Air Distribution Impacts in Net-Zero Energy Homes

Summary

The residential building sector accounts for approximately 21 % of the total energy consumption in the United States[1]. Reducing residential energy consumption aids homeowners by lowering their monthly utility bills while aiding the nation with more readily allocating energy resources to the commercial and industrial sectors. A multitude of construction processes, equipment selections, and operating strategies exist that can reduce home energy use. Needed work remains to quantify the savings and overall merit of different approaches, including capturing how operating one home subsystem may affect another subsystem, while also accounting for impacts on costs other than the utility bill, thermal comfort, user utility, and maintenance duties. To research these issues and provide industry guidance and data on the best approaches to achieve savings that, with the inclusion of on-site energy generation, can achieve net-zero energy operation, NIST has developed the Net-Zero Energy Residential Test Facility (NZERTF).  The NZERTF is a well-constructed, well-instrumented, controllable and reconfigurable single-family house. The NZERTF provides a field laboratory to test and evaluate technologies and operating strategies at both the subsystem and whole house levels.

Key issues for low-energy consuming homes include the design and operation of the space conditioning and ventilation systems, particularly their air distribution components. This project’s primary research focus is to quantify and provide information on the energy consumption and comfort impacts from using different duct configurations and operating strategies. The current research effort uses two of the three supply duct systems installed within the NZERTF, each of which is comprised of an excess number of supplies and oversized returns from which different subset systems are created and then evaluated over multiday test intervals. The NZERTF also readily accommodates multiple equipment options to heat, cool, dehumidify, and humidify the air. During FY25, the setup will be switched from an air-source heat pump and a separate water heater to a ground-source integrated heat pump system. The field data and information gained from the shorter-term air distribution studies and the longer-term integrated appliance evaluation will be shared with industry and standards-setting organizations and used for computer model validations. These models will then be exercised to explore “what if” scenarios, such as predicting the performance of the same or similar systems and operating strategies in other housing types and climate zones.

A secondary focus of this project is to maintain, facilitate, and where merited, upgrade the NZERTF to maximize its use for other research projects. The facility is used to explore research in the areas of indoor air quality and air chemistry, water quality, assessing the potential for indoor photovoltaics, and whole building economics, among others. Considerable time in FY25 will be allocated to facilitating a multi-month study at the NZERTF that is investigating how gases and particles are captured, transformed, or unaffected as air circulates through filters and heat exchangers of heating, ventilating, and air conditioning equipment.

Description

Objective 
To acquire high-quality data from well-controlled field configurations that inform residential building designers and equipment standards organizations about space conditioning and ventilation system performance characteristics, especially the air distribution system(s), when applied in well-insulated, well-sealed homes. The experimental data will also be used for evaluating, improving, and validating building energy, indoor air quality and indoor air chemistry computer models.

Technical Idea
The NZERTF serves as a test bed to help address the lack of detailed field-test performance data that slow the progression to net-zero energy homes. For example, the NZERTF provides a means to investigate how different duct configurations and strategies for controlling ventilation air and the recirculation and exhausting of conditioned air affects energy consumption and comfort for applications where the building loads are lower, the sensible heat ratio is shifted lower, and the ventilation rate is controllable, all while the volume of the conditioned space remains unchanged. The NZERTF allows these air distribution investigations through the use of its multiple duct systems, most being reconfigurable, and multiple variable-speed air handlers that can implement a wide range of user-specified control strategies. For example, recent testing quantified the impacts from using (1) a cooling optimized damper configuration when heating, and vice versa, (2) an indoor blower continuous fan operation versus a stand-alone air redistribution system, (3) small-duct supplies versus conventional supplies, (4) dedicated dehumidification by the heat pump versus a stand-alone whole house dehumidifier, and (5) four fully open returns versus configurations where the main level returns were substantially restricted. The mechanical ventilation system, in addition, allows comparisons of heat recovery ventilation versus energy recovery (via changing the heat exchanger’s core). The NZERTF also offers unique features and capabilities to evaluate the field performance of efficient equipment options (e.g., CO2 heat pump water heater, ground-source heat pump that offers integrated water heating) and to conduct whole-house studies (e.g., demand ventilation control, thermal envelope and fenestration performance, residential indoor air quality and chemistry). In many of these cases, the measured field performance is used to develop and/or evaluate models and testing and rating methods that are used to predict installed performance.

Research Plan
The FY25 work is centered on initiating and then conducting a 12-month, whole-house study in which a ground-source, integrated heat pump (GSIHP) is used to meet the space conditioning and water heating needs of a virtual family of four “living” in the NZERTF. As part of a separate EL project, the steady-state and transient space conditioning and water heating performance of the integrated appliance is being quantified via laboratory testing in advance of the system installation in the NZERTF. The combination of the lab characterization and field monitoring will provide data for evaluating an existing ASHRAE test standard that estimates seasonal efficiency. The combination will also provide a means for expanding and “tuning” an existing TRNSYS model of the NZERTF and its major subsystems. The laboratory and full-scale testing will provide information helpful to the next revision of the standard, while the test results coupled with the TRNSYS modeling will allow more informed decisions on the merits of using ground-source integrated appliances as a mechanism for achieving low-energy consuming homes for a variety of building designs and climate zones.

Prior to transitioning to the GSIHP study, the duct configuration work will be ramped down. Measurements are planned to focus on two lingering research topics: the performance differences between heat and energy recovery ventilation, and the differences between conventional and small duct air supply systems. Efforts will otherwise focus on organizing, mining, and analyzing large amounts of data and then writing two publications. A NIST Tech Note will serve as an archival reference for the daily setup and operating characteristics of the heating, ventilation, and air conditioning systems – hardware and controls – when different research efforts were conducted at the NZERTF over the past few years. Unique adaptations tailored to meet the research study’s specific needs will be covered.  Following the analysis of a subset of the duct configuration test results, a journal paper will report the energy consumption and comfort impacts of using an alternative duct distribution system in a well-insulated, well-sealed home.

Short term tests and calibration checks will then be conducted to close out the multi-year duct distribution systems study and to prepare for the ground-source integrated heat pump 12-month field study. For example, tests will be conducted to measure the component and total air volume rates of the ventilation, exhaust, and heat pump duct distribution systems.

The 12-month field study will be carried out under two High Performance Buildings projects, with this project being more responsible for the water heating, air distribution and comfort performance evaluations and associated instrumentation and measurements. During the second half of FY25, researchers will install, instrument, and commission the ground-source integrated heat pump in the NZERTF. Thereafter, the house and the GSIHP, in particular, will be closely monitored. The goal is to identify any anomalies as soon as possible so that they may be considered and, if merited, addressed in a timely manner.

This project also retains the responsibility for operating the NZERTF in a manner that facilitates the primary research effort, which changes over time. In FY25, a multi-month study investigating how gases and particles are captured, transformed, or not affected as air circulates through filters and heat exchangers of heating, ventilating, and air conditioning equipment requires operating the NZERETF as a house on some days and as a quasi-environmental chamber on others. Consequently, creating a load pattern within the house that is representative of a typical family of four is the typical goal for the former while maintaining prescribed indoor temperatures and humidity levels is the primary objective for the latter. Various NZERTF operational duties – such as coordinating and aiding the use of the NZERTF by other researchers, tours, access to past data, repairs to any hardware and instrumentation that are not directly tied to the GSIHP’s space conditioning system – are the responsibility of this project. Finally, steps will be taken to create an inhouse competency for conducting building energy simulations using the modeling tool TRNSYS.

---

[1] United States Energy Information Administration, “Total Energy”, https://www.eia.gov/totalenergy/ Last accessed June 11, 2021.