ECOVENT WHEN BIGGER ISN T BETTER HOW ECOVENT HELPS RIGHT-SIZE HOME HVAC SYSTEMS A WHITEPAPER PUBLISHED AUGUST 2014
TABLE OF CONTENTS 01 INTRODUCTION 03 02 WHY RIGHT-SIZING IS SO DIFFICULT 06 03 THE PROBLEMS WITH OVERSIZING 09 04 GETTING BEYOND RULES OF THUMB 12 05 ROOM LEVEL COMFORT: THE NEXT FRONTIER 15 06 REAL DATA, NOT JUST ESTIMATES 18 07 CONCLUSION 19 08 SOURCES 20
01 INTRODUCTION Imagine the hottest day of summer. The sun is beating down, the air is still, and the humidity is so bad that it s nearly impossible to go outside. Now imagine that your air conditioner breaks down. If you are a heating, ventilation, and air conditioning (HVAC) technician, you have probably received dozens of calls from homeowners at just this moment of frustration and panic. And since everyone s A/C unit works hardest during a heat wave, you have probably received more than one of these calls on the same day. It s no wonder then that most contractors try to avoid these distress calls by installing HVAC equipment that is powerful enough to withstand the very hottest days of summer or the coldest days of winter. However, these rule of thumb methods for sizing heating and cooling equipment can be far from ideal. 03
WHEN BIGGER ISN T BETTER Study after study has found that residential HVAC systems are routinely oversized by as much as 200 percent. A survey of homes built between 1994 and 1999 in Fort Collins, Colorado found that cooling systems were significantly oversized, ranging from 143 percent to 322 percent of the design requirements. 1 And according to a large-scale survey cited by the National Renewable Energy Lab (NREL), nearly 40 percent of HVAC contractors report purposefully oversizing equipment to reduce call backs or due to customer demand. 2 That may translate into fewer distress calls for HVAC contractors, but it is costly for homeowners. For example, Boston-area HVAC contractors report that a fully-installed 2.5 ton air conditioning system, including the air handling unit, condenser, refrigerant lineset, ductwork, thermostat, and wiring, costs the homeowner approximately $12,000, while the same system in a 5 ton size will be about $20,000. In other words, a double-sized air conditioning system costs the homeowner approximately 66 percent more a high price for comfort. With better planning, design, information, and technology, it is possible to right-size home heating and cooling equipment. This process can reduce homeowners upfront costs and save money on long-term operations. It can also help HVAC technicians reduce the peak season blitz while retaining happy, loyal customers. FIGURE 1 : AVERAGE HVAC SYSTEM COSTS BASED ON SIZE 20000 SYSTEM COST IN $USD 15000 10000 5000 0 2.5 5.0 SYSTEM WEIGHT IN TONS 04
A double-sized A/C system is a high price for comfort.
02 WHY RIGHT-SIZING IS SO DIFFICULT Equipment sizing is a key factor in designing residential climate control systems. An appropriately sized system can deliver energy-efficient comfort for years and years. However, in most cases, equipment sizing receives minimal attention in the design process. This is partly because it is so difficult to do well. In order to appropriately size an HVAC system, a technician must develop a clear model of heat loss and heat gain throughout the home. This requires the technician to account for a variety of variables, including building characteristics, construction materials, air sealing techniques, window materials, installation quality, and many other factors. 06
WHEN BIGGER ISN T BETTER The HVAC industry has developed manuals and software to assist contractors in managing these variables for the sizing of heating and cooling systems. For example, the Air Conditioning Contractor s Association of America s (ACCA) Manual J offers procedures for determining heating and cooling loads based on the amount of wall, floor, ceiling, and window area, as well as the relative insulation value of these materials, the observed duct leakage, and other characteristics of the building envelope. Once this process is complete, ACCA s Manual S is used to select the appropriate equipment. Unfortunately, due to the competitive nature of the industry, many HVAC contractors are unwilling to spend the time necessary to measure and investigate the home, input the data into a software program, and adapt their designs accordingly. In a widely-cited survey of Florida air conditioning contractors, researchers revealed that only 33 percent of contractors size air conditioning systems using a Manual J calculation. 3 And even the authors of the survey admit that estimate may be high. They wrote: It seems likely that any bias in the sample is probably toward those with a greater interest in the subject, and possibly in the profession, hence the answers may be skewed towards more detailed methods of sizing. In other words, the contractors who avoid entering data into Manual J software weren t likely to fill out a survey, either. IN A COMPETITIVE INDUSTRY, TIME IS AT A PREMIUM. 07
03 THE PROBLEMS WITH OVERSIZING Although over-sized equipment is often installed as a preventative measure, it can create a variety of problems on its own. 09
