In new homes, poor HVAC design and installation
practice accounts for more energy waste than any other single factor. Unfortunately, when it comes to
HVAC, it’s easier to sell high efficiency boxes than high efficiency systems, a distinction invariably lost in
a competitive marketplace. As a result, HVAC remains the weakest link in most high performance homes.
As homes have become more efficient, oversized HVAC equipment has emerged as one of the more
serious problems in building science. Although there’s general awareness of this issue among industry
practitioners, few understand the full extent of the problem or its consequences.
There are numerous reasons why oversizing is a bad idea:
• Oversized equipment costs more and requires larger electrical circuits
• Oversized compressors have a shorter life expectancy
• Excess capacity compromises comfort (larger temperature swings)
• Excess cooling capacity compromises moisture removal, a big deal in humid areas
• Larger compressors and blowers produce more noise
• Excess capacity compromises indoor air quality (less run time = less filtration)
• Excess cooling capacity increases the potential for structural damage from moisture
• Oversized equipment is less efficient, thus increases operating costs
Said differently, right-sized HVAC systems cost less up front, last longer, provide better
comfort, improve moisture removal, run quieter, provide better filtration, and cost less to operate.
Oversizing, endemic in code-built homes, is becoming an epidemic in high performance homes. Even though
most mechanical contractors have been taught the virtues of right-sizing, they can’t seem to break free from
their ‘bigger is better’ bias. Moreover, most are unfamiliar with the nuances of designing for high
performance homes. As a result, the HVAC equipment is often grossly oversized. This not only undercuts
potential energy savings but makes these homes especially vulnerable to comfort and moisture problems.
The best way to ensure proper sizing is to have an independent specialist size the equipment according to
Manual J procedures. To this end, a growing number of energy raters are offering load calculations in
conjunction with Energy Star and other home performance programs. Builders who understand the rationale
for right-sizing appreciate the value of this service. Moreover, an independently prepared load and equipment
specification makes it easy for the builder to obtain apples-to-apples HVAC bids.
Optimal Building Systems was established to provide load calculations1 and HVAC
design guidance to energy raters and other home performance professionals who either don't have HVAC
expertise on staff or simply don't have the time. All loads are performed by David Butler, a building systems
engineer with 25 years of experience in residential energy efficiency.
Please check out The Elephant in the Room. It will open your eyes.
1 Homes must participate in Energy Star or
other third-party verification program that includes thermal bypass and insulation inspections, blower door and duct
leakage tests. A completed Rem/Rate file will be provided to the HERS Rater for an additional fee.