To ensure HVAC equipment meets space conditioning loads during peak conditions, systems that serve those loads are significantly oversized. In fact, the Air Conditioning Contractors of America (ACCA) guidelines in Manual S recommend equipment be sized 15% to 40% greater than the expected peak loads depending on equipment type. Therefore, HVAC equipment capacities are typically much larger than the loads they serve most of the time. As such, they operate at low part-loads (the ratio of space conditioning load to equipment capacity). When systems serve part-load conditions, cycling and other factors negatively impact performance; typically, the lower the part-load, the shorter the heating cycles, and the lower the efficiency. Systems that operate efficiently at low part-load conditions improve comfort and cost-effectiveness. Therefore, it is imperative the industry understands how well systems perform in part-load conditions.
A novel approach has been developed for determining part-load performance of residential appliances. The approach is referred to as the Virtual Test Home (VTH) method and it consists of multiple test rigs and associated algorithms that simulate real world conditions in a controlled, repeatable laboratory environment. The VTH can be used to characterize the part-load performance of gas and electric space and water heating equipment. Unlike the Annual Fuel Utilization Efficiency (AFUE) and Heating Seasonal Performance Factor (HSPF), which are method of tests (MOTs) based on one set of specific conditions, the VTH provides performance characterizations across a wide range of system capacity part-loads ranging from 1-100% and various ambient conditions and building envelope/type. The performance characterizations are then incorporated in building energy modeling software, such as EnergyPlus, to estimate annual appliance efficiencies, and quantify potential annual energy, emissions, and cost savings for various equipment in various buildings and climates. Field data are then used to ground truth the VTH results and ensure accuracy of the resulting modelling work. This paper describes how the VTH test methods and modeling, combined with measurement and verification data collected in the field have resulted in a detailed and flexible approach to assessing as-installed performance of HVAC and water heating equipment and quantifying potential energy and cost savings in buildings.
