HVAC Equipment Sizing Calcs

"Problems are messages."
- Shakti Gawain

Sam Dardano, a Boulder-based code official who chairs the committee of statewide mechanical and plumbing inspectors, reports that by early next year roughly 75 percent of the building jurisdictions in Colorado will be operating under the International Codes. If that's true, here's a key item from the code that can help, not just hurt.

The International Energy Conservation Code (IECC) requires that load calculations be used to size heating and cooling equipment. 11' properly implemented, this could reduce the widespread tendency to oversize equipment. Yet both builders and code officials are uncertain how to evaluate such calculations to assure the results are accurate.

This article presents 10 top items to look for when evaluating HVAC sizing calcs.

Background

An article titled "Bigger is Not Better," Published in the May-June 1995 Home Energy magazine, was one of the first to draw attention to the widespread problem of residential equipment oversizing. A study of design. construction and performance issues in northern Colorado hones built in the mid- to late1990S ( fcgov.com/utilities/es-performancestudy.php ) was the most recent to confirm that heating and cooling equipment tends to be oversized by substantial margins in this part of the country. The Colorado study showed heating systems were moderately oversized while air conditioning systems were nearly twice as large as needed - averaging 158 percent and 208 percent of design loads, respectively.

Furnace sizing ratios ranged from 106 percent to 234 percent of design heating requirements. Greater oversizing factors were typically observed in homes with insulated basements versus homes with uninsulated basements, suggesting that furnace-sizing practice had not yet reflected the reduction in heating loads due to basement insulation.

Cooling systems ranged from about 143 percent to 322 percent of design cooling requirements.

Note that for every hour of the year when heating and cooling requirements are less demanding than design conditions, the equipment is even further oversized.

Over-sized equipment requires more air flow and larger ductwork; without this, equipment will not operate within manufacturer specifications. Even if ductwork sizing is increased, the oversized equipment will short-cycle. These problems decrease efficiency and equipment life while compromising homeowner comfort. Utilities may be burdened with higher summer peak loads and more blown transformers. Builders and homeowners pay more for oversized systems.

Over-sizing typically occurs when contractors use "rules of thumb," such as "I toil of AC needed per 600 square feet" or other simple sizing approach based on their own experience. In 2000, Hank Rutkowski. author of ACCA Manual J: Residential Load Calculation, estimated that only 5 to 10 percent of' HVAC systems had calculations performed to help size systems. Furthermore, even when load calculations were performed, contractors were inclined to include fudge factors based on past customer complaints about comfort. "I've never been sued for installing too large a system," contractors have stated repeatedly.

In the 8th edition, published in April 2002, Rutkowski wrote, "Manual J calculations should be aggressive, which means the design should take full advantage of legitimate opportunities to minimize the size of estimated loads. In this regard, the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring internal and external window shading devices, and then applying an arbitrary 'safety factor' is indefensible."

It should be noted that oversizing does not address many other related problems that cause homeowners to complain. As noted in the Colorado study. these include problems with excessive solar gain, insulation and air sealing flaws, lack of' ductwork design and many compromises in duct installation (constrictions. leakage, pressure imbalances, no way to balance air flow among branch ducts).

Does the above sound a little academic'' It doesn't have to be. Aspen Homes now installs 40,000 Btu to 60,000 Btu furnaces in all their high-performance homes, replacing 100,000 and 120,000 Btu units, respectively, saving $40 to $50 a pop: their air conditioners are similarly downsized, saving at $250-$500.

Ten key sizing factors

1. Use acceptable sizing calculation tool: Most jurisdictions allow calculations based on Manual J (Air Conditioning Contractors of America - an industry trade group). Manual J methods are based on the ASHRAE Handbook of Fundamentals. The 8th Edition of Manual J is the most current; it has been modified to reduce Manual Fs past tendency to enable over-sizing.

