Benefits: Lower HVAC Costs

High-performance windows not only provide reduced annual heating and cooling bills; they reduce the peak heating and cooling loads as well. The peak load for a building is the maximum requirement for heating or cooling at one time. These loads determine the size of the furnace, heat pump, air conditioner, and fans that must be installed.

Reducing peak load may allow homeowners to install a smaller heating or cooling system. Smaller HVAC systems cost less and as such can offset some of the cost of the efficient windows. The graphs below show HVAC system sizing for an average size home in Phoenix and Minneapolis. In the Phoenix example, using low-solar-gain low-E windows may allow cooling equipment to be two tons smaller than with single-pane windows and at least one ton smaller than with clear double-pane windows. Even in the Minneapolis example, window choices can impact cooling equipment size by up to a ton. In any homebuilding or remodeling project it is important to properly size the HVAC system to ensure that the equipment runs efficiently and provides the best comfort. When efficient windows are installed, peak HVAC loads are often lower than commonly expected.

Note: The peak system sizing figures shown were generated from simulation results using RESFEN provided by Lawrence Berkeley National Laboratory ( Results assume a typical new construction 2600 sq ft house with 15% window-to-floor area distributed equally on all four sides. U-factor and SHGC are for the total window including frame. Conversion between air conditioner electric load and system sizing assumes an 11.2 EER.

Right-Sizing HVAC Systems

Properly sized HVAC systems allow for a number of benefits, including:

•    Health and comfort – By running more constantly, smaller HVAC systems provide the best air quality and comfort.

•    Mold prevention – In climates with humidity issues, HVAC systems that are more closely matched to 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.

•    Energy savings – Oversized units may have inefficient stop-and-go cycles; the best system efficiency is realized when HVAC units are sized appropriately.

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.

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. Where internal shades and blinds will be actively used, these should also be accounted for in load calculations.