Field Performance of Mini-Split Heat Pumps in Southeastern Low-Load Homes

As building codes get stricter, homes are getting more efficient, but one thing remains the same: Heating, ventilation and air conditioning (HVAC) systems continue to use a substantial amount of energy. Fortunately, like homes, HVAC technology is improving, with the hopes that it becomes less of a burden for energy users and utilities.

Our residential team regularly conducts research to understand how new and up-and-coming home components reduce energy usage and peak demand — the times of day and year when energy consumption is at its highest. Over the last two years, Advanced Energy senior consultant Jonathan Coulter led a group to explore one type of HVAC system that has been growing in popularity: the mini-split heat pump.

Jonathan recently discussed this work at the Behavior, Energy & Climate Change (BECC) Conference, an annual meeting focused on understanding human behavior and decision-making to accelerate the transition to a low-carbon future. The conference is put on by the American Council for an Energy-Efficient Economy, the California Institute for Energy and Environment at U.C. Berkeley and the Environmental and Energy Policy Analysis Center at Stanford University.

Mini-split heat pumps function like traditional heat pumps, by absorbing energy in the form of heat from one location and moving it to another. But they differentiate themselves by being highly efficient and ductless, meaning they don’t require ducts to transport air. While much is known about mini-splits’ technical characteristics and operation, less is understood about how they actually behave in the field.

Our study was able to fill that gap. Thanks to partnerships with the Habitat for Humanity of Washington, D.C., and Habitat for Humanity of Catawba Valley, it evaluated the real-world performance of nine low-load homes that contained mini-split heat pumps. Low-load homes are designed, built and fitted to be highly efficient (e.g., they are well insulated with significantly reduced air leakage), and for this reason, their energy consumption is often minimally influenced by outdoor weather. Importantly, what’s considered “low-load” today will likely be the standard building code in the future, so it is essential to make sure these homes provide the comfort and outcomes that they promise.

The research team’s primary goal was to understand the mini-splits’ energy usage, but Jonathan and colleagues also wanted to evaluate how they affected comfort in the form of interior moisture. The study was particularly interested in exploring these issues in a mixed-humid climate, so the Southeast was a good fit.

Along with the mini-split system, each home included sensors to track indoor temperature and relative humidity. Energy performance, in both kilowatts and kilowatt-hours, was recorded by current transducers located in the home’s breaker panel.

Overall, the field data acquired from the study provided valuable insight into the everyday functioning of mini-split heat pumps. It also showed the importance of sensors that can provide continuous measurements.

Indeed, because of some sensor issues, and with only nine participating homes, there was high variability in the data, but patterns did emerge. For example, the homes experienced moisture accumulation in the summer months, suggesting that mini-splits cannot be relied on to provide moisture control below indoor dew points of 55 degrees Fahrenheit. Therefore, a drying strategy, such as kitchen and bathroom exhaust ventilation, or supplemental dehumidification would be recommended.

In addition, and consistent with expectations, the mini-splits were found to be highly energy efficient. Heating and cooling energy usage remained low, demonstrating that low-load homes are useful for reducing peak load.

Using a mix of field studies and computer-generated energy modeling, Jonathan and others on our residential team continue to study the role of heating and cooling technology in managing and limiting peak impacts. Ongoing projects are exploring auxiliary-free variable capacity HVAC proof of concepts in new and existing homes, performing energy modeling on 12 HVAC systems in four North Carolina cities, and creating templates of two simulated homes for use in above-code programs to understand the impacts of various home components throughout North Carolina.