HVAC Is All About Air
Dispelling Myths About Whole-Home Efficiency
For decades, the residential-environmental air conditioning community has delivered a less-than-stellar performance on installation quality.
The HVAC industry suffers the stigma, not from intentional wrongdoing, but rather adoption of rules of thumb that have little basis in fact. Prove to the industry they are on the wrong path and they will adopt right thinking. When marketed correctly, the home-performance, house-as-a-system model is being accepted across the country with great success.
Follow the Leader
We must remember that our industry, in hot, humid climates, began only in the mid-1960s. The path the industry took in past years is understandable. While meeting housing demand for the sheer number of baby boomers, and a higher standard of living, engineering values were sidestepped with shortcuts.
With cheap energy prices, no one was the wiser. As energy prices began to rise, and with the oil embargo of the late 1970s, it became apparent a more-efficient housing stock had to be developed. Once a path was established to improve construction methods, the emphasis would shift to higher-performing HVAC equipment. In the wake of this rapid demand, equipment developers and installers began seeking shortcuts on materials and installation methodology. As a result, everyone started asking questions:
Manufacturers questioned warranty returns on parts that seemed to fail prematurely with little reasoning; green-energy gurus scratched their heads when efficiently built homes failed to perform at acceptable comfort levels; and contractors pondered why some systems operated poorly, for no apparent reason.
For rightly deduced reasons and solid concerns above, Austin Energy’s staff, as far back as 2005, has advocated for the study of delivered interior home efficiency.
In 2011, Austin Energy began a systematic search for homes where the staff and a group of contractors might be able to impact air-delivery systems with minimal expense and improved performance.
While what has been discovered to date is extremely encouraging, more work will need to be done. The staff at Austin Energy is comfortable in recommending changes in day-to-day installations. At Austin Energy, we see improvement by following a few rules. These rules are not actually rules at all, but a rediscovery of what the HVAC industry practiced in its infancy.
Setting Up the System
Well over half of the systems tested in our system-performance pilot were set up improperly. Incorrect fan speeds were selected on standard blower motors; timed on-and-off fan operation (for harvesting evaporator energy) was left at factory defaults; incorrect fan settings remained on variable-capacity equipment; and thermostats for high-end equipment were left at default programs in
Several studies across our country have proven more than 80 percent of systems are oversized and deliver only 60 percent of rated capacity. We should propose lasting improvements. There are methods to identify the issues quickly. The homes we surveyed proved this study is true in our area. The opportunity for improvement should be granted to the homeowner. Failure to inform the homeowner only perpetuates problems in the HVAC industry.
Monitoring Static Pressure
We should begin monitoring static pressure upon arrival at a residence. This practice should take precedence over hooking up gauges. In most every home we tested, pilot static pressure was a persistent issue. Rules of thumb on static pressure should be avoided. Refer to manufacturer specifications and work to obtain required static. Industry engineers who design the equipment know what system output is required. If we short cut static pressure requirements with generic information we need forget static pressure all together.
Temperatures can identify issues early, however, don’t be fooled. A ΔT (Delta T) without validated airflow requirements can lead us down a meaningless path. This is similar to superheat and sub cooling. Without verified airflow present, readings are bogus.
Return air continues to challenge our industry. We continue to follow a trend of increased return air size to provide adequate flow for our systems. This includes a practice of increasing filter area. Seldom will systems of 3 tons (nominal size) or larger perform with only one high-efficiency air filter. Some systems require commercial-grade returns to obtain flows while others (platform return) may need extensive alteration.
Supply Plenum Design
Substantial gains in airflow per-
formance and comfort can be achieved with simple changes in plenum design. These include increased plenum length to accommodate air stabilization off the blower, a pressure-rebound zone, and adequate take-off surface. We must also include more-efficient transitions when installing evaporators or adapting existing plenums and ductwork. Austin Energy’s pilot found unexpected gains by avoiding taps at specific areas of the plenum formerly utilized without hesitation.
Duct Design/Proper Installation
Duct flows and system performance have been overrated for quite some time. New duct-performance flow criteria are available through a few sources. Even more important than equipment selection is the fact that actual duct installation quality will determine performance outcomes. We must pay attention to air duct installation.
Very little attention is paid to grille location and the type of grille used. With Austin Energy’s pilot, we found same-size grilles with differing manufacturers flowing upward of 20 percent more air. Some homes could be fixed with grille replacement. With homes located in hot-and-humid climates, it is extremely important to avoid air delivery to exterior walls at velocities above 50 feet per minute. During mild weather, the practice can cause condensation and deterioration of construction material.
In the U.S., the overarching goal has been to weatherize the structure by sealing the envelope and adding insulation. With less than 15 percent of homes in the Austin, Texas, area without a central air conditioning system, the need to adopt a change becomes more apparent. Austin Energy is looking to provide a program that considers sustainability through a process of fixing the air-delivery system, and then sealing the envelope and adding insulation. Fixing the air-delivery system later only calls for additional weatherization measures to be repeated. Consumers may be slow to accept the idea. After all, we put the air conditioning in. There must be an honest effort to advance a system-performance approach to inform the homeowner so an intelligent decision can be made on possibly his or her most valuable investment.
In conclusion, poor system performance in the retrofit market left unaltered, costs everyone. Obviously it costs the consumer in higher utility bills, poor comfort, and premature replacement. It cost manufacturers with increased warranty and lost wholesale revenue. It costs the contracting community with poor customer retention and lost revenue when servicing callbacks for both service and retail sales.
Most importantly, system performance answers the question of, how do we get the install right?
Publication date: 6/10/2013