Getting the Right Minisplit

Curious readers regularly post questions on how to size minisplit heat pumps and how to select and install the right equipment. Although Fine Homebuilding and Green Building Advisor contain dozens of articles on the topic, the questions keep coming—so it’s time to provide some straightforward guidance on this issue.

First, calculate the heating and cooling loads

The first required step for anyone who wants to install HVAC equipment is to perform a room-by-room load calculation. In other words, the designer needs to calculate the anticipated heating load for each room under the coldest likely conditions, as well as the anticipated cooling load for each room under the hottest likely conditions. Although most people have heard of heating and cooling load calculations, which are code-required, this step is often ignored.

Getting an accurate load calculation isn’t easy; most HVAC contractors grossly overestimate design heating loads and design cooling loads. For more information on obtaining an accurate load calculation, see these articles:

• “Saving Energy With Manual J and Manual D”
• “How to Perform a Heat-Loss Calculation — Part 1”
• “How to Perform a Heat-Loss Calculation — Part 2”
• “Who Can Perform My Load Calculations?”

Installing a separate minisplit head in each bedroom is a mistake

The typical bedroom has a design heating load of 2,500 BTU/h or less — far less than the rating on a typical ductless minisplit head, which usually ranges from 9,000 to 12,000 BTU/h. If your HVAC contractor is advising you to install a separate minisplit head in each bedroom, it’s probably time to choose a different contractor. (Of course, an unusually large bedroom that is poorly insulated, leaky, or equipped with very large windows may have a much higher design heating load than average — so it always pays to do the math.)

Instead of installing a ductless minisplit head in each bedroom, a better approach would be to install a minisplit head in a nearby common room — perhaps the living room or hallway — or to select a ducted unit instead of a ductless unit.

According to GBA reader Dana Dorsett, a self-taught minisplit expert, “In most houses the ‘head in every room’ zoning approach leads to gross oversizing to the point of lower efficiency.”

So what’s the best way to handle bedrooms?

If you’re thinking of omitting heating and cooling units (or registers) in your bedrooms — for simplicity’s sake, let’s call this the “point-source heating” approach — you should keep these facts in mind:

• The point-source approach works better for heating than cooling.
• The point-source approach works best in compact rectangular two-story houses with upstairs bedrooms — but not so well in stretched-out single-story houses.
• Occupants who are irritated by room-to-room temperature variations may not like the point-source approach.
• If you adopt the point-source approach, make sure that your home’s thermal envelope is tighter than average, with above-code insulation levels and high-performance windows.
• The approach works better in bedrooms with small windows than bedrooms with big windows.

If these limitations make you nervous, go with a ducted minisplit.

In a recent telephone conversation, John Semmelhack, a certified Passive House consultant in Charlottesville, Virginia, explained, “It comes down to client expectations. Most of our clients are used to central heating and cooling. If you deviate from what they’re used to and provide worse comfort in bedrooms, they aren’t going to be too happy. Up north, some homeowners have never had air conditioning at all. For them, point-source cooling — maybe with one head per floor — is an approach that works for them, because any cooling at all is better than what they’re used to. It’s a lower-cost approach. But in my opinion, it all depends on the client.”

Ducted minisplit units often make more sense than ductless units

In most cases, a ductless minisplit will be more efficient than a ducted system, due in part to the fact that it takes energy to move air through ducts. That said, it often makes sense to select a ducted unit, in spite of the energy penalty.

A designer who plans ahead can usually find a way to specify a ducted minisplit unit that can serve three or four bedrooms. If you’re interested in this approach, you’ll need to provide a location for the indoor unit. (A closet or mechanical room will work; or, if the house has high enough ceilings, an indoor unit can be hidden behind a dropped ceiling.) You’ll also need to plan your duct layout carefully, providing plenty of room in your plan for oversized ducts that are short and, if possible, unencumbered by elbows.

If you want a ducted unit, choose Fujitsu over Mitsubishi

There seems to be broad consensus that when it comes to ducted minisplit units, Fujitsu equipment is superior to Mitsubishi equipment.

For a single-family home, the most useful models are these:

• Fujitsu Halcyon 9RLFCD (the outdoor unit is model AOU9RLFC; the indoor unit is model ARU9RLF);
• Fujitsu Halcyon 12RLFCD (the outdoor unit is model AOU12RLFC; the indoor unit is model ARU12RLF).

