Measured Space-Conditioning Energy and Indoor RH in a Mechanically-Ventilated Lab Home with Fixed and Variable-Capacity Cooling Systems Located in a Hot and Humid Climate

Residential whole-house mechanical ventilation has become more important as the impetus has been made to construct homes with less air leakage. Requirements for homes to meet minimum air tightness requirements and to be equipped with mechanical ventilation have even been made mandatory under certain building programs and codes. Homes mechanically ventilated during warm and humid weather will have elevated indoor relative humidity (RH) during low cooling load periods. This requires supplemental dehumidification for at least some low load periods to maintain acceptable RH control. Herein lies a challenge to balance acceptable RH with minimal energy use. A research project was completed to evaluate three specific types of space cooling equipment test configurations in a controlled research lab home. The lab home was furnished and had three bedrooms and two bathrooms located in central Florida. It had automated internal sensible and latent loads, and was ventilated in accordance with ASHRAE 62.2-2013. The main goal of the project was to contribute to the limited body of research seeking to balance space conditioning energy efficiency with good RH control in continuously mechanically ventilated homes, particularly in the hot and humid climate zones of the United States. The focus of the testing was to evaluate space cooling and dehumidifier energy use as well as the resulting indoor RH throughout the home. The three primary test configurations covered in this paper involved: 1) a central ducted fixed-capacity SEER 13 rated system 2) a central ducted variable-capacity SEER 22 rated system, and 3) a SEER 21.5 ductless variable-capacity minisplit. The minisplit was operated as the primary cooling system with central system used for cooling backup during near peak cooling load periods. The project found that the SEER 22 central system configuration used 20% less energy than the SEER 13 central system, and the mini-split configuration used 25% less energy than the SEER 13 system under typical seasonal conditions. Limited supplemental dehumidification was needed to maintain indoor RH below 60% during some low cooling load periods. When needed, dehumidifier use was typically only 1-3 cycles per day (0.18 – 0.58 kWh/day). This paper shares greater details on the variability of indoor RH among the test configurations, factors that resulted in the very limited need for supplemental dehumidification, and recommendations to improve latent performance of variable capacity cooling systems.