2. Outdoor design temperatures: There is considerable room for error here; check to assure the proper winter/summer outdoor design temperatures are used. The IECC specifies using `°97.5 percent values for winter and 2.5 percent values for summer, from tables in the ASHRAE Handbook of Fundamentals." (97.5 percent means during the average winter, the temperature will remain above that temperature 97.5 percent of the time.) Unfortunately, 97.5 percent and 2.5 percent values aren't available in the ASHRAE Handbook any longer. Contact E '-Star (see contact info below) for the comparable list of design temperatures.

In most Denver areas. the winter design temperature should be within a few degrees of 0 (leg. F, and the summer design temperature should he about 92 degrees.

3. Indoor design temperatures: Check to assure that proper indoor design temperatures are used (70 deg. F winter and 75 deg. F summer).

4. Window orientation: While heating equipment sizing is unaffected by window orientation. the impact of orientation on cooling loads can be substantial. In fact, in a new home built to the TECC standard, solar gains through windows are typically the home's largest contributor to peak cooling load up to 50 percent. For production builders, orientation should he considered when calculating cooling equipment size for the same model home placed on lots with different orientations. It should he noted that some homes with predominantly west-facing glass will not be comfortable. during some climate conditions. regardless of system size, without very smart window choices.

5. Reasonable air infiltration assumptions. A few jurisdictions insist that high air-leakage rates be assumed. Many contractors assume high leakage rates. Often, projected house leakage is overestimated, again contributing to over-sizing. House tightness testing results for geographic locations and specific builders should he factored in. A reasonable air leakage assumption: between 0.35 to 0.50 natural air-changes per hour, Unless a builder has data specific to their construction practices indicating they build tighter (or looser). (Engle Homes averages 0.12 air changes - four times tighter than the average home.)

6. Proper energy features. The R-values. U-values and window Solar Heat Gain Coefficients (SHGC) specified on the plans should match those used in the calculations. Foundation insulation and window values are prone to incorrect entry.

7. Duct losses. One figure is entered in the calculation to represent conductive losses from ducts in unconditioned spaces. It is otherwise specified and assumed that ductwork will be "substantially leak free," per code. (The IECC specifics this as being, "5 percent or less of the air handler's rated air-flow when the return grilles and supply registers are sealed off' and the entire distribution system-including the air handler cabinet is pressurized to 0.1-inch w.g. 125 pascals. Unfortunately, random testing in the northern Colorado showed that ductwork leakage averaged 130 percent of the average air-handler's rated air flow). Today, a small but growing number of Colorado HVAC contractors are developing the expertise to design and build tight ductwork. then buying equipment to perform pressure measurements that confirm their results. Duct losses are highly dependent on duct location. The number of ducts in exterior walls, garage ceilings, vented crawl spaces and attics is a critical factor, with respect to losses from both duct leakage and air infiltration. Ducts in the exterior of the envelope must be effectively insulated to a minimum of R8. (IECC 2003)

8. Climatic moisture load factor. The difference between the moisture content of the outdoor air and desired interior humidity is referred to as "design grains." Calculations should use "design grains" applicable to a particular jurisdiction (see Manual J). Latent loads are typically a tiny part of the design cooling load in this climate. In the metro area. designs grains are approximately -40. Latent loads for summer cooling typically in the 1.000 to 2.000 Btu/hr range (varying with house size).

9. Assume shading devices. Even for new homes. the presence of reasonable internal shading devices should be assumed. People can he expected to close their window cover day. Built-in external shading (overhangs, adjacent buildings, etc.) should also be factored in.

10. Capacity margin of selected equipment. This maximum sizing guideline should be followed: "The total capacity (sensible plus latent) of the cooling equipment should not exceed the total load (sensible plus latent) by more than 15 percent for cooling-only applications and warm-climate heat pump applications: or by more than 25 percent for cold-climate applications." (Manual J. 8th Edition)

If your HVAC contractor uses the correct factors for the above 10 items, you've got a sharp one.

Finally, don't forget to' educate your clients: they think having the biggest AC unit on the block is a good thing. One thousand times: no!

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007