Ductless Heat Pumps
By Paul Cutting, Energy Planner
Ductless heat pumps are exciting for their potential to serve as low-cost, straightforward and simple solutions to home heating electrification. As the name implies, ductless heat pumps use interior units that have incorporated evaporator coils and blowers to deliver heating and cooling to interior spaces. You’ve likely seen interior mini split style wall units that hang close to a ceiling. Along with wall units, there are floor units that resemble radiators, ceiling cassettes that are incorporated into finished ceilings or commercial drop ceiling grids, and low and mid static ducted units that serve small branches of ductwork in small spaces and individual rooms. These interior units are paired with either single or multi zone outdoor air source heat pump compressors. Single zone compressors serve one interior unit whereas multi zone compressors serve multiple indoor units.
Wall unit, floor unit, ceiling cassette and low static unit
The Virtues of Ductless
Beginning January 1, 2023 homeowners will be eligible for a 30% federal tax credit worth up to $2,000 for the installation of air-source heat pumps. Though at face value the credit seems small, it can be used annually and there isn’t a lifetime cap, meaning homeowners can tiptoe into whole-home electrified space heating over time. A homeowner could do one zone one year, and another the next, repeating until the home has transitioned away from fossil heating.
Equipment eligible for the federal tax credit must meet CEE Advanced Tier standards: HSPF2 rating of ≥10.0, a SEER2 rating of ≥17.0 and a COP (coefficient of performance; performance relative to electric resistance heat) of ≥1.75 at 5°f. Most ductless equipment already meets these standards, but some larger capacity and multi zone units will not. In addition to the tax credit, sometime late next year, or at the start of 2024, an income-dependent rebate program will come into effect that will pay up to $8,000 for heat pumps.
If our goal is to transition space heating away from fossil fuels as fast and cheaply as is humanly possible, ductless air source heat pumps are a pretty exciting technology. First, air source heat pumps work in really any climate, and potentially without the need for secondary backup heat sources (for issues of backup heat source, see my prior ducted heat pump explainer). Ductless is—in theory—a low cost and straightforward technology to install. Much of the world already uses ductless for heating and cooling. Just look at any photo of a developing country urban environment and you’ll likely spot countless units hanging off building facades. In these markets, most ductless units are purchased much the way you’d buy other consumer electronics. Pricing is transparent and straightforward, installation contractors bill their services by the hour, and overall all-in costs tend to be low.
In the US, the major manufacturers like Daikin, Fujitsu, Mitsubishi, and LG have developed contractor-network models where units are sold, installed and serviced exclusively by contractors who partner with specific manufacturers. This type of arrangement is good for service, warranty assurance, and consumer satisfaction, but probably results in higher than necessary installation costs. As the technology has improved—especially on the heating side of things—Chinese manufacturers like Gree, Midea, and others have developed products that match—and in some cases exceed—the products of the major manufacturers (and as an aside both Gree and Midea already make ductless equipment rebranded for “major brands” like Carrier, Lennox, Bosch and others). These off-brand products can be purchased online from places like Homedepot, Amazon and a slew of other retailers.
In some cases, consumers in the US are able to purchase major manufacturer equipment through sites like ecomfort.com and others—albeit for a higher price compared to what a contractor would pay from a distributor. But some manufacturers, like Fujitsu, won’t honor warranties of products purchased online, even if the equipment is installed by a licensed HVAC technician. Other manufacturers like Mitsubishi offer parts warranties that are less than half the length of time for equipment purchased online and installed by a licensed HVAC professional. Service operates much the same, with parts available only to contractors. In addition, if you were to purchase equipment online, it’s hard, if not impossible, to find a contractor willing to hook it up. Much like the vehicle dealership model or carrier-tied phone contracts, the manufacturer-dealer model is lucrative.
Why bring all this up anyway? I’m definitely not pushing the idea that contractors are overcharging customers or acting dishonest in any way, but as someone who desperately wants to see wide-scale and mass heat pump adoption, in order for that to become reality, costs have to come down. In our market, contractor-installed ductless equipment seems to be running about $4,000 to $5,000 per 12,000 BTU of capacity per indoor unit, but I’ve recently seen estimates as high as $7,000 per 12,000 BTU of capacity. For a home needing 36,000-48,000 BTUs of heating, costs on that order are a total loser compared to the conventional gas furnace.
Hopefully the new heat pump tax credit has some effect in bringing costs down by making pricing more transparent, encouraging new contractors to enter the market, creating more service and support options for DIY inclined customers, and making ductless installs more of a service call and less a relationship buy-in. A larger, better developed and more transparent market is better for everyone. And the yet-to-be-implemented Inflation Reduction Act HEEHRA rebate program provides $8,000 for heat pump installation costs up to 100% of project costs for those under 80% of area median income. Those funds were intended to support whole-home conversions, but if pricing is 2x, 3x, or 4x that, the program will not fulfill its goals.
