Air Distribution Impacts in Net-Zero Energy Homes

Summary

A Net-Zero Energy Residential Test Facility (NZERTF) has been constructed and extensively instrumented. The facility incorporates a high-performance envelope, a mechanical ventilation system with energy recovery, three distinct ground-source heat exchangers, a whole house dehumidifier, and three separate supply duct systems for distributing conditioned air throughout the house. Two of these supply duct systems include an excessive number of outlets, which can be individually opened or closed (in one case by using electronic dampers) to permit duct system reconfigurations and zoning.  The heat loads, the hot water draw profile, and the electrical plug loads generated by a typical family are replicated using computer-controlled devices located throughout the facility.

The NZERTF provides a platform for comprehensive measurements to explore various designs, technologies, and control strategies for achieving net-zero energy performance. This project’s primary research focus is to quantify and understand the energy consumption and comfort impacts of the configuration and operation of the forced air distribution systems within the house – multiple configurations of the  heat pump supply and return duct system, the mechanical ventilation system, and the whole dehumidifier’s dedicated return duct. Moreover, in FY21, the prospects for using a ceiling fan(s) and/or a stand-alone duct system to independently circulate air between floors will be explored. The roles of coordinating and facilitating the effective use of the NZERTF for other research projects, and in some cases, serving as a support researcher, are also elements of this project. In FY21, these other projects vary from conducting whole-house EnergyPlus evaluation testing to facilitating an intensive study of residential indoor air chemistry issues by a large group of collaborators. Overall, the insight gained, lessons learned, and data generated directly and indirectly from this project will provide information to the building industry to enable advances in efficient residential construction and will result in measurement science that accurately characterizes the performance of several technologies.

Description

Objective - Develop the measurement science necessary to evaluate various technological approaches, with a focus on the air distribution systems, to achieve thermally comfortable and healthy net-zero energy residential buildings via innovative monitoring techniques, methods of test, performance metrics and high quality experimental data for the validation and improvement of building energy and indoor air quality models.

What is the new technical idea? 
NIST’s “Measurement Science Roadmap for Net-Zero Energy Buildings” emphasized the need for improved monitoring techniques, metrics, and models to assess the energy performance of net-zero energy buildings (NZEB).  The NZERTF, which has been used to demonstrate net-zero energy performance using different equipment and operating configurations, is now being used as a test bed to address the noted measurement science needs. For example, the NZERTF provides a means to investigate how different 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 to latent ratio is shifted, 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 blowers and air handlers that can implement a wide range of user-specified control strategies. 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 performance, residential indoor air chemistry). In nearly all 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.

What is the research plan?
During the FY21 heating season, project members will coordinate a series of week-long tests at the NZERTF that quantify the performance of the house’s thermal envelope. The test configurations and operating schedule are devised by a group of collaborators that includes researchers from ORNL, NJIT, and EL’s Indoor Air Quality Group. The collaborators are in the midst of a 3-year project that seeks to evaluate how well the widely-used whole-building simulation tool EnergyPlus models highly-insulated, well-sealed homes like the NZERTF. As occurred during FY20, project members will answer questions that arise pertaining to the test measurements and specific equipment operational observations while also helping review and interpret the results.

In addition, during FY21 this project will include the completion of investigations and publications on the impact of the air distribution systems within the house. The effects of reducing the flow at one or more of the four returns will be quantified. The fan power penalty and comfort changes created from using only the supplies of the small-duct, high velocity (SDHV) system will also be investigated. This testing is being pursued because SDHV ductwork is easier to install and so offers a lower installation cost. This testing will help determine the “cost” with respect to thermal performance and equipment efficiency. Another atypical use of the SDHV supplies that will be investigated is use two subsets of these ducts, in combination with the blower of the otherwise dormant SDHV heat pump, to transfer air between different levels in the house. For example, during the cooling season, the basement manifold will be connected to the return side of the blower while the blower’s outlet is connected to the plenum that feeds the SDHV attic manifold. The resulting dedicated air redistribution system is expected to yield the best option at minimizing the persistent stratification that occurs for all the duct configuration and operating strategies tried during the cooling season (when continuous fan operation of the home’s air conditioner or heat pump should not be used because such operations evaporate water that collects in the condensate pan). More conventional ways of approximating the dedicated air redistribution system – by using the heat pump’s indoor blower in combination with the on/off electronic air dampers and/or via the deployment of one or more ceiling fans will also be explored. Finally, the potential for maximizing the ventilation rate for the sole purpose of operating in an economizer mode will be tested. An economizer mode saves energy during those times when the outdoor temperature and relative humidity create an opportunity for natural cooling/heating in lieu of the thermostat calling for mechanical space conditioning of the same form. The economizer option will likely be explored as an offshoot of a separate research study into demand ventilation control, for which members of this project will serve in its normal supporting role of implementing the researcher-specified virtual family schedule and preserving the whole-house measurements. The data generated from this air distribution study will contribute to improving industry duct design procedures and to quantifying the merits of incorporating zoning within net-zero energy homes.