The smaller of these two units is the 9RLFCD. It has the following specifications:

• Rated cooling capacity, 9,000 BTU/h.
• Cooling capacity range: 3,100 to 12,000 BTU/h.
• Rated heating capacity: 12,000 BTU/h.
• Heating capacity range: 3,100 to 18,000 BTU/h.

The larger these two units is the 12RLFCD. It has the following specifications:

• Rated cooling capacity, 12,000 BTU/h.
• Cooling capacity range: 3,100 to 13,600 BTU/h.
• Rated heating capacity: 16,000 BTU/h.
• Heating capacity range: 3,100 to 19,400 BTU/h.

Although Fujitsu provides performance data for these units at an outdoor temperature of -5°F — but no performance data at colder outdoor temperatures — several energy experts (including Marc Rosenbaum and Gary Nelson) report that the equipment works in New Hampshire and Minnesota, at outdoor temperatures as low as -20°F.

“We prefer the Fujitsu ducted units to the Mitsubishi equipment,” says Semmelhack. “We like their really great heating capacity at low outdoor temperatures, and they have an extra little bit of fan power compared to other slim duct air handlers. We like the price. Another great thing is that the Fujitsu indoor units can be installed vertically as well as horizontally. Mitsubishi currently has a one-to-one single split unit with good heating capacity, but the smallest unit is rated at 30,000 BTU/h. Mitsubishi has been promising to come out with something with smaller capacity, but it’s not available yet.”

According to Dana Dorsett, “At maximum speed the Fujitsu AOU/ARU9RLFCD is good for 14,000 BTU/h at [an outdoor temperature of] -5°F (the lowest it goes in the extended temperature capacity chart).  … That series is specified at 0.4 inch water column static pressure, compared to 0.2 i.w.c for the comparable Mitsubishi mini-duct cassettes (which also have lousier low-temperature capacity), which makes the ductwork design [with the Fujitsu] a bit easier.”

Summing up, Dorsett notes, “Fujitsu’s mini-duct units have the flexibility of being able to mount them vertically (in the side of a closet or something), and also have more capacity and higher efficiency at 0°F than most of the competition.”

Ducted units require careful duct system design

Ducted minisplits from Asia are more efficient than heat pumps from American manufacturers, in part because their air handler fans aren’t as powerful. As long as your duct system is well-designed, these efficient air handlers from Asia work perfectly. But if you try to marry a ducted minisplit unit to the typical sloppy American duct system, performance will disappoint.

To achieve rated air flows, ducted minisplit units require close attention to maximum permissible static pressures (usually listed in manufacturer’s data sheets). To achieve these specs:

• You’ll need to choose larger duct sizes than you’re used to installing;
• You’ll need to keep duct runs as short as possible; and
• You’ll need to minimize the number of elbows.

According to Semmelhack, “For the Fujitsu, the maximum static pressure is 0.36 i.w.c at the rated air flow. For the other manufacturers, it’s 0.20 i.w.c. for the most part. That’s not a lot of fan power.”

If you have access to Manual D software from ACCA, then you’re all set: Manual D is perfectly capable of designing a duct system for a ducted minisplit. If you don’t have access to Manual D, you might want to check out these relevant Q&A threads on GBA:

• “Help me understand static pressure in ducted minisplit systems”
• “Horizontal ducted minisplit ducting”

According to Semmelhack, the problem with most installed ducted systems is that “No one asks for duct design services. They just blindly trust their HVAC contractor to do it.”

Multisplit systems have an Achilles’ heel

The term “multisplit” is used to describe a multi-zone minisplit system with one outdoor unit connected to several indoor units. My advice: don’t install a multisplit system unless you are absolutely painted into a corner. As Semmelhack notes, “There can be weird control and efficiency issues with multisplits.”

When multiple minisplit heads are connected to a single outdoor unit, both performance and efficiency can be disappointing. For technical reasons, as long as one indoor head calls for heating or cooling, refrigerant continues to flow to all the indoor heads — and, as a result, rooms that don’t need heat or cooling can become too hot or too cold.

Dorsett explains, “When a zone of a multi-split is nominally ‘off,’ there is still refrigerant flowing through the coils whenever any other zone is running, it is still heating/cooling the ‘off’ zone. In cases of extreme oversizing that can even lead to overheating or overcooling comfort issues.”