The DIY and off-brand equipment market is growing significantly, and hopefully these improvements will also help drive down prices. Major manufacturers like Fujitsu and Mitsubishi were early entrants into the cold climate market, but now there exist many brands with respectable–and in some cases excellent–cold climate equipment. Brands like Midea, Gree, Blueridge (rebranded Gree equipment), Tosot (also rebranded Gree equipment), Senville (mostly rebranded Midea equipment), Mr. Cool (rebranded Gree and Midea equipment), Pioneer, and a slew of others have products with respectable cold weather performance, and sometimes at half the cost (or less) of the major manufacturers. And each of these companies freely sell equipment and repair parts directly to the consumer.
Another advancement with interesting potential are pre-charged line sets, which eliminate the need for vacuuming and purging of refrigerant lines, making installation (short of the high voltage hookup) entirely possible for the end consumer, carpenter, or handyman. Pre-charged units have been used in military applications for a while, but the only consumer-facing pre-charged products on the market today are from Mr. Cool. Unfortunately, the cold weather performance of these particular models is pretty terrible, but the concept is intriguing. It’s doubtful any major manufacturer with a contractor network will develop this market, but hopefully the lesser name, DIY-friendly manufacturers will.
Configuration and Arrangement
There are a few general principles to consider if you’re looking to go ductless. First, generally speaking, shy away from placing indoor units in every room, and instead think strategically of how you can best move air throughout spaces. If you look at marketing material from the major manufacturers like Mitsubishi or Fujitsu, they pitch the concept of highly personalized comfort and control, where every room is served by its own indoor unit. For example, Billy keeps his bedroom at 72 degrees while his sister, Sally, keeps hers at 68. Generally speaking, putting ductless units in every (or nearly every) room is a really bad idea. Heating loads of individual rooms, (especially small bedrooms, interior rooms, rooms without many windows, or really any small room in new construction), tend to be really small, like on the order of a few thousand BTUs per hour, whereas the smallest indoor ductless units tend to be 6,000 to 9,000 BTU capacity, well above the heating and cooling loads of the rooms they are placed in. And placing ductless units in each individual room is expensive, and often results in poor operating efficiencies and comfort issues.
A better approach is to position ductless units so that they blow air through entire areas, like sets of bedrooms via a hallway or through spaces with an open floor plan that connect kitchens, dining rooms, and living rooms. Four years ago I converted my old, rambling 2,900 square foot house to ductless, and the entire house is heated and cooled by just four indoor units.
The number and size of units required to heat and cool a home is entirely dependent on building square footage, air tightness, insulation levels and floor plan layout. There’s no hard-fast rule to determine how big a system should be, other than to have someone perform a heating and cooling load calculation or analyze past fuel usage. Unlike gas furnaces where there isn’t much of a performance or efficiency hit for oversizing equipment, heat pumps, especially multi zones, need to be closely sized to heating and cooling loads.
Shy Away from Multi Zone Units
If your objective is primarily heating, try to avoid multi zone units. Multi zone units are plagued by a few persistent issues, namely high minimum modulation levels, overall relatively poor efficiencies (as compared to single zone equivalents), and lack of redundancy. A good single zone heat pump will have a minimum modulation level of less than 25% of its rated output, while multi zone units tend to have minimum modulation levels closer to 50% of their rated outputs. A heat pump with a low minimum modulation level will be able to match heating loads and run continually, even in the shoulder heating seasons of October, November, March and April when the number of BTUs required to heat a space is relatively small. If a unit has a minimum modulation level that exceeds the load of the space it is attempting to heat, it will short cycle. In this situation, the unit reaches setpoint and shuts off, turns on again when the thermostat calls for heat, and shuts off again once the setpoint has been reached. The beauty of modern inverter-driven heat pumps is that everything is controlled by variable speed direct control technology. But invariably, even with this type of equipment, there is a bottom threshold.
For example, let’s compare Mitsubishi’s multi zone 20,000 BTU cold climate heat pump (assuming you connect two 9,000 BTU wall units to it) with two Mitsubishi 9,000 BTU single zone cold climate heat pumps. Since we care most about minimum modulation levels in the shoulder heating season, let’s look at how the multi zone compares to two single zone units at 47°f (engineering data is provided at 47°, 17°, 5° and -13°). At 47° the multizone has a minimum output of 11,400 BTUs and consumes 850 watts of power. On the other hand, the two 9,000 BTU units have a combined minimum output of 3,600 BTUs and collectively consume just 220 watts of power. Now assume both the single multi zone unit and the two single zone units were each heating a hypothetical living space of 800 square feet, and let’s say the heating load of that space at 47° is 5,000 BTUs. The two single zone units would run continually at a low output whereas the multi zone unit would short cycle at 850 watts, resulting in horrible efficiency with less than desirable heating performance.
