Ducted Variable-Speed Split System Heat Pump
Residential Heat Pumps: Variable-Speed vs. Constant-Speed Air-Source Heat Pump or Electric Furnace
A ducted heat pump leveraging the successful inverter-driven technology found in ductless heat pumps.
Item ID: 332
Technical Advisory Group: 2014 Residential Building TAG (#10)
Average TAG Rating: 3.21 out of 5
TAG Ranking Date: 04/10/2014
TAG Rating Commentary:
I would encourage research in this and also encourage caution when engaging consultants who might over complicate the issue. There is a big conflict of interest. Hire somebody you don't deal with all the time with the understanding it is a one off job so we get a quick analysis that isn't over priced or drawn out.
Caveat: must deal with the duct system. It needs either to be located inside, or it needs to be fully sealed and well insulated to make it worthy of conducting air most all of the time.
We already have research and measures for this.
Makes mini-split systems that much more adaptable. Seattle City Light has already decided to provide incentives for them.
Technical Advisory Group: 2015-1 Commercial HVAC TAG (#11)
Average TAG Rating: 3.5 out of 5
TAG Ranking Date: 03/10/2015
TAG Rating Commentary:
- Ductless with DOAS is better.
- Similar to VRF systems, the applications of this technology vary widely, resulting in custom projects.
- Wide-modulation range (inverter-drive, VRF, or whatever) compressors are the ultimate high-SEER devices, since they can give optimum part-load performance - and humidity control where needed by also regulating the air handler speed with an ECM fan.
- Very good efficiency and control.
- We see little potential for this technology in commercial buildings.
Traditional ducted split-system heat pumps have constant-speed compressors. Ducted variable-speed split system heat pumps incorporate variable-speed compressors, variable-speed indoor and outdoor blowers, and linear expansion valves (versus thermostatic, two-position valves) into traditional ducted split systems. The variable speed features allow the equipment to operate at the actual load and not cycle on/off, resulting in about 20% to 66% energy savings for the HVAC system. The variable speed features also substantially extends the equipment life.
This variable speed technology has been available in ductless split system configurations for about 30 years abroad, and dominates the HVAC industry in Japan, China, and Europe. Original equipment manufacturers began introducing ducted variable speed compressor technology into the U.S. HVAC market in 2006 (Nielson, 2014). However, although there are an estimated 13,000 ductless heat pumps installed as of 2011, far fewer variable-speed ducted systems have been installed. This imbalance may change as more variable split systems become available. However, ductless systems are inherently more efficient due to their lack of duct losses and electric resistance strip heat, so the long-term trend will likely favor variable refrigerant flow systems. Split systems should be designed and controlled to minimize the use of strip heat and make sure the ductwork is well sealed.
This technology is best used in a retrofit situation with existing air ducts in good condition. Incorporation of a variable speed compressor into a ducted system will save significant energy over a constant-speed unit with energy savings as high as 40% (Nielson, 2014).
Energy savings occur due to less cycling on and off and improved operation during light-load conditions. Each time a cycle restarts, the heat pump has to consume energy to balance pressure and to bring the refrigerant to the correct temperature before heat transfer can begin (Beach, 2015).
Baseline Description: Constant-speed air-source heat pump or electric forced-air furnace
Baseline Energy Use: 4.4 kWh per year per square foot
According to the Residential Building Stock Assessment: Metering Study Table #37 (Ecotope, 2014), the typical energy use for an air-source heat pump is 3.1 kWh/sf/year and an electric forced air furnace (FAF) uses about 6.85 kWh/sf/year. About 6.1% of the residential stock has electric forced air furnaces while 11.4% use air-source heat pumps. Both technologies are candidates for retrofit with variable speed ductless heat pumps (and variable speed blowers). The weighted average annual energy use is: 6.1% x 6.85 kWh/sf/year + 11.4% x 3.1 kWh/sf/year /(6.1% +11.4%) = 4.41 kWh/sf/year.
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 39%
Low and High Energy Savings: 20% to 66%
Energy Savings Reliability: 6 - Approved Measure
Per the Air-Conditioning, Heating, & Refrigeration Institute (AHRI) this technology has an integrated energy efficiency ratio (IEER) that ranges from about 17-25%. Existing equipment typically has IEERs less than 15. Energy savings will depend on weather conditions for a given site, with an average savings of about 25% of the heating and cooling system energy use.
Ecotope conducted a field study that measured the performance of 2- to 4-ton air-source variable speed heat pumps installed in six houses in Bend, OR over an eight-week period. They used the field data in a modeling study to show that variable capacity heat pumps in a ducted system provide a 25% to 30% improvement in energy efficiency. They also found that with reduced airflows, duct losses account for a greater percentage of the delivered energy (Ecotope, 2013).
A 25% reduction from the typical residential air-source heat pump annual energy use of 3.1 kWh/sf/year yields a reduced energy use of 2.32 kWh/sf/year. Assume that residential electric forced air furnaces will reduce their annual energy use from 6.85 kWh/sf/year to 2.32 kWh/sf-year (a 66% reduction) given a retrofit with a ductless variable speed air-source heat pump. The weighted average energy savings are thus: (66% x 6.1) + (25% x 11.4)/ (6.1% +11.4%) = 39.3%.
Energy Use of Emerging Technology:
2.7 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:
The estimate of technical potential for this technology assumes a "baseline" ducted air-source heat pump (ASHP) heating system, then calculates the energy savings due to replacement by variable-capacity equipment. (Energy savings that result from replacing baseboard electric and electric forced-air-furnaces by ductless, mini-split heat pumps are given in ET #300.) This ET examines the savings from upgrading the existing ASHPs that are located in 11.4% of single-family houses (Baylon, et. al., 2012) (2011 Residential Building Stock Assessment: Single-Family Characteristics and Energy Use, Table #50) plus this technology can also be used to replace an electric forced-air furnace heating system. Making the simplifying assumption that electrically heated homes are the same average size as each category of home with all heating sources, the total square footage of each type of home is multiplied by the percentage of homes that are electrically heated in that category to get an estimate of electrically heated square footage. It can also be used in small commercial applications, so the technical potential below is conservatively under-stated. The total square footage that can be retrofitted with this technology is
Regional Technical Potential:
| Type of Home || No. of Homes || Avg. Size || Total s.f. ||% with Air Source Heat Pump || s.f. Electrical |
| Single-Family || 4,023,937 || 2,006 || 8,072,017,622 || 11.4% || 920,210,009 |
| || || || || % with Electric FAF || s.f. Electrical |
| Single-Family || 4,023,937 || 2,006 || 8,072,017,622 || 6.1% || 492,393,075 |
| Total || || || || || 1,412,603,084 |
2.60 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): $4.72
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $3.45
A replacement 3-ton variable-speed heat pump serving an existing ducted system would have a total installed cost of about $8,500 (M. Karr, WSU Energy Program). This is equivalent to about $8,500/1800 sf = $4.72/sf. When replacing a force-air furnace system, additional costs would be incurred for installed condensing unit pad, power, and refrigerant lines. (Note: A rule of thumb for residential heating system sizing is that a one ton heat pump can service about 600 sf). A conventional 3-ton air-source unitary split system heat pump condensing unit would have a total installed cost (including overhead and profit) of $3,490. A fan coil unit would cost an additional $2,725 for a total cost of $6,215 (RS Means, 2015 Facilities Construction Cost Data). The baseline cost is equivalent to $3.45/sf.
Simple payback, new construction (years): 8.2
Simple payback, retrofit (years): 30.6
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.