WHEN BIGGER ISN T BETTER SHORT CYCLING For one, over-sized equipment can lead to inefficient short cycling. If a furnace is too big for the home it is heating, it only needs to run for a few minutes before it reaches the target temperature. At that point, the system shuts off, only to turn back on moments later when the heat dissipates. The same issue occurs when an over-sized air conditioner is installed. The system runs long enough to achieve the target temperature, but not long enough to achieve optimum dehumidification. That leaves homeowners running to the thermostat to reduce the temperature even further, believing that that will improve their comfort, when the real culprit is humidity. Short cycling also reduces the efficiency of A/C operations. The effect is similar to driving a car in stop-and-go traffic. It s inefficient to slam on the gas, slam on the brakes, then slam on the gas again. Your car is much more efficient when it is cruising along on the highway at a steady 60 miles per hour. HVAC equipment is similar: It works best when it s operating consistently. Short cycling puts unnecessary wear on the system, and many people also find it irritating to hear their system constantly turning on and off throughout the day. INCREASED ENERGY USE Oversized HVAC equipment also requires more energy to operate. NREL cites a Florida study that showed an average increase of 9 percent of annual space cooling electricity usage for units that were oversized by 50 percent or more. A 2005 study by Proctor Engineering yielded similar results, noting that reducing a system s size by approximately 50 percent saved nearly 9 percent of the energy consumed by the A/C unit. The following table is reproduced from that study. Given these drawbacks, it s no wonder that the HVAC industry advises that oversized air conditioners force the consumer to pay more for equipment and maintenance while providing lower levels of comfort and wasting more energy. TABLE 1: EFFECT OF A/C UNIT SIZING REDUCTIONS ON INDIVIDUAL UNIT ENERGY CONSUMPTION 4 SIZING REDUCTION (% OF ORIGINAL SIZE) ENERGY SAVINGS (INDIVIDUAL UNIT) 13% 23% 31% 37% 43% 47% 1.6% 3.1% 4.6% 6.0% 7.4% 8.7% 01 10
Short cycling is like driving a car in stop-and-go traffic.
04 GETTING BEYOND RULES OF THUMB Many HVAC professionals have already moved beyond rules of thumb and are using load modeling to generate more accurate equipment sizing calculations. New code amendments may soon require that others follow in their footsteps. 12
WHEN BIGGER ISN T BETTER BUILDING CODE IMPROVEMENTS Local and state governments are beginning to mandate Manual J software calculations on all new buildings or extensive remodeling projects through new building codes. These local code changes may take years to be implemented nationwide, but they tend to be a bellwether for the industry as a whole. VARIABLE ELECTRICITY RATES Indeed, peak demand is becoming a bigger and bigger concern for homeowners and the electric power utilities that serve them. In order to offset the cost of high-priced peak power, many utilities are beginning to offer incentives to customers who can cut their peak power consumption. TECHNOLOGY ADVANCES Beyond regulatory requirements, there are other reasons why the rule of thumb system sizing method may no longer cut it. Technological advances now allow for improved comfort with greater energy savings. Today s heating and cooling equipment is capable of modulating or operating in multiple stages in order to manage efficient system operation during partial load conditions. These adaptive systems can enhance life expectancy of equipment while reducing energy use. Most multi-stage equipment has two stages: low and high. Low stage capacity can be set as close to the load at an average summer temperature for cooling, while heating levels should be set for average winter conditions. This ensures efficient operation during the majority of the year. Under peak load conditions, the system would operate on high stage capacity to meet the increased demand. In 2012, approximately 24 percent of U.S. utilities offered such price-based incentives to residential customers. 5 For example, in 2014, Baltimore Gas and Electric offered an optional, variable Time of Use rate plan that charges nearly 12 cents per kilowatt-hour (kwh) during summertime peaks, and as little as 7 cents per kwh during off-peak hours. 6 Proctor and Pira s study indicates that right-sizing an HVAC system also has an impact on the homeowner s peak demand, as illustrated in Table 2. For customers who have opted into variable electricity rate programs, peak demand for power will have an even bigger impact on their electricity bills. Therefore, these customers will have an even stronger desire to choose a smaller, right-sized A/C unit. 01 13
WHEN BIGGER ISN T BETTER TABLE 2: EFFECT OF A/C UNIT SIZING REDUCTIONS ON DIVERSIFIED PEAK LOADS 7 SIZING REDUCTION (% OF ORIGINAL SIZE) PEAK ENERGY SAVINGS (INDIVIDUAL UNIT) 13% 23% 31% 37% 8% 10% 12% 14% RIGHT-SIZING HVAC EQUIPMENT REDUCES PEAK DEMAND FOR ENERGY. 14
05 ROOM-LEVEL COMFORT: THE NEXT FRONTIER In addition to these advances in codes, technology, and electricity rates, the delivery system for conditioned air has also evolved. Room-by-room zoning is now possible via interventions within the ductwork or at the vent register. The propagation of this technology will further enhance comfort and energy savings for homeowners with forced air systems. 15
