Ductless mini-split systems have been around North America for almost 20 years. Mainly considered a niche item, they worked great in old houses or historic buildings that needed cooling but had no space for ductwork. Or, they were used when spot cooling was required, or perhaps when one room needed a little more cooling than the rest of a structure.

But in the last two or three years, mini-splits have enjoyed wider popularity, due in part to the fact that a whole generation has now lived with them and found them to work well. Word of mouth has spread, inducing more people to check out the benefits of mini-splits. Some owners of new houses are even requesting multiple mini-split systems as opposed to one central a/c system.

Contractors, too, have been looking at mini-splits with renewed interest, learning about their benefits as well as understanding how they function. In addition, the profitability on a mini-split system can be very good.

These mini-splits are inverter-driven systems. The inverter decreases power consumption a further 20%. Inverter-type systems use only the necessary capacity needed to maintain their setpoint.

BENEFITS ARE MANY

Another mini-split advantage is its energy efficiency. “People are understanding that true indoor comfort costs include operation and longevