Duane Gentry is not your HVAC contractor. If he were, the Asheville, N.C.–area sub likely would supply your latest air-tight and insulated home with a ground-source heat pump that distributes conditioned air through short runs of sealed ducts located only in conditioned space, supplemented with a whole-house energy recovery ventilator and an active return air scheme.
If that setup doesn’t sound familiar, or simply sounds expensive, chances are you haven’t confronted the latest version of the Energy Star Qualified Homes Program or the similarly stringent 2012 edition of the International Energy Conservation Code (IECC), or competitive challenges and consumer demand that might attract your attention.
Those drivers, though, are “leading to more changes in HVAC system design and performance than any time in my 30 years in the housing industry,” says Dean Potter, vice president of quality for K. Hovnanian Homes in Red Bank, N.J., one of relatively few builders out in front of them. “Builders are being challenged to do something better, but what’s the best way to do that?”
Certainly not the old-school ways practiced by HVAC contractors since the advent of air conditioning and central forced air systems in the 1950s. That is, with rules of thumb and rolls of duct tape.
Better Practices
Space heating and cooling historically consumes the most energy of any system in the house besides the building shell. So after tightening up and insulating the envelope to reduce the home’s overall energy load, the HVAC is the next and best bastion for energy-saving efforts.
Achieving IECC-compliant or Energy Star–acceptable efficiency, however, requires a holistic rethinking of everything from equipment sizing and efficiency to duct design and installation, beginning with a Manual J residential energy load calculation designed by the American Air Conditioning Contractors Association (ACCA). The checklist considers heat loss through the envelope, forced-air duct leakage, and heat gain from the sun, lighting, appliances, and occupants, among other conditions.
The results of a Manual J (and companion blower-door test to determine envelope air infiltration) translate to the proper sizing (or capacity) of the heating and cooling equipment and necessary airflow to condition the indoor spaces. “The more you can precisely size the system, the higher the energy savings,” says Drew Smith, president of Two Trails Consulting, a building science expert and home energy modeler, auditor, and rater in Sarasota, Fla.
But while “rightsizing” the heat pump or A/C unit and air handler per the home’s actual energy load is far better than the simple per-foot guess made by unenlightened or stubborn HVAC subs, “the efficiency of the unit and the ability to have variable speed operation is critical to achieving the highest energy savings,” Smith says.
For A/C condensers and heat pumps, that means seasonal energy efficiency ratings of 13 or higher and the equipment’s ability to automatically toggle between a lower (and more energy-efficient) maintenance speed and an as-needed higher speed to boost the system in extreme conditions, instead of a one-speed-suits-all approach.
Meanwhile, furnaces require an annual fuel utilization efficiency (AFUE) rating of at least 78 percent, although those with an AFUE rating of more than 90 percent are not only readily available but also preferable for low-load homes—and soon will be required among codes and standards. Minimum efficiencies for other HVAC equipment, including oil boilers, radiant systems, and geothermal heat pumps, also are on the rise in an effort to reduce their energy consumption.
Don’t Forget Ducts
While the Energy Star program estimates that 5 percent of a typical A/C or heat pump’s efficiency is lost because of unit oversizing, upward of 18 percent can be attributed to the air distribution system, specifically leaks and kinks in the ductwork.
As a result, the heating and cooling system must work harder to condition and deliver the necessary airflow to maintain the desired indoor temperature, if not comfort. “In the past, the answer from HVAC contractors was to boost the system’s capacity and airflow rate,” says Potter—or basically to throw good money after bad building practices. “The quality of duct design and installation needs to improve by multiple factors.”
This is where Gentry’s methods shine. In addition to conducting energy audits and Manual J calculations to rightsize the equipment, his crews routinely install ductwork with 2 percent or less leakage, which is the 2012 IECC standard. “It’s almost a game with our guys, and a point of leadership in our market,” Gentry says, adding that they haven’t failed a duct blast test (how leakage is measured) in a year.
