How is High Performance Air Conditioning Different?
When a house or component of a house is “built to code”, that can often mean that it is as bad as it is legally allowed to be, not that it is the best it can be or utilizes current technology. Air conditioning is one system in most homes where minimum code requirements fall far behind what should be expected in terms of system performance. “High Performance” Air Conditioning refers to an air conditioning system that utilizes superior equipment technology and installation best practices to achieve much improved energy efficiency, superior comfort, and better durability compared with conventional code-minimum air conditioning systems.
Parts of a Typical Air Conditioning System
Conventional Air Conditioning Systems consist of several pieces of equipment:
- The condenser unit is the unit that sits outside the home. The condenser unit contains a condenser coil, a compressor, and a fan. The condenser unit removes heat from the air conditioner refrigerant that is coming from inside the house, before compressing and condensing the gaseous refrigerant into liquid form and sending it back into the home
- The evaporator coil is connected to the condenser unit by a refrigerant line. Refrigerant that comes into the house from the condenser is in a high-energy liquid form, but it expands rapidly into a gaseous form as it enters the evaporator coil, causing the temperature of the coil to drop rapidly to far below the temperature inside the house so that it cools and dehumidifies air that passes it
- The air handler consists of a large fan that moves air across the evaporator coil. Air handler fans are most commonly packaged with gas furnaces so the fan can do double duty of distributing cooling in summer as well as heating in winter.
- The duct system uses one or more return ducts to move temperate air from inside the home, past the air handler, through the evaporator coil where the air is cooled and dehumidified, and back into the home through a network of supply ducts.
Air Conditioning Efficiency
Condensers are rated for efficiency according to Seasonal Energy Efficiency Ratio (SEER). SEER measures the number of British thermal units (Btus) of heat removed by the air conditioner per the amount of wattage required to remove the heat, averaged over the entire cooling season. Currently, SEER 13 air conditioners are the least efficient air conditioners which can be legally sold. The most efficient air conditioners perform at around SEER 21. Because the equipment utilizes existing technology, these units can be purchased inexpensively and are popular because a contractor can charge a larger margin when selling to a homeowner, or come in very inexpensively as a subcontractor on a new construction project. Interestingly enough, the price difference between a SEER 13 air conditioner and a SEER 16 unit, which is 23% more efficient, is pretty small at the wholesale level when comparing apples to apples in terms of features. The several hundred extra dollars are recouped quickly in energy savings. Beyond SEER 16, the efficiency gains relative to the additional price become less pronounced. A number of features can further increase the operational efficiency of an air conditioning system, but aren’t necessarily accounted for when calculating SEER:
- Two-Stage or Modulating Capability: Most air conditioners are single stage, meaning that they are always either turned completely off, or running full blast. Two stage air conditioners are capable of switching between a high and a low cooling output. Modulating air conditioners are capable of running at a variable output ranging from very low to full blast – kind of like burners on your gas stove. Two stage and modulating air conditioners are typically more efficient than single stage furnaces, because by running at lower outputs, they can run at peak efficiency for longer and avoid energy losses associated with cycling on and off. Two stage air conditioners are very good for small to medium size energy efficient homes that only need a small amount of heat in all but the coldest weather. Modulating air conditioners are great for large homes, and especially large homes with zoned duct systems that may only be providing cooling to certain parts of the house at any given time. In fact, it is practically impossible to design and install a high performance air conditioning system with multiple duct zones that does not also utilize modulating equipment. Two stage AC usually costs between $500-$1,000 extra to install compared with single stage. Modulating AC often requires proprietary thermostats and other controls, and is generally installed as a component of a larger, more complex HVAC system, so a system featuring modulating equipment can cost thousands of dollars more than a basic single stage system once all additional installation costs are factored in.
- Fans: The air handler fan can use a lot of electricity. Multi-speed or variable-speed fans can save a substantial amount of electricity, but are commonly only available on higher-end dual stage or modulating equipment.
- Controls: With the increased proliferation of smartphones, tablets, and other devices which keep us constantly connected to myriad aspects of our lives, home automation technology has rapidly improved and increased in popularity in the last five years. While almost any HVAC system can be paired with a traditional programmable thermostat or a Nest thermostat, major furnace manufacturers are developing sophisticated proprietary control systems designed to be paired with their higher end furnaces. For example, the Trane XV20i air conditioner is designed to run optimally when paired with the Trane ComfortLink II, in a system that provides advanced air filtration, manages multiple heating and cooling zones, monitors a variety of factors to calibrate the system to run most efficiently , and provides service reminders, all while being controlled from the device of your choice. If you plan to have modulating equipment and/or multiple duct zones, a manufacturer’s proprietary system is worth considering.
- Filtration: Like controls, filtration can be provided either by a stand-alone accessory such as an Aprilaire or by a manufacturer’s proprietary product. Both proprietary and aftermarket filtration systems may provide a wide range of options, including timer or automated controls, service reminders, and electrostatic or ultraviolet purification in addition to filtration. Once again, if you are planning to install the high-end HVAC system with proprietary controls, it’s best to use the proprietary filtration for seamless integration. Also, if someone in your household has athsma, allergies, or other air quality concerns are present in the home, it’s worth installing a fancier filtration system. However, for basic systems, a stand-alone filter is sufficient, provided it filters at least MERV 11 and is installed properly. Filtration should always be installed right at the air handler intake, not at the return duct grille.
The Problem with Oversizing Air Conditioners
Proper sizing is very important to the overall performance of an air conditioner system. Depending on a wide range of factors, including where you live, how large your home is, which direction it faces, and how well insulated your home is, the correct size of your HVAC system may vary drastically. Like jeans, every model of air conditioner is available in a number of sizes, and won’t do its job properly if you choose the wrong size.
Traditionally, HVAC contractors have installed drastically oversized heating and cooling equipment, which leads to a number of problems (LINK TO HVAC OVERSIZING). To ensure your home gets the correct size system, worth with a qualified home performance contractor who will use the ACCA Manual J method to properly size your heating and cooling equipment. In addition to being sized to match the home’s cooling load, the air conditioner will perform best when it is paired to a forced air fan that is size matched to move the correct amount of air past the evaporator coil. A general rule of thumb is that the air handler fan should move about 350 Cubic Feet per Minute (CFM) of air per Ton (12,000 Btu) of cooling. When heating and cooling loads differ dramatically, such as in the San Francisco Bay Area, where heating loads are much higher than cooling loads, it can be difficult to find a furnace/air handler combo that also moves the correct amount of air for the air conditioner. Once an approporiate fan is selected, it will require careful commissioning to ensure that the fan runs at proper speeds – which are often very different – during heating and cooling seasons.
High Performance Means Better Than Code
High efficiency, properly sized, well integrated equipment will still perform poorly and return poor efficiency if best practice installation methods are not used. The installation makes or breaks the project. By far, the duct system is the most difficult component of the system to install well. Ducts should be well insulated to avoid condensation from forming on the outside of the ducts during cooling season, as well as to ensure that energy performance targets are met. Duct runs should be properly sized, thoughtfully laid out with the shortest possible distance between the air handler and the inside of the living space. Ducts should be free of kings and sags, and installed so as to minimize sharp turns, which make the ducts perform as though they are much longer. The area where most installers fall short is in making the ducts airtight. Nationwide, duct systems leak about 30% of the air they condition before that air arrives in the living space of the home. Today, more cities and states are requiring that duct systems leak no more than 15%, but these does are not yet being enforced well in many places. In any case, 15% leakage is still not a very lofty goal. A high performance system should leak no more than 6%, and some systems are leaking less than 2% – virtually airtight.
Using a higher efficiency unit, sizing equipment to match the engineered cooling load, adhering to best practices during installation, and thoroughly sealing the duct system will yield a high performance air conditioning system that will run quieter, dehumidify better, provide more even temperatures throughout the home, last longer, AND save 30% or more on energy consumption compared with a code-minimum AC system. To review, an installer who professes to install a high performance AC system should practice all of the following guidelines:
- Be properly licensed and insured
- Obtain permits and inspections for the installation to ensure code compliance
- Choose equipment that is a good value in terms of both purchase price and energy costs
- Size the system according to ACCA Manual J
- Install air filtration at the air handler intake
- Provide a guarantee that all ducts are tightly sealed and confirm duct airtightness with a duct blaster test once installation is complete
- Check static pressure and charge
- Educate the homeowner on how to operate the system
If all of these criteria are met, your high performance air conditioner will work better, consume less electricity, and last longer than a code-minimum system. The true metric of performance is value over time.
Copyright September 2014 by Advanced Home Energy
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