For Designers

Right-Sizing HVAC Systems

The energy performance of windows has a direct impact on heating and cooling loads and thus on the demand placed on heating, cooling and air conditioning (HVAC) systems. It is important to take into account this impact on heating and cooling loads when determining the adequate HVAC system size for homes. Oversized HVAC systems should be avoided for several reasons:

• Energy savings - The energy savings from better performing windows can fully come to bear if HVAC units are no larger than needed. Oversized units may have inefficient stop-and-go cycles if the demand placed on them is far lower than what their tonnage can provide;
• Health and comfort - By running more constantly, smaller HVAC systems provide the best air quality and comfort if their size does not exceed what is necessary for meeting the heating and cooling load;
• Mold prevention - In climates with humidity issues, HVAC systems that are more closely matched to both peak and non-peak cooling loads achieve better dehumidification;
• First cost savings - Smaller HVAC units cost less. If, for example, down-sizing the HVAC system by half a ton saves $275, the cost premium of energy-efficient windows does not present as big an up-front investment anymore.

Efficient windows and compact HVAC system design
Including the potential for HVAC downsizing with energy-efficient windows in design considerations brings more cost reduction than just the reduced price of smaller HVAC units alone. Compact design with smaller HVAC distribution systems reduces installation cost and reduces the potential for leaks and losses. Significantly smaller distribution systems are possible with highly insulating windows in cold climates and windows with superior solar control in warm climates. Such advanced windows combined with good insulation may provide that occupants are comfortable with registers or baseboards located closer to the center of the building, rather than the standard practice of delivering heating and cooling near exterior windows.

HVAC sizing tools
Several computation procedures exist for proper sizing of HVAC equipment. The most prominent ones, which are also recommended by the ENERGY STAR Homes program, are ACCA Manual J and the ASHRAE Handbook of Fundamentals.

Fenestration factors to be considered:

• The energy performance of the windows themselves must be considered in load calculations. NFRC-certified window performance values significantly increase the accuracy of these calculations.
• Window orientation and overhangs must be taken into account. Overhangs are an important factor influencing solar gains through windows, and failure to include them in the calculation can lead to an oversized cooling capacity of one ton for a mid-sized home.
• Where internal shades and blinds are to be assumed, these should also be accounted for in load calculations.