A while back I was offered a tour of a recently constructed 1,200 square foot addition. The HVAC contractor installed four indoor units, each between 6,000BTUs and 12,000BTUs of capacity connected to a single 45,000 BTU-rated outdoor unit. The building was well built and the heating loads of the addition were small, even on the coldest days of the year. The outdoor multi zone compressor, which all four units were connected to, had a minimum modulation level of about 22,000 BTUs. That minimum modulation likely exceeded the heating load of the entire 1,200 square foot addition, even on the coldest days of the year. In this case, any capacity not absorbed by the individual indoor units calling for heat (all of which were between 6,000 and 12,000 BTUs of capacity) is bled through the entire system, unnecessarily warming rooms with units that are turned off. In the shoulder heating season, this sometimes results in uncontrolled and uneven heating. And with any multi zone unit, if one part of that setup were to fail unexpectedly, all zones would be out of commission.
When I converted my house from gas forced air to ductless air source heat pumps, I kept the existing 80,000 BTU gas furnace for backup situations when the heat pumps aren’t able to keep up. Taken together, the heat pumps at my house have 45,000 BTU of heating capacity, just over half the capacity of the furnace they replace. How did I go from an 80,000 BTU furnace to a 45,000 BTU heat pump, you might ask? It’s that the heat pumps are—in theory—sized to meet the load of the building on all but the absolutely coldest days of the year. They run continually, sometimes at high output, and don’t shut off, unlike the gas furnace that starts and stops. Given performance and comfort issues with short cycling, and the fact each installed BTU of capacity adds to the overall project cost, it’s important to have some understanding of how many BTUs are required to heat your home. This information can be gleaned from different sources, like a design load calculation or through an analysis of your past fuel usage.
Cold Climate Performance and Efficiency
Until the last decade or so, air source heat pumps performed pretty poorly in cold temperatures, with poor efficiency. Heat pumps were relegated to warmer, more temperate climates. That has completely changed, and new cold climate models are able to provide excellent outputs at relatively decent efficiencies even at really cold temperatures. If heating is your primary objective, be sure that what you’re installing is cold climate capable. Some manufacturers have branded certain model lines as cold climate equipment, for example, Mitsubishi H2i, Fujitsu XLTH and LG LGRed, while others have not. Generally speaking, cold climate heat pumps have larger compressors, base pan heaters that melt ice buildup in the base of the outdoor unit, and larger heat exchangers capable of extracting more heat at cold temperatures.
There’s no hard-and-fast rule for distinguishing a heat pump that is cold climate appropriate from one that isn’t, but generally speaking, cold climate units are capable of producing 100% of their rated heating capacity down to 5°f, and about 80% of rated output at -15°f. As an example, a 12,000 BTU cold climate heat pump should be able to produce 12,000 BTUs of heat at 5°f and about 9,000 BTUs at -15°f.
And as I alluded to in the previous ducted heat pump explainer, in our climate you’ll likely need some sort of backup heat, whether that’s space heaters or baseboard electric, a woodstove or an existing gas furnace. And unlike ducted heat pumps where backup can be added to the indoor air handler, ductless heat pumps provide no opportunity for integrated electric resistance backup. Some sort of secondary heating backup system is required.
Many of the name brand manufacturers provide heating output information down to -13°f or -15°, but some are starting to provide information down to -22°f and -31°f. Regardless of model or manufacturer, a heat pump at -30° isn’t going to operate with an efficiency much greater (or sometimes even lower!) than electric resistance heat. And the only manufacturer that has a thermal cutout (where the machine simply turns off to protect itself) is Mitsubishi, which usually happens at -18°f (model dependent). These units will not start again until the outdoor temperature has risen to -13°f.
If you’re really interested in digging into the efficiencies of specific heat pump models, head over to ashp.neep.org. There you’ll find a wealth of information on heat pump specifications, efficiencies and outputs at various operating temperatures, and an incredibly useful heat pump sizing tool.
With soon to arrive heat pump tax credits and rebate programs, now’s the time to begin thinking how you might incorporate ductless heat pumps into your home. The most exciting aspects of ductless systems are that they are relatively cheap and simple, cost competitive to most alternative fossil fuel sources, work well in cold climates, and can be phased in over time. If you’re needing help selecting equipment, developing a heating and cooling load calculation, or just want some general information of where to begin, reach out to us at the Winneshiek Energy District.