Properly Size Cooling Equipment and Delivery System

Bigger is not always better! In the case of space cooling, equipment that is larger than the building load requires actually does harm. Oversized equipment can lead to moisture problems since the air conditioner may not run long enough to adequately dehumidify indoor air during summer cooling months. This can lead to mold and condensation on surfaces and possibly in wall cavities, not to mention the comfort issues this creates from cool but clammy air.

• Use proper methods such as the Air Conditioning Contractors’ Association (ACCA) Manual J to determine the design heating/cooling loads. Equally important, use simple QA checks to assure what looks like “right sizing” isn’t really “wrong
  sizing”.
  • Load sizing has been required by code for some time. It’s been made simpler and faster via load sizing software too. So having a load calculation for a home is a bare minimum at this point—now we need to make sure it’s actually accurate! As the example in Figure 5–1 shows, seemingly reasonable—but inaccurate—values for just six basic inputs to a load calculation can cause under- or over-sizing errors approaching 25%. The combined impact of the six inaccurate inputs is actually a bit less than the total of their individual impacts, because the inputs interact with each other when they’re modeled together. In real-world examples, more egregious errors throw off design loads by a factor of 2 or more.
  • For this reason, builders and their contractors should employ the ENERGY STAR Homes HVAC checklists. The HVAC Contractor checklist includes written documentation of key design variables including outdoor design temperature and corresponding location; home orientation; predominant window Solar Heat Gain Coefficient (SHGC); etc.
  • For added assurance of a good load calculation, the HVAC Rater checklist requires a rater to double-check that the outdoor design temperature is appropriate and accurate; that the home orientation matches the actual home; that the predominant window SHGC is within 0.1 of the installed windows; etc.
  • Builders and their trade partners can take advantage of these resources regardless of participation in the ENERGY STAR Homes program. Links to both checklists are found in Section 5.4.
  • Multiple software packages for calculating building heating and cooling loads are available. Several residentially oriented packages can also be used to model energy performance as well as code compliance. A link to DOE’s software directory can be found at the end of this chapter.

• Use ACCA Manual S for selecting proper equipment sizes.
  
  • Central air conditioning equipment should be within 95–115% of design cooling load or the next nominal size
  • Central warm air furnaces should be within 100–140% of design heating load or the next nominal size, unless cooling load dictates larger equipment
  • Central boilers should be within
    100–115% of design heating load or the next nominal size

It is equally important that ductwork is also properly sized, designed, and of course, installed per design.

• Design and size the duct system in accordance with ACCA Manual D and blower performance data from manufacturer equipment specs.
• Airflow at each register to be within 20% or 25 cfm of design airflow, whichever is greater. Provide balancing dampers at each register. Measure air flow at each register and balance accordingly.

Ductwork that is not properly sized will result in a blower that is starved for air and/or very high or low flow rates. Excessively high or low flow rates can set up pressure imbalances that induce infiltration or exfiltration through the building envelope. Such flow rates also put undue wear and tear on the compressor and shorten the life of the equipment.

A link is provided at the end of this chapter that serves as a good resource for proper air distribution system design.