The funny thing is, doing ducts right isn’t a mystery, it’s just paying attention to details. Using an ACCA-approved Manual D calculation to properly design and size his duct network, Gentry combines sheet metal primary runs with Class 1 (insulated) flexible branches, where necessary; the main runs also are wrapped in insulation, but only after each connection—including to the air handler—is sealed inside and out with mesh and mastic. He also eliminates toekick supply registers and limits the number of zoning dampers, which he calls “very leaky.”
Few HVAC contractors are as attentive or precise. Despite specifying the ductwork details down to the brand of mastic for their reference, Smith says most subs fall back to old habits on the job. “It doesn’t do any good to mastic the outside if you haven’t sealed the inside” of each connection, he says.
A pressurized, pre-drywall smoke test through the system usually gets the message across. “When they see smoke coming out of their ducts all over the house, that’s when they really learn,” Smith says. “It’s an ongoing education.”
An equally hard nut to crack is during the duct design phase. Ideally (and routinely in the latest codes and certification standards), ductwork should be contained in conditioned or “semi-conditioned” areas, such as sealed and insulated attic or crawl space. Doing so nearly eliminates temperature and pressure differences between the distributed air and its environment that cause heat loss through the ducts and their connections.
In addition, the system’s air handler should be centralized within the floor plan of the house to enable shorter and straighter duct runs to further reduce leakage and allow unrestricted airflow. “There’s a negative impact on airflow every time you turn a duct,” Potter says. “The system gets less airflow than what it is designed to deliver,” perhaps requiring adjustments in equipment sizing and airflow rates to compensate.
Problem is, builders aren’t savvy to that building science. “The challenge is that builders and architectural designers often don’t have mechanical design expertise,” Potter explains. “They haven’t learned the importance of integrating the HVAC system into the house.”
For builders seeking Energy Star or similar certification, Gentry works with local independent energy raters to nudge his clients toward reworking their home designs for centralized and short-run systems. “We seldom see air handlers in the garage anymore,” he says.
Unfortunately, only a relative handful of residential HVAC contractors are following Gentry’s lead into the next frontier of heating and cooling systems, an obstinacy that has forced high-performance builders to turn elsewhere. “The answer for K. Hovnanian is to bring mechanical design in-house,” Potter says. “It’s a different model for the industry.”
Cost Considerations
That model comes at a cost, of course, from hiring a mechanical engineer or consultant like Smith to using higher-efficiency equipment and supplemental components for heat recovery and filtration to achieve energy savings and deliver better comfort and healthier indoor air in a low-load home.
To date, builders and contractors who do it right have been able to balance less-expensive, right-sized equipment against upgrades in building envelope and unit efficiencies.
But there are points of diminishing returns, such as SEER ratings of 18 or above that take perhaps twice as long to pay back their investment. To help builders and their homeowners gauge value, both Smith and Gentry offer energy modeling to show costs, energy savings, and paybacks for different scenarios, factoring in tax credits and utility rebates, if available, to reduce the return on investment.
Of course, Gentry charges a premium for this extra work (and the resulting performance improvements), but he may not have to for long. As of July 1, all homes seeking Energy Star qualification (about 28 percent of all new homes in 2011, a record share) must not only boost their HVAC specs, but also hire an Energy Star–certified contractor to install it. “It raises the bar for everyone and will keep pricing competitive,” he says.
As a production builder always looking to balance performance, value, and price, Potter worries that codes and certification standards may be tipping too far and sees the industry—especially HVAC subs—falling behind.
“The required upgrades from the 2009 to the 2012 IECC [a 30 percent improvement compared with its 2006 edition] cost more than the energy savings you can likely realize now, and the 2015 IECC [which seeks a 50 percent improvement] will likely cost more than the resulting benefit,” he says. “My expectation is that most HVAC contractors aren’t aware of what’s happening and haven’t done a lot of training to prepare for it.”
Because of that specter, and what Potter calls “narrowing options” to achieve the latest code and certification standards of HVAC efficiency, today’s technology and tactics for delivering conditioned air are likely just the beginning of a new era.
“This is by no means the end game,” Potter says. “You are going to see mechanical systems continue to evolve as building energy efficiency improves.”
