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Space-Conditioning Options for Energy-Efficient Buildings Project


Space-conditioning equipment in buildings accounts for 37% of the Nation’s primary energy consumption in buildings and 15% of the Nation’s overall primary energy consumption1. For this reason, reaching the goal of net-zero energy buildings requires high-efficiency air-conditioner and heat pump designs. These designs must accommodate the shifting building load paradigm brought about by better insulation, lower infiltration, and more efficient appliances with smart controls.

The project focuses on evaluating space-conditioning options for energy-efficient residences, where the NIST Net-Zero Energy Residential Test Facility (NZERTF) will serve as the test bed for experimental verification. Heating and cooling loads in the NZERTF will be recorded and documented in detail and serve as the benchmark for comparing alternative configurations. Through data analysis and simulations, the project will explore space-conditioning options for different climatic locations in search of optimal solutions for these localities. This project will also develop simulation models for vapor compression systems, the dominant technology for comfort space conditioning. These models will be used to evaluate different system configurations for energy-efficient homes.


[1] 2011 Buildings Energy Data Book, US Department of Energy, Office of Energy Efficiency & Renewable Energy,



Objective: By FY2016, develop and deploy the measurement science to support the development and implementation of the most efficient and cost effective space-conditioning options for energy-efficient buildings.

What is the new technical idea? The unique aspects of high-efficiency homes dictate new solutions for space conditioning equipment. Because of tighter envelopes and better insulation, high-efficiency homes are less sensitive to the external environment and more sensitive to internal loads. As a result, cooling systems must be able to cope with higher moisture removal demands (higher latent loads) as a percentage of the overall cooling demand. An additional consideration for heat pumps in new homes is the integration of outdoor air mechanical ventilation systems, which are required to maintain indoor air quality for buildings with tighter envelopes.

The overall concept of the project is to document the cooling and the heating loads measured in the Net-Zero Energy Residential Test Facility (NZERTF), and to use these load profiles to explore various energy technologies in search of the most effective and economical options for energy-efficient homes. The main focus of the study is on enhancing the efficiency of the NZERTF. However, a broader impact of the study will be attained by extrapolating the NZERTF results to different climatic regions and longer timeframes using a building simulation model developed on the TRNSYS platform. Special consideration will be given to exploring and understanding the performance of geothermal heat exchangers (GHXs). The project will also advance simulation tools for designing high-efficiency heat pump systems, and will involve some considerations for proper equipment commissioning since faulty equipment installation can negate efficiency benefits otherwise attainable with high-efficiency air conditioners and heat pumps.

What is the research plan? Task 1 will focus on documenting the space-conditioning loads of the NZERTF with special attention given to the latent/sensible load fraction. Heat pump performance will be analyzed to explore new design options that would make a heat pump more suitable to the elevated latent load expected in energy-efficient homes. Results of this analysis – aided with simulation tools of Task 2 and Task 4 – will guide the selection of the next space-conditioning system to be tested in the NZERTF. The FY2015 Technical Note will constitute the U.S. contribution to the IEA Annex 402. In FY2016, a new space-conditioning configuration will be tested in the NZERTF.

Task 2 involves using a TRNSYS building simulation model to broaden the impact and guide the direction of the experimental research conducted on the NZERTF. In FY2014, the model will be validated against the NZERTF annual test data. Then, the energy and economic merits of different energy technologies will be evaluated to provide guidance for NZERTF space-conditioning options beyond those used in the initial baseline tests. Also, separate TRNSYS simulations of the NZERTF GHXs will be conducted. They will expand on the measurements taken during the FY2013 GHX tests to develop improved design guidance and to select the GHX for future NZERTF tests. In FY2015, the NZERTF space-conditioning options will be made more broadly applicable through their simulations across the U.S. climate-zone spectrum. Work in FY2016 will explore the potential benefit of integrated appliances. 

Task 3 entails the development and implementation of novel optimization methods in the EVAP-COND tool for designing air-to-refrigerant heat exchangers. Version 4.0, released in FY2013, included the predefined “hair pin” pattern option, which improved the manufacturability of generated designs. The FY2014 Ver. 4.1 will include a method for optimizing heat exchangers that operate both as an evaporator and condenser, depending on the season. In the next two years a model of a microchannel condenser and evaporator will be included in the EVAP-COND package.

Task 4 entails the development of HPSIM, a first-principles-based vapor-compression heat pump model. In FY2013, HPSIM Ver.1.0 was prepared, and its release is scheduled for the 1st quarter of FY2014. In FY2014, several upgrades to the model (Ver. 1.1) will expand its utility for equipment manufacturers and will make HPSIM suitable for use in fault detection and diagnostic studies conducted within the Embedded Intelligence in Buildings Program.

Task 5 will involve considerations for proper installation of split air conditioners and heat pumps. NIST will participate in the completion stage of the IEA Annex 36 by contributing to the annex’s final report and the closing workshop.


[2] International Energy Agency, Heat Pump Program, Annex 40, “Heat Pump Concepts for Nearly Zero Energy Buildings”