Maximizing Energy Savings for Ductless Heat Pumps

Many Northwest utility rebate programs for ductless heat pumps find it challenging to keep rebates cost effective with the ever-increasing equipment prices and labor rates. For this reason, utilities seek to ensure installation practices deliver maximum energy savings for their programs and for their customers.

To support utility ductless rebate programs, the NW Ductless Heat Pump Project commissioned a research project called Maximizing Mini-Split Heat Pump Performance. The research included: tests performed on ductless heat pump installations at the Pacific Northwest National Laboratory (PNNL); literature review of work performed by Energy Trust of Oregon and the Regional Technical Forum (RTF), along with roughly 40 other published papers; and interviews with 30 subject matter experts from all over the country.

The research identified five opportunities for energy-saving enhancements, which are outlined below. When combined, they can nearly double utility program savings. For many utilities, this savings increase could allow them to continue offering rebates.

The following table shows projected incremental savings identified. These are regional average weighted incremental savings values over the RTF’s 2019 published UES value of approximately 1709 kWh/year for zonal heated homes and 3,498 kWh/year for electric forced air furnace (EFAF) heated homes.

Enhancement #1: Integrated Controls of Backup Heating System

The research looked very closely at what opportunities might exist to apply controls to homes with both a ductless heat pump system and an electric forced air furnace. The incorrect configuration of an integrated control system can lead to a costly error. When a ductless heat pump is operating in cooling mode, it can trigger the thermostat of the electric forced air furnace, causing the furnace to heat the home while the ductless heat pump tries to cool the home. This problem occurs when the homeowner forgets to turn off their furnace thermostat in the summer months.

Contractors can help to mitigate this risk by installing lockout controls. This class of controls can "lock out" the electric furnace for outdoor temperatures at which the ductless heat pump can easily meet a home’s heating requirements. In addition, a new class of integrated controls can provide a single interface for customers. The interface coordinates the operations of both the ductless heat pump and the electric furnace, so they work together to more efficiently heat the home. In fact, utility programs are already supporting these new controls in other areas of the country, including Mass Save in Massachusetts.

While these controls and strategies are seeing increased market adoption, research found lockout controls are expensive and produce relatively lower savings. Keep in mind this technology is still new and is improving.

Enhancement #2: Design Installations to Maximize Displacement

Field studies indicated that the indoor unit of a ductless heat pump system is not consistently located in a room that allows for the greatest electricity savings. In these instances, the ductless system becomes underutilized, and the homeowner relies more heavily on the original electric resistance heat. Ideally, installers place the indoor unit of the ductless system in the living room, family room, or other primary living space where occupants spend most of their waking hours. The living room typically has a relatively large floor area, so it represents a larger percentage of the total heating load. If utilities decide to begin requiring this current recommendation for their rebate programs, a combination of increased site inspections and enhanced contractor training could help deliver improved savings.

Enhancement #3: Use Recommended Installation Practices

Heating, ventilation, and air conditioning (HVAC) installations are relatively complicated. They are subject to many requirements that, if not met, can result in sub-optimal equipment performance. Many rebate programs account for a certain number of underperforming installations when forecasting their achieved savings. But there are currently enough underperforming installations in the Northwest that a meaningful gap exists between the actual electricity savings and the potential savings of all installed systems. This gap represents an opportunity for Northwest utilities.

Installers could address this problem by increasing attention to proper placement, head location, refrigerant charge, and control strategies. Utility researchers around the country are also developing a more advanced solution: a technology framework for remote diagnostics and post-installation quality control that would be able to access the internal computer controls of nearly any ductless heat pump currently available to the market. Internet-connected heat pumps that provide remote performance data to a local utility could reduce or eliminate the need for onsite inspection and utility quality assurance/quality control site visits. While still in the early stages of development, this technology framework has the attention of several leading heat pump manufacturers who are participating in joint research with utilities.

Enhancement #4: Homeowner Education

In one of the more surprising research findings, the research uncovered that a significant percentage of homeowners do not know their ductless heat pump can operate in a heating mode to produce indoor comfort and savings year-round. Therefore, many consumers are not using their system during the winter months. Without education from their contractors, many homeowners have purchased ductless systems to only cool their home despite receiving a rebate for displacing electric resistance heat. The solution could be as simple as improved initial homeowner education.

Enhancement #5: Target Homes with Electric Heating Loads

Research found that a significant number of homes that receive ductless heat pump rebates have a relatively small electric resistance heating load. In these cases, the homes never could have achieved cost-effective electricity savings because they were not consuming the assumed electric load of a home their size prior to installing the ductless heat pump. A low-cost solution exists for this problem: calculating a home's electricity use during the winter months and comparing it to months in late spring or early fall can reveal the heating portion of the home’s energy use. Targeting marketing efforts to homes with high heat loads will produce the greatest energy savings.

Also, research results show that utility programs would improve rebated savings by eliminating rebates offered on homes that currently use very little electricity for heating. Utilities would need to identify homes with large electric heating bills and ensure that rebate money is only (or primarily) available to those homes. Currently, most utilities allow contractors to offer a rebate for any home that has electric resistance heat installed, regardless of how much electricity it uses for heating.