Ductless Mini-Split Heat Pumps for Residential Applications
Residential Ductless Mini-split Heat Pumps: Variable-Speed vs. Constant-Speed
Ductless split systems with non-inverter driven compressors have been available in the U.S. for decades. Within the last few years, the market has begun to shift to using inverter-driven compressors. Non-inverter driven compressors are still used however, typically due to lower first cost.
Item ID: 300
Technical Advisory Group: 2009 HVAC TAG (#2)
Equipment that is capable of operating only as much as needed to meet the load is more efficient than equipment that can only operate at full capacity or off.
HVAC split systems, including ductless split systems, utilize variable speed (inverter-driven) compressors and fans. Constant speed on-off only compressors are good for locations where the load is constant, such as a small data or server room. However, for most applications, the load changes with the seasons and time of day. For these applications, inverter-driven motors on HVAC compressors can save significant energy.
Heat pumps with variable speed compressors have been available in other countries for many years, commanding more than 80% of the commercial and residential market combined in Japan, Europe and China. This technology is now widely available in the U.S.; manufacturers are offering more options to fit the U.S. market.
NEEA conducted a pilot program in 2009, training 900 installers and succeeding in getting 4,000 ductless heat pumps (DHPs) installed. The heat pumps were provided by 5 different manufacturers. By 2011, NEEA indicates that more than 13,000 ductless heat pumps had been installed in the Northwest, savings at least 40,500,000 kWh of energy (3,115 kWh/unit). Research showed that 90% of customers were satisfied with their ductless heat pumps and that customers reported a 25% to 50% reduction in their heating bills.
Baseline Description: Typical electrically-heated single-family home in the Northwest
Baseline Energy Use: 5.5 kWh per year per square foot
The NEEA Residential Building Stock Assessment: Metering Study (April 2014) indicates an annual heating EUI for homes in the Northwest with electric baseboard heating of 17.74 kBtu/sf-year. This is equivalent to about 5.19 kWh/sf-year. About 12.3% of the region's homes are equipped with baseboard heaters as their primary source of heating. Note that the savings from homes with heat pumps (air source, ground source dual-fuel) will be much less than for homes heated with electric furnaces or baseboard systems. For residential AC, the mean region-wide annual energy use is about 0.3 kWh/sf-year (personal communication, Tina Jayaweera, Northwest Power and Conservation Council). Therefore, the combined heating and cooling electrical energy use is taken as 5.49 kWh/sf-year.
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 32%
Energy Savings Reliability: 6 - Approved Measure
Per the Air-Conditioning, Heating and Refrigeration Institute (AHRI) this ET has Integrated Energy Efficiency Ratio's (IEER) in the high teens to mid twenty's. Existing equipment is in the low teens. Energy savings will depend on weather conditions for a given site. As the bulk of the energy used is for heating, a ductless heat pump COP of 3.5 is assumed (Daiken values range from 3.2 to 3.8). This results in a heating and cooling energy savings of about 71%---given that the whole house electrical heating requirements are served by multiple ductless heat pump heads. But, ductless heat pumps typically are defined as a zonal heating source and do not serve the entire household. In fact, Baylon reports that a ductless heat pump typically provides only 45% to 80% of the space heating in residential DHP retrofits. Geraghty studied 14 existing single-family homes with zonal heating systems and found that small, split-system heat pumps saved about 4,000 to 4,500 kWh/year. (from Baylon, "Ductless Heat Pump Engineering Analysis", December, 2012).
In 2008, BPA and the Northwest Energy Efficiency Alliance (NEEA) initiated a DHP pilot program targeting 2,500 single-family, site-built homes with zonal electric heating. The study drew from a broad range of sites across the Northwest and found average savings of approximately 3,049 kWh/year to 3,850 kWh/year across three heating climate zones. With an initial heating load of 5.5 kWh/sf-year, and assuming an average savings of 3,500 kWh/year, we obtain a post-installation annual energy use of 3.75 kWh/sf-year with a DHP energy savings of 31.7%. This savings value will be used in this analysis.
Note: This is a deemed measure under the October 1, 2014 BPA "Energy Efficiency Implementation Manual". The deemed amount is $800 to $1200 per unit depending upon existing heating source (baseboard versus electric forced air furnace).
Energy Use of Emerging Technology:
3.7 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
We can only count in the technical potential of those homes currently using electrical energy for space heating. Ductless heat pumps in the manufactured home and multifamily home markets are covered in ET #301 "Ductless Mini-Split Heat Pump for Manufactured Homes" and ET#302 "Ductless Mini-Split Heat Pump for Multi-Family Housing." According to estimates in the Northwest Energy Efficiency Alliance's (NEEA's) 2011 Residential Building Stock Assessment (RBSA), 34.2% of single-family homes in the Northwest are heated with electricity (Baylon, 2012 Pg 53, Table 51) but only 12.3% with electric baseboard heaters.
Homes with electric forced air furnaces have the option of switching to a conventional air-source heat pump and thus capture energy savings by utilizing the same ducted delivery system for heating and cooling needs rather than installing a single zone heating and cooling system. A BPA funded study does find that energy savings due to installation of ductless heat pumps in single-family homes with electric forced-air furnaces actually saves more energy---5,214 kWh/year---than a comparable installation in a home with electric baseboard heat. This is likely due to the reduction in duct leakage and losses through insulation. In such homes, the DHP tended to provide an average of about 77% of the heating energy (Baylon, 2012).
We make the simplifying assumption that electrically-heated baseboard heated homes are the same average size as each category of home with all heating sources, so to get an estimate of square footage for houses with baseboard heating, we multiply the total square footage of each type of home times the percentage of homes that are electrically heated in that category. The Residential Building Stock Assessment also indicates (in a roundabout manner) that about 6.1% of the single-family homes in the region use electric forced air furnaces. As these homes have a greater energy savings potential than do baseboard resistance heated homes, the baseboard home square footage is increased by an additional 6.1% x (5,214/3,500) = 9.08% of the total single family home sf.
Regional Technical Potential:
|Type of Home || No. of Homes || Avg. Size || Total s.f. || % Electrically Heated || s.f. Electrical |
| Single-Family || 4,023,937 || 2,006 || 8,072,017,622 || 12.3% baseboard || 992,858,167 sf |
| Single-Family || 4,023,937 || 2,006 || 8,072,017,622 || 9.1% forced-air furnace || 734,553,603 |
| || || || || Total baseboard equivalent sf || 1,727,411,771 |
3.04 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $5.33
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $0.56
Installed costs for single zone systems are around $3,500 for a 1-ton system and up to around $8,500 for a 5-ton system. This technology is available in up to eight zones on a single condensing unit, adding about $1,200 per zone. Prices vary greatly by contractor, and seem to depend on control systems selected, contractor familiarity with this technology, general availability of this technology within the given market (ie, where there are fewer qualified contractors, prices tend to be higher) the size of building, etc. The total installed cost for a 1.5 ton ductless heat pump (in mid-2014) was about $4,800 including wireless programmable controls. These costs are approximately 20% higher than an equivalent constant-speed system. Given that a one-ton unit will serve a residential area of 600 sf, the 1.5 ton unit captures some economies of scale and has a total cost of $5.33/sf.
The assumed baseboard heating system cost is $0.56/sf. (From Means, an 8-foot baseboard has a cost of about $193 and has a power output of 2 kW).
Simple payback, new construction (years): 30.1
Simple payback, retrofit (years): 33.7
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Cost effectiveness is a complicated subject for this technology. It is difficult to account for reduced maintenance and increased comfort and reliability. The cost effectiveness of this technology also depends on weather/climate conditions at the site and operational differences between owners.
Split systems can come with either a constant speed on-off compressor or an inverter-driven motor on the compressor. The on-off style does not last as long and is more costly to operate. Compressors with inverter-driven motors have a soft-start feature that allows the compressor to ramp up, avoiding the high inrush of current inherent with the on-off style. Compressors with inverter-driven motors have the ability to match the load rather than cycling on and off like the traditional style.
Manufacturers offering inverter-driven motors on compressor HVAC equipment include Daikin, Mitsubishi, Fujitsu, LG, and Sanyo.
The inverter-driven motor on a compressor is available with many styles of indoor air handlers, although different manufacturers offer different options: wall-hung ductless, ceiling cassette, floor radiator style, ducted fan coil, etc.
Traditionally, HVAC systems have constant speed on-off capacity compressors; constant speed fans on the indoor and outdoor units; and an expansion valve that can only operate at the fully open or fully closed position.
This technology has been used in the residential and commercial sectors for over 20 years in Japan, China and Europe. In fact, it dominates those markets, capturing more than 80% of the markets. To accommodate the U.S. market, the indoor unit options expanded from a ductless wall-hung unit to many other styles, including ducted, floor mount, ceiling mount and cassette.
Currently in the U.S., these systems are readily available from several large, well-established manufacturers who are well-positioned to expand capacity as needed due to increased interest in this technology. As the result of utility incentives and extensive marketing efforts by manufacturers and regional energy efficiency advocates, the number of installations have increased significantly. The Northwest Energy Efficiency Alliance estimates that as of 2013, over 65,000 of these systems have been installed in the Pacific Northwest (NEEA, 2014).
End User Drawbacks:
There is about a 20% premium on the first cost for this equipment. Until contractors get comfortable with the technology, it will continue to cost more than conventional equipment.
This technology performs very well if constantly operated, maintaining a steady temperature over long periods. They function less well in setback mode (turned off when the home is unoccupied, then turned on again when the home is occupied, or heat is desired.) (Harley, 2014.) Homeowners dissatisfied with the speed of heating with this technology might resort to electric resistance heat to bring the home to the desired setpoint, offsetting the savings for this technology. If the technology is to be operated in the most effective manner, the homeowner needs education on proper operation.
Operations and Maintenance Costs:
Maintenance costs are not significant - the only regular maintenance required is filter cleaning, which can easily be performed by the homeowner.
The conventional standard existing technology without inverter-driven compressors usually has compressor failure and requires replacement in five to six years and system replacement in 12 to 15 years. Compressor failures are usually the result of the initial rush of current that is hard on equipment and the constant cycling on and off. Compressors with the inverter-driven motors have a “soft start” and shut down less frequently. Inverter-driven equipment modulates to meet the load. In addition to the soft start on the compressor, there is a similar variable speed feature for the indoor and outdoor unit fans, and the expansion valve is a modulating type. It is expected that inverter-driven systems should be operational for over 20 years, although the technology is new enough that this estimate cannot be verified with certainty.
Equipment with constant speed compressors, constant speed indoor and outdoor fans, and open or closed only expansion valve.
Reference and Citations:
Ductless Heat Pump Impact & Process Evaluation: Billing Analysis Report
Performance Rating of Variable Refrigerant Flow (VRF) Multi-Split Air-Conditioning and Heat Pump Equipment
Air-Conditioning, Heating, and Refrigeration Institute
Ductless, Mini-Split Heat Pumps
U.S. Department of Energy
Field Test of Ductless Heat Pumps in Residences and Small, Commercial-Use Buildings
Bonneville Power Administration
Baylon, et. al.,
Residential Building Stock Assessment Reports
Northwest Energy Efficiency Alliance & Ecotope
NW Ductless Heat Pump Project
Northwest Energy Efficiency Alliance (NEEA)
Ductless Heat Pump Engineering Analysis: Single -Family and Manufacturerd Homes with Electric Forced-Air Furnaces
BPA Energy Efficiency Emerging Technologies Initiative