In another thread, Dorsett noted, “With multi-splits you need to be very careful about sizing the heads proportionally to their loads. A half-ton head in a room with a tiny 1500 BTU/h design load will overcool the zone even when it’s off if the other zones are right-sized for the loads. The multi-split compressors don’t modulate continuously with load, instead operating in steps based on the size of the zone calling for coolant. There is always a modest amount of ‘extra’ that gets distributed to the other heads, even if they’re ‘off,’ to avoid returning liquid coolant to the intake side of the compressor. For tiny-load rooms even without the blower running the coolant running in the coil can overcool (or overheat, in winter) the room.”

A minisplit system works best when the compressor as well as the fan can modulate. Dorsett notes, “The downside to multi-head solutions on a single compressor is that the heads don’t modulate with load. You can set the fan speeds up/down, but it will always cycle on/off, which is less efficient and less comfortable. A mini-split head married to its own modulating compressor can change both the compressor speed and blower speeds to adjust to the load, and run very long, nearly continuous and quiet cycles at low speed.”

One final point: a multisplit isn’t necessarily any cheaper than better alternatives. According to Dorsett, “It’s often/usually cheaper to install two separate minisplits than a 2- or 3-zone multisplit, and it’s more efficient and comfortable to boot.”

If you can’t avoid installing a multisplit system, here are two key points to designing a multisplit system:

• The indoor units must be sized to match — but not exceed — the indoor design loads; and
• The outdoor unit must be sized to match — but not exceed — the sum of the rated output of the indoor units.

Consult these web sites for performance specifications

If you are selecting a ductless minisplit or a ducted minisplit, you need to know the rated output of the equipment at a variety of outdoor temperatures. There’s a problem, though: it’s not always obvious where that information can be found.

Here are three online resources where homeowners, builders, and HVAC professionals can find equipment specifications for minisplits:

• One of the best available databases for cold-climate air-source heat pumps is maintained by Northeast Energy Efficiency Partnership (NEEP). Here is the link.
• Mitsubishi maintains several web sites. Here is the link to the one that is actually useful.
• Submittal sheets and operating manuals for Fujitsu equipment can be found here.

Choose equipment with a high turndown ratio

Although HVAC systems are usually sized so that the rated capacity matches the design heating or cooling load, builders need to remember that these design loads only occur for a few hours per year. Most of the time, HVAC systems are operating under partial load conditions. That’s why modulating equipment is vastly preferable to single-speed equipment.

For air-source heat pumps, the usual definition of the turndown ratio is the maximum heat output capacity at 5°F divided by the minimum heat output capacity at 47°F.  Some Mitsubishi minisplits (for example, the Mitsubishi FH09NA) have turndown ratios as high as 11-to-1. That means that under partial load conditions, the Mitsubishi FH09NA can operate continuously while delivering just 9% of the heat that would be required when the unit is operating at maximum capacity — for example, on the coldest day of the year.

If you look at a unit’s turndown ratio from the perspective of heating performance alone, it’s easy to make the case that a high turndown ratio improves comfort and efficiency under partial load conditions. But since cold-climate homeowners often size an air-source heat pump to meet the peak heating load rather than the cooling load, a high turndown ratio also affects cooling performance. (In a cold climate, a house might need a 3-ton air-source heat pump to meet the peak heating load, while it only needs a 2-ton unit to meet the peak cooling load.)

In general, multisplit systems have a worse turndown ratio than one-to-one minisplit systems — but there are exceptions. Look at the specifications before making your final equipment selection.

Miscellaneous advice

• A compact two-story house can often be heated with a single ductless minisplit located on the first floor. However, a single downstairs minisplit won’t work for cooling the second floor.
• In snowy climates, outdoor units should be located on the gable end of a building and should be mounted above the expected snow depth.
• In cold climates, outdoor units may require a pan heater to avoid icing problems. Consult equipment manufacturers on this issue before ordering equipment.
• Mitsubishi equipment has a heating capacity equal to 100% of its rated output at an outdoor temperature of 5°F. At -4°F, the heating capacity drops to 82% of the rated capacity. At -13°F, the heating capacity drops to 62% of the rated heating capacity.
• Wall-mounted controls for indoor units, though expensive, do a better job of maintaining the thermostat setpoint than a handheld remote.
• With a minisplit, thermostat setbacks won’t save much energy unless you plan to be out of the house for several days. “Set it and forget it” is the usual advice.
• Since indoor units can get clogged up with household dust or pet hair, homeowners should plan on cleaning their indoor units once per year. Most homeowners will prefer to hire an HVAC contractor to perform this annual chore.

Originally published on GreenBuildingAdvisor.com. Martin Holladay is a retired editor who lives in Vermont.