During the latter half of FY21, a ground-source heat pump with integral water heating will be installed, instrumented, and commissioned in the NZERTF. Project members will serve as research participants of this field study. Lab testing to fully characterize the performance of the ground-source heat pump, including the operating modes where it provides domestic water heating, will be completed in advance of installing the unit in the NZERTF. The combination the lab and field results will be used for evaluating the applicable ASHRAE testing and rating method, Standard 206, and for validating a TRNSYS model covering this category of combined appliance.

Finally, maintaining and operating the NZERTF and coordinating the multiple research projects that use the NZERTF are key responsibilities of this project. In FY21, for example, participants on this project will assist members of the Indoor Air Quality Group and their many collaborators with conducting a series of indoor air chemistry tests at the NZERTF. Other projects that will be utilizing the NZERTF, its reconfigurable activity schedule of the virtual family, and the daily data of minutely measurements include:  Ventilation and Indoor Air Quality in Low-Energy Buildings, Space-Conditioning Options for Energy-Efficient Buildings, and carryover elements within Measuring the Performance of Net-Zero Homes.

During the FY21 heating season, project members will coordinate a series of week-long tests at the NZERTF that quantify the performance of the house’s thermal envelope. The test configurations and operating schedule are devised by a group of collaborators that includes researchers from ORNL, NJIT, and EL’s Indoor Air Quality Group. The collaborators are in the midst of a 3-year project that seeks to evaluate how well the widely-used whole-building simulation tool EnergyPlus models highly-insulated, well-sealed homes like the NZERTF. As occurred during FY20, project members will answer questions that arise pertaining to the test measurements and specific equipment operational observations while also helping review and interpret the results.

In addition, during FY21 this project will include the completion of investigations and publications on the impact of the air distribution systems within the house. The effects of reducing the flow at one or more of the four returns will be quantified. The fan power penalty and comfort changes created from using only the supplies of the small-duct, high velocity (SDHV) system will also be investigated. This testing is being pursued because SDHV ductwork is easier to install and so offers a lower installation cost. This testing will help determine the “cost” with respect to thermal performance and equipment efficiency. Another atypical use of the SDHV supplies that will be investigated is use two subsets of these ducts, in combination with the blower of the otherwise dormant SDHV heat pump, to transfer air between different levels in the house. For example, during the cooling season, the basement manifold will be connected to the return side of the blower while the blower’s outlet is connected to the plenum that feeds the SDHV attic manifold. The resulting dedicated air redistribution system is expected to yield the best option at minimizing the persistent stratification that occurs for all the duct configuration and operating strategies tried during the cooling season (when continuous fan operation of the home’s air conditioner or heat pump should not be used because such operations evaporate water that collects in the condensate pan). More conventional ways of approximating the dedicated air redistribution system – by using the heat pump’s indoor blower in combination with the on/off electronic air dampers and/or via the deployment of one or more ceiling fans will also be explored. Finally, the potential for maximizing the ventilation rate for the sole purpose of operating in an economizer mode will be tested. An economizer mode saves energy during those times when the outdoor temperature and relative humidity create an opportunity for natural cooling/heating in lieu of the thermostat calling for mechanical space conditioning of the same form. The economizer option will likely be explored as an offshoot of a separate research study into demand ventilation control, for which members of this project will serve in its normal supporting role of implementing the researcher-specified virtual family schedule and preserving the whole-house measurements. The data generated from this air distribution study will contribute to improving industry duct design procedures and to quantifying the merits of incorporating zoning within net-zero energy homes.

During the latter half of FY21, a ground-source heat pump with integral water heating will be installed, instrumented, and commissioned in the NZERTF. Project members will serve as research participants of this field study. Lab testing to fully characterize the performance of the ground-source heat pump, including the operating modes where it provides domestic water heating, will be completed in advance of installing the unit in the NZERTF. The combination the lab and field results will be used for evaluating the applicable ASHRAE testing and rating method, Standard 206, and for validating a TRNSYS model covering this category of combined appliance.

Finally, maintaining and operating the NZERTF and coordinating the multiple research projects that use the NZERTF are key responsibilities of this project. In FY21, for example, participants on this project will assist members of the Indoor Air Quality Group and their many collaborators with conducting a series of indoor air chemistry tests at the NZERTF. Other projects that will be utilizing the NZERTF, its reconfigurable activity schedule of the virtual family, and the daily data of minutely measurements include:  Ventilation and Indoor Air Quality in Low-Energy Buildings, Space-Conditioning Options for Energy-Efficient Buildings, and carryover elements within Measuring the Performance of Net-Zero Homes.