lower speeds during these part load conditions improves the units ability to remove moisture (control humidity). Furthermore, and probably the biggest perceived benefit, is that the blower ramps up rather than starting at full speed, resulting in a quiet startup that often goes unnoticed.
Variable Speed Air Handler / Multi-Stage Compressor
Multi-Stage compressors matched with variable speed air handlers have drastically improved the part-load performance and humidity control of residential air conditioning. It is like having two units in one. Since most of the time operating conditions are less than the design day, the multi-stage unit can more accurately match actual conditions by staging the compressor(s) and adjusting the variable speed air handler accordingly. The result is a highly efficient system with greatly improve part load moisture removal capabilities.
Variable Refrigerant Volume (VRV)
VRV technology offers the most advanced system for residential and light commercial applications, providing variable capacity, advanced zoning and unmatched energy efficiency. The VRV system has a truly variable capacity at both the condensing and air handling units, thereby running at the exact capacity as required at any given time. This type of operation is inherently efficient and drastically increases moisture removal. Furthermore, temperature for the VRV system is sensed at the air handler’s return, allowing much more flexibility for controller location and enhanced temperature control. The VRV system has advanced zoning capabilities and enables multiple fan coils to run off of one condensing unit, containing an inverter “variable speed” compressor. The system basically has the ability to treat every room as a separate zone. Additionally, the condensing units for the VRV system are extremely quiet compared to standard air source equipment and the air handlers are specifically designed for quiet operation.
Indoor Environmental Control in the Hot, Humid South
Building Codes require that residential cooling systems be designed to maintain an interior temperature of no less that 75º on a design day as defined by ASHRAE. Design day temperatures for Charleston, South Carolina are 92º dry bulb (thermometer reading) and 78º wet bulb (indication of relative humidity). These “standard” design conditions have been developed by ASHRAE and are incorporated into ACCA’s (Air Conditioning Contractors of America) Manual J Load Calculation Procedure. The design conditions reflect average comfort conditions and take into account the limited moisture removal capabilities of standard residential cooling equipment.
Although it is certainly conceivable that the summertime exterior temperature in Charleston can exceed 92º, ASHRAE warns against sizing equipment to meet the cooling requirements during the most extreme conditions. Rather, ASHRAE recommends that in addition to temperature control, cooling systems be sized to provide adequate comfort dehumidification control as well, particularly in humid climates such as coastal South Carolina. To accomplish this, they recommend the use of an outdoor design temperature for which only a small percentage of predicted seasonal temperatures are expected to exceed. Since standard residential equipment only removes moisture when the compressor is running in an attempt to satisfy the thermostat, this method provides for improved dehumidification via longer run times and improves part-load performance.
However, this approach does little to remove moisture during high humidity/mild temperature periods such as during the Spring or Fall. This shortcoming of standard air conditioning equipment needs to be understood if humidity control is desired.
Low Operating Temperatures
Although environmental control system are designed to maintain an interior temperature of 75º on the design day, systems are often capable of achieving temperatures below that setting, particularly during off-peak days. Temperatures below 75º can have negative implications, the most common of which are sweating ducts and elevated substructure/crawl space moisture. Excessive substructure moisture can lead to mold and decay, result in cupped hardwood floors, invite termite activity and is a major contributor to poor indoor air quality. If air conditioning ducts are located in the crawl space, health effects are amplified as mold, moisture and other crawl space contaminants are readily drawn into the living space.
Substructure Moisture Control
Traditional building practices in the south have incorporated natural ventilation into homes built on crawl spaces. Although ventilated craw spaces are still common, great improvements have been made in how to realistically address substructure moisture concerns before they result in damage. The most common alternative to natural ventilation (and frequently used to correct excessive moisture in existing construction) is a closed and dehumidified crawl space. Properly installed and maintained, closed crawl spaces prevent wood floors from cupping (moisture is equalized above and below the floors), prevent wood decay/rot (wood stays dry, 10-15% wood moisture content), prevent mold (no excess moisture, relative humidity is maintained at 50-60%) and deter termite activity.
Infiltration is one of the most unaccounted for aspects of indoor environmental control. Often associated with wasted energy, high rates of infiltration can significantly impact the performance of heating and air conditioning equipment and negatively impact indoor air quality via the introduction of unconditioned, unfiltered, moisture-laden outside air.
High or fluctuating interior humidity is often traced back to excessive infiltration. Although leaky windows are often blamed for high infiltration, in many cases they are not the most significant contributor. Particularly prone to excessive infiltration are homes with vented attics and numerous canned lights or other ceiling penetrations. Homes with volumous “knee wall” attic spaces are also particularly susceptible to excessive infiltration.
Although mechanical fresh air ventilation is recommended (ASHRAE Standard 62.2-2003) for today’s tighter homes, fresh air can not be directly introduced into a home in the southeast without first considering outdoor humidity and the added moisture load to the interior. Although energy recovery ventilators may provide satisfactory results, ventilating dehumidifiers are available that are capable of bringing in filtered outside air, putting the house under a slight positive pressure and dehumidifying the air as necessary. In addition to dehumidifying the air brought in for ventilation the units also maintains interior humidity 24/7 independent of the air conditioning system.
Although it seems to make sense to adjust the thermostat when we leave for an extended period of time, doing so can have negative implications. What limited moisture capabilities the air conditioning system has when it is running are non-existent when the thermostat is raised (or unit is cut off) for an extended period. Dehumidification systems that run in conjunction with (but independent of) the central air conditioning system offer the best option for maintaining humidity control even during unoccupied times.
Building Design Considerations
Glass – Windows and glass doors are the largest single source of heat gain in most houses. Houses with extensive glass exposures are particularly difficult to control due to the thermal performance of glass and high peak-to-part load ratios
Vaulted Ceilings – Since heat rises, vaulted or two-story ceiling make proper air mixing difficult and are particularly problematic when adjacent to living spaces such as a catwalks or lofts.
Open Returns – Many commercial buildings have air handlers located above a drop acoustical ceiling without a ducted return. This arrangement places the space above the drop ceiling under a significant negative pressure, possibly resulting in unwanted outside air (unfiltered and humid) to being drawn into the conditioned space.
Canned Lights – Although sealed canned lights are available, many are not and often contribute to a significant “stack” effect, particularly in homes with ventilated attics, ultimately resulting in unfiltered warm humid air being drawn into the living space.
FROGs – Without a separate AC system or zoned controls, and due to the thermal envelope consisting mostly of roof and knee walls, FROGs often experience large temperature swings.
Stack Effect – Also called the Chimney effect, naturally ventilated attics (during hot weather) often draw air out of the living space through access opening, canned lights, and other ceiling penetrations. This puts the living space under negative pressure and the air is replaced with unfiltered warm humid air from the outside.
Knee Walls – Knee walls are the walls between the living space and an attic space. As with an overhead attic, temperatures can exceed 120º during hot weather, making temperature control difficult unless properly insulated.
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