WHEN BIGGER ISN T BETTER Many homes have been equipped with two-stage equipment but do not have the room-byroom control that would optimize this design. Operating two-stage equipment on low stage capacity without modifying the duct volume with room-by-room zoning creates a whole different set of issues. Room-by-room zoning solutions, like ecovent s, can modulate airflow in response to user input, occupancy, and the dynamic environmental conditions of each room as they change throughout the day or over the course of the year. ecovent addresses airflow management at the vent register level. Other room-by-room zoning solutions use mechanical or inflatable baffles to achieve the same goal. In effect, room-level zoning reduces the amount of space that is heated or cooled, which reduces runtimes, improves the overall efficiency of HVAC operations, and enables the HVAC technician to select a smaller air conditioner or furnace for a customer s home. ecovent is in the process of validating its technology through a beta program involving dozens of homes across the United States. These homes are distributed across different climate zones, feature distinct design characteristics, and are experiencing different issues with their heating and cooling equipment. We plan to publish case studies based on the results of this program, including information on the energy savings and comfort improvements facilitated by room-level zoning. Room-level zoning reduces run-times and increases overall efficiency. 01 16
06 REAL DATA, NOT JUST ESTIMATES Another benefit of advanced zoning is the installation of environmental sensing equipment across multiple points in a home. For example, ecovent s system includes a sensor for each room in the home. This sensor constantly measures the temperature and humidity conditions at the room level. Not only does this ensure occupant comfort, but also, it represents a new opportunity for HVAC technicians to right-size equipment. If a homeowner has ecovent installed and needs to replace the home s heating or cooling equipment, the technician may utilize the system s sensor data to tailor the equipment purchase to the actual heat loading conditions in the home. Even the best Manual J calculation is an approximation. But with room-level sensor data, ecovent can report on actual heat loss and heat gain conditions throughout the home under real-life conditions. This new data source is a potential boon to contractors and homeowners alike. 18
07 CONCLUSION Today, the process of HVAC system sizing is an educated guess at best. Most heating and cooling takes place during average weather conditions, but most HVAC systems are sized for the hottest and coldest days of the year. In that light, most systems are significantly over-sized in relation to their average load. With better design tools and technology, there is plenty of room for improvement. Proper system sizing technique can enable HVAC technicians to design systems that meet the demands of rare peak load days while operating efficiently on average load days. These optimized systems would include room-byroom control and two-stage equipment that can adapt to heating and cooling based on the demands of that zone of the house and particular weather conditions. As sensing equipment becomes more widespread, comfort control systems will be able to monitor operating conditions both indoors and outdoors at multiple points. These smarter systems could eventually calculate the exact capacity required at both the seasonal average days as well as the coldest and hottest days. For now, there are tools readily available for HVAC technicians to improve the comfort and efficiency of their customers homes. And by doing so, they can ensure that when the phone rings, it s a happy customer on the end of the line. 19
SECTION 08 SOURCES 1. As cited in Dagostino, Frank and Joseph B. Wujek, Mechanical and Electrical Systems in Architecture, Engineering and Construction, Prentice Hall, 2009 2. http://www.energy.wsu.edu/documents/aht_energy%20efficient%20home%20cooling. pdf 3. Vieira, Robin K., Danny S Parker, Jon F. Klongerbo, Jeffrey K. Sonne, Jo Ellen Cummings. How Contractors Really Size Air Conditioning Systems. Presented at the 1996 ACEEE Summer Study on Energy Efficiency in Buildings. American Council for an Energy-Efficient Economy, 1001 Connecticut Avenue, Washington, DC http://www.fsec.ucf.edu/en/publications/html/fsec-pf-289-95/ 4. Proctor, John, Joe Pira. System Optimization of Residential Ventilation, Space Conditioning and Thermal Distribution Proctor Engineering Group. San Rafael, CA. October 2005. http://www.proctoreng.com/dnld/systemoptimizationofresidentialventilation.pdf 5. U.S. Energy Information Administration. 2012 Electric power sales, revenue, and energy efficiency Form EIA-861 detailed data files October 29, 2013. http://www.eia.gov/electricity/ data/eia861/index.html 6. Baltimore Gas & Electric Company. Standard Offer Service (SOS) Rates/Miscellaneous Charges http://www.bge.com/myaccount/billsrates/ratestariffs/electricservice/electric%20 Rates%20Information%20Documents/POLR_Rates_PTC_MiscCharges.pdf 7. Proctor, John, Joe Pira. System Optimization of Residential Ventilation, Space Conditioning and Thermal Distribution Proctor Engineering Group. San Rafael, CA. October 2005. http://www.proctoreng.com/dnld/systemoptimizationofresidentialventilation.pdf 20
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