|Air handler and duct layout: A simplified air-return system cuts down duct leakage.|
Houses are complex systems. Realize that when you change any significant house part, say from standard double-glazed to low-E windows, the system will respond. Sometimes the house works better, sometimes worse.
With HVAC systems, the unintended consequences can be huge. Consider oversize equipment. Then connect that equipment to leaky ductwork. Among the big headaches this pairing causes are higher first cost, lower homeowner comfort, reduced equipment life span, more noise, higher energy bills and possible health risks. Plus utilities have to build more power plants. What’s to like?
Oversizing isn’t overstated
Remodeling clients will probably point to the HVAC systems in the new homes where their friends or relatives live. A recently released housing performance study by Fort Collins (Colo.) Utilities showed that air-conditioning equipment in local new homes was sized at 208% of design load requirements (see chart, page 116); the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ recommended target is only 115%.
“We could cut residential heating and cooling equipment size in this country by 30% and as much as 50%,” says Ohio-based HVAC engineer Hank Rutkowski, author of Manual J: Load Calculations for Residential Winter and Summer Air Conditioning.
How did it get this way? Rutkowski estimates that only 10% of HVAC contractors calculate heating and cooling loads. Even then, he says, many fudge the sizing results upward although a safety factor is already built into the Manual J calculation tool. Tradition and rules of thumb, such as 1 ton per 500 square feet, predominate. Some HVAC contractors figure that with larger equipment they’ll have fewer callbacks. Rutkowski reports that installed equipment often is rounded up a half-ton.
Toward sizing sanity
Misinformed homeowners don’t help. Their typical questions: “Is the heating and cooling equipment big enough? Why do I have a 3-ton system when my neighbor with the same-size house has 5 tons?” Wrong questions.
Here’s what they should ask: “For my high-performance building envelope, have you sized your heating and cooling equipment according to engineering calculations?”
Because they probably won’t ask that question, try presenting clients with a question of your own: How would a Lexus sedan perform if you dropped a D-8 Caterpillar’s engine in it?
Not well at all, according to engineer Joe Lstiburek, principal author of the Energy & Environmental Building Association’s Builder’s Guide series.
“For many years, ducted distribution systems have been used to compensate for poor envelope design and installation,” says Lstiburek. “In the process, we ended up prematurely breaking those mechanical systems. Call that stupid squared. Now we have to fix both.”
In his work with builders, Lstiburek starts with the envelope. Using spectrally selective windows is critical. Then he advises adding thicker wall insulation, tightening the envelope and bringing all the ductwork indoors. During a whole-house remodel or a major addition, relocating existing ductwork is practical only when you are gutting the original structure.
After improving the envelope, right-sizing the HVAC equipment is step two. Engle Homes of Colorado, one of Lstiburek’s clients, is coordinating with Rheem equipment distributor Dave Shrock and its heating contractors to do so.
“The contractors all flinched when Engle told them what good sizing calculations showed they needed,” Schrock says. “But the numbers don’t lie.”
The ductwork connection
Just as important to equipment right-sizing is a complete makeover of the return-air system. HVAC contractors in most northern states, or any market with ductwork in basements, use building cavities for returning air back to the HVAC equipment. Field testing shows that those cavities are extraordinarily leaky. Engle recently switched to a simplified return-air system, similar to that shown in the EEBA details (see air handler and duct layout on page 114). It’s helping Engle cut duct leakage by more than 75%, enabling further downsizing.
Lstiburek’s team has recommended for years that air conditioners be substantially downsized. A few years ago, Pulte Homes/Las Vegas took the bait. Two homes, 1,787 and 2,260 square feet, respectively, had their equipment aggressively downsized to 3 and 2.5 tons, respectively. That’s about 40% smaller than normal installations. One home worked well; on the hottest days of the year, the second home’s AC got the job done with no margin for error. Major updating of gut-rehabbed homes can lead to similar downsizing in existing homes.
Denver-based freelance writer Steve Andrews has consulted in the energy world, primarily in the residential sector, for more than two decades, and he serves on EEBA’s board of directors.
The Energy & Environmental Building Association (Westford, Mass.) is a nonprofit organization dedicated to offering tools and resources for constructing homes that are safer, more affordable, durable, efficient and environmentally responsible.
EEBA’s five Builder’s Guides highlight design issues and solutions tied to specific climates: hot-humid, hot-dry/mixed-dry, mixed-humid, cold and severe cold. For more information, visit www.eeba.org or call 952/881-1098.