Bigger Is Not Better
Previously, some builders may have selected oversized HVAC systems in the hopes of guaranteeing comfort and limiting callbacks but this approach is costly and inefficient, particularly in homes with tight thermal envelopes. Instead of providing comfort, an oversized system creates temperature swings and wastes energy by short cycling: constantly turning on and off. Short cycling also diminishes the system’s ability to remove moisture.
A traditional system sized for ASHRAE design temperatures — the coldest and hottest days in a particular region — will be especially vulnerable to performance issues related to short cycling. During the “shoulder season,” — the greater part of the year where temperatures are between the design day extremes — the fixed speed compressors of traditional systems leave them unable to go from maximum capacity to low-capacity output. Additionally, oversized central air conditioners can produce negative air pressure that acts as a vacuum pulling in volatile organic compounds (VOCs) and other particulates from outside the home.
A Better Approach
Builders apply split-ductless and ducted systems to successfully address the particular loads and challenges of high-performance homes. Available in a range of capacities to match the size and loads of any home, these highly efficient systems feature either a single outdoor compressor or multiple small outdoor units. The outdoor equipment is connected via refrigerant lines to a single indoor unit or to multiple indoor units. The variety of indoor units include ductless options such as ceiling cassettes, wall-mounted units and floor-mounted units as well as ducted options such as compact, low-profile air handlers and larger, multiposition air handlers. A combination of these units may be applied together according to the home’s design, efficiency goals and customer preferences.
Additionally, the low amperage draw of these units compared to traditional systems makes modern split-ductless and ducted systems an ideal choice for homes that use solar, wind or hydroelectric power. Builders are now operating in an environment where global, national and local initiatives toward reducing greenhouse gas emissions has led to Strategic Electrification — the movement to replace fuels like oil, gas and propane with electricity produced from renewable and sustainable sources. This makes it increasingly important to consider choices like high-efficiency heat pumps that perform optimally with solar power.
These more sophisticated HVAC systems are still sized according to ACCA guidelines for load calculations, using their Air-Conditioning, Heating and Refrigeration Institute (AHRI) rated capacities but are equipped with variable-speed compressors that seamlessly adjust performance to maintain set points. This technology conserves energy compared to traditional systems that run at full power until reaching the set point and then stop, only to turn on again, sometimes multiple times within an hour as rooms become uncomfortable. In addition to saving energy by only using the precise amount of energy needed to meet set points, there’s an added benefit of increased durability and longer life because of the minimized wear and tear. Additionally, modern split-ductless heat pump systems may incorporate air-source hyper-heating technology engineered to perform efficiently in cold climates with design temperatures below freezing