Part 2: Evaluation of Mini-Split Heat Pumps as Supplemental and Full System Retrofits in a Hot-Humid Climate

December 05, 2017
Winter 2017
A version of this article appears in the Winter 2017 issue of Home Energy Magazine.
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This is part 2 of a feature article on retrofitting homes with mini-split heat pumps in hot and humid climates. For more information, read Part 1 here.

Complete System Replacement with Mini- and Multi-Split Heat Pumps

Another important objective of the PDR project was to evaluate a high-efficiency, single and multi-unit inverter-driven heat pump as a full replacement to the existing central system.

This evaluation was conducted at two sites, to investigate possible space-cooling and heating energy use reductions when a traditional-type central system is replaced. Testing before and after allowed detailed insight into the impact of the replacement on energy use and comfort. Two different replacement schemes were tested. The first design consisted of a multi-split with a single condenser and two fan coils—a ductless unit to condition the main living area, and a ducted component to condition the rooms remote from the main living space. This design was referred to as Multi-Split Heat Pump. The second design consisted of one MSHP with interior ducts to condition the whole house. This design was referred to as Ducted MSHP.

Multi-Split Heat Pump: Materials and Methods

Site 11, the home for the multi-split design, is a three-person occupancy, single-story, three-bedroom, two-bathroom home with 1,672 square feet of living space, located in Cocoa Beach, Florida. The existing system was a 12 SEER, 7.5 HSPF rated, 3-ton heat pump. A single, centrally located return feeds into the interior-located fan coil. Supply air is distributed via ducts to the bedrooms and bathrooms through the unvented attic, and to the remainder of the building inside a chase. The flex ducts (~R-4 hrft2-F per Btu) are poorly sealed (Qn, out = 0.13).

The retrofit consisted of a single 3-ton compressor tied to both a 1.5-ton wall-mounted fan coil installed in the main living area and a 1-ton ceiling-mounted fan coil in the central hallway. A Carrier model set (manufactured by Toshiba) was chosen, consisting of the 38MGQF36 variable-speed condensing unit, the 40MAQB18B wall-mount, and the 40MBQB12D ceiling-mount. The performance rating on the system varies from a high of 18 SEER, 10.0 HSPF with nonducted units to 15 SEER, 9.2 HSPF with ducted units. The rating for use with a combination of ducted and nonducted units is 16.5 SEER, 9.7 HSPF. The system has a nominal AHRI-rated cooling capacity of 35,000 Btu per hour at an outdoor temperature of 95°F, ranging from 9,500 to 37,000 Btu per hour; and a heating capacity of 36,000 Btu per hour at an outdoor temperature of 47°F.

The multi-split heat pump installation took place between July 1 and July 8, 2015. The wall-mounted fan coil was installed in the dining area on an exterior wall of the house to service the kitchen, dining room, living room, and Florida room. The ceiling-mounted fan coil was installed at the far end of the hallway to service the bedrooms, office, and second bathroom.

The dropped ceiling above the hallway provided the best location to house the fan coil and run very short supply ducts through the knee walls to the adjacent bedrooms with single-assembly ceilings, although configuration space was limited. While initial postretrofit airflow and duct leakage test results were poor, the mechanical contractor revisited the installation, resealing duct work and sealing the return plenum with satisfactory results. The total cost for the installation was $8,100.

Multi-Split Heat Pump: Results and Discussion

The occupants complained of comfort issues after the multi-split system was completed. They experienced temperature imbalances across rooms during the height of the cooling season; the office was often too cool while the master bedroom was often too warm. Figure 1 shows the hourly average daily temperature profile for four rooms during two summer weeks. The occupants at the ducted mini-split site made no such complaints. After learning about the interior temperature issues, Carrier technicians conducted a site visit. They uncovered two possible contributing problems. The first problem was with the sensing location. There was a discrepancy depending on whether the temperature was being sensed at the fan coil, which is located in a semi-conditioned attic buffer space, or at the thermostat located in the hall, an area only indirectly conditioned by the unit. The second problem was that there was a temperature response lag. There are little or no monitored data regarding the ability of multi-split systems to effectively dehumidify homes in humid climates. Upon start-up or set point change, the delta between set point and the temperature that would trigger the fan coil system to cycle off was about 4°F.

Site 11 Average Hourly Temperature Profile by Room and MS Power

Site 11 Average Hourly Temperature Profile by Room and MS Power
Figure 1. Average hourly temperature profile of four interior locations.

The room temperature and RH were specifically examined before and after the duct repair. While the additional run time of an inverter-driven system may provide enhanced latent removal, it is restricted in the multi-split case by the maximum turn-down ratio, which is by the width of the operational range, defined as the ratio from maximum to minimum capacity. For example, a 3-ton single-unit can typically provide about 1 ton (33%) of the maximum outdoor compressor capacity, while a multi-unit design consisting of a 2-ton and a 1-ton unit is capable of turning down to 4,000 Btu per hour (33% of the smallest unit). Thus, a multi-split system with a single outdoor unit is less capable of operating during low sensible-load conditions than would be the case if zoning were accomplished by two independent mini-split systems delivering the same total capacity. This is a known issue that the industry is working to solve.

While these points may be related to the comfort issues at Site 11, RH appeared to be a problem only in the rooms serviced by the ceiling mount, and not in the main living area. (See Table 1 for the postretrofit change in the main living area RH). This again points to a specific limitation in the multi-split arrangement, where the sizing of the single outdoor compressor may be critical to the potential degree of control at low cooling loads.

Table 1. Multi-Split Heat Pump Space Conditioning Energy Savings

Table 1. Multi-Split Heat Pump Space Conditioning Energy Savings

Estimated space-cooling energy savings are 37%, 2,250 kWh per year, or 7.8 kWh per day, while the home is being kept slightly (0.5°F) warmer postretrofit - than preretrofit. Meanwhile, the average indoor RH in the main living area is 2.4% lower postretrofit than preretrofit, amid similar pre- and postretrofit ambient conditions. While RH in the main living space decreased, it is important not to overlook increased RH in the rooms served by the ceiling-mounted fan coil.

Evaluating the impact of the multi-split heat pump on a utility peak summer day, HVAC power demand at the peak hour in 2014 was compared to 2015, showing a reduction of 0.24 kWh (11%). Space-heating energy savings were slightly negative (1% or 5 kWh per year). Occupant take-back was the likely explanation: Selected post-retrofit main area temperature was about 4°F warmer than selected preretrofit main area temperature.

The projected energy savings from the multi-split heat pump are impressive at 35% or 2,245 kWh per year. Still, with a cost of $8,100, the economics are only attractive at time of replacement. Also, identified comfort issues arising from this design need to be resolved before broad recommendation. A two-compressor design may solve these comfort issues, but at greater cost.

Ducted Mini-Split Heat Pump: Materials and Methods

Site 61, the home for the ducted MSHP design, is a two-person occupancy, single-story, ranch-style, three-bedroom, one-bathroom house with 875 square feet of living space, located in Cocoa Beach, Florida. The existing air conditioner at Site 61 is a 13.5 SEER rated, 2.5-ton system with electric-resistance heat. A single, centrally located, nonducted return feeds into the interior-located fan coil. Supply air is distributed through the vented attic in insulated R-4.2 rigid ducts with poor airtightness (Qn, out = 0.14).

The retrofit abandoned the leaky attic ducts and associated conductive losses. Contractor-suggested sizing (99% of design day) indicated a 1.5-ton unit. A single 20 SEER, 11.5 HSPF, 1-ton, ceiling-mounted MSHP was installed in the hallway of the subject home, with short duct runs below the ceiling plane distributing conditioned air to the main rooms. A Fujitsu model set was chosen, consisting of the ARU12RLF fan coil fan coil unit and matched AOU12RLFC condensing unit with variable refrigerant flow. The system has a nominal AHRI-rated cooling capacity of 12,000 Btu per hour at 95°F, and a heating capacity of 13,500 Btu per hour at 47°F.

The installation of the ducted MSHP began on June 18, 2015 and spanned three weeks, including finish carpentry work. The new ductwork and fan coil was installed in the centrally located hallway, inside conditioned space below the ceiling plane. All three bedroom doors are accessible from the approximately 12-foot-long hallway. Supply air is distributed to all three bedrooms, the living room, and the kitchen, with each room designed to receive between about 50 and 100 CFM. The total cost of the ducted MSHP installation was $9,100.

Ducted Mini-Split Heat Pump: Results and Discussion

The ducted mini-spit design significantly improved latent control and provided large energy savings. Much of this improvement was probably due to the abandonment of leaky attic ducts and the use of an interior duct system. Figure 2 plots the RH for all rooms and the exterior dew point before and after the retrofit. Though RH remains variable from pre- to postretrofit, RH in all rooms is sharply reduced. In fact, the average RH for all rooms during this snapshot was 55% preretrofit and 45% postretrofit, while the average ambient dew point was higher between these same periods. Follow-up conversations with the homeowners indicated that they are very pleased with the interior comfort provided by the new system. Greater savings would probably have been achieved without the apparent, (and occupant-acknowledged) take-back behavior. However, it is clear from the homeowners’ comments that the abandoned central system was unable to satisfy their comfort needs. Space-heating energy savings are large, which was to be expected, given that the preretrofit heating was electric resistance. The heating savings of 76%, 601 kWh per year, or 6.7 kWh per day also include some behavioral take-back, as the occupants preferred a warmer postretrofit period by about 1°F. However, a portion of the postretrofit heating evaluation period is post attic insulation.

Site 6 Pre- and Post-Retrofit Single-Ducted MSHP Room-by-Room RH

Site 6 Pre- and Post-Retrofit Single-Ducted MSHP Room-by-Room RH
Figure 2. Pre- and post-retrofit room-by-room relative humidity.

Observed space-conditioning energy savings from the ducted MSHP were impressive at 38% or 1,548 kWh per year (see Table 2). However, with a cost of $9,100, the economics are only attractive if the new system is installed at the end of the existing system’s life. Assuming that incremental costs over a new central system are about $3,000, the savings combined with the markedly improved interior conditions make the ducted MSHP an attractive option.

Table 2. Ducted MSHP Space Conditioning Energy Savings

Table 2. Ducted MSHP Space Conditioning Energy Savings

An evaluation of the ducted MSHP on utility peak summer hour showed HVAC power demand was reduced by 0.70 kWh or 41% between 2014 and 2015. All things being equal, this appears to be a superior option to the multi-split strategy—at least until latent removal issues are addressed. However, the findings here are based on single case studies, and further field evaluation is critically needed.

Both retrofits were expensive, but both would be competitive if you were replacing a conventional unitary air-conditioning system at burn-out. Note, however, that in a full-sized house, two to three ducted mini-splits would be needed, depending upon room layout, to achieve similar results. Still, the potential to provide excellent energy savings with improved interior RH control could make this an attractive choice.

Conclusions

The Phased Deep Retrofit project in Florida evaluated high-efficiency ductless mini-split heat pumps. Results for the novel supplemental MSHP configuration suggest cooling energy use savings of 33% (6.7 kWh per day), and heating energy use savings of 59% (6.5 kWh per day). The daily heating energy percent savings were significantly greater than the cooling energy percent savings in the six homes with electric-resistance central heating. Electrical demand reductions during utility peak system hour were 16% for summer and 56% for winter. With similar pre- and postretrofit weather conditions, the supplemental MSHP showed similar moisture control characteristics pre and post.

Economics using median savings and a current installation price of about $3,900 are potentially attractive, with a suggested payback of about 14 years and a 7.3% annual rate of return. As the MSHP market matures and installation costs fall, the economics will further improve. A redundant heating-and-cooling system is a large added benefit to the consumer, which is highly desirable given the 7–10% annual failure rate of central air-conditioning systems.

An additional central system replacement involved installation of a multi-split with one condenser and two fan coils—a ductless unit to condition the main living area, and a ducted component to condition the rooms isolated from the main living area. Cooling energy savings were excellent at 37% (7.8 kWh per day) with electric demand reductions during utility peak summer system hour of 11%. With an installed cost of $8,100, the economics are attractive at the time of old system replacement. Nevertheless, this installation created comfort issues.

The design for a second complete HVAC solution consisted of one MSHP ducted to condition the whole house—a modest, compact, single-story design. Space- conditioning savings totaled 38% (1,548 kWh per year), comprising of 29% (3.5 kWh per day) for cooling, and 76% (6.7 kWh per day) for heating. Large (5.3%) reductions were seen in RH during the cooling season with slightly higher ambient dew point conditions. Electrical demand reductions during peak summer system hours were 41%. With an installation price of $9,100, economics are attractive if the new system is installed upon existing system failure.

In two cases, the performance of a mini- and a multi-split system were compared to that of a standard conventional ducted unitary system at the same site in the hot-humid climate. Both systems showed impressive cooling energy savings; however, the multi-split system did not adequately control indoor humidity and zone temperatures. The ducted mini-split approach showed significantly reduced indoor RH. However, a split-plan house would require installation of added ducted MSHP units, greatly increasing cost.

See resources in Part 1.

Karen Sutherland is a research analyst at the Florida Solar Energy Center (FSEC). Danny Parker is a principal research scientist at FSEC. Eric Martin is a program director at the FSEC. All three of them work in the Buildings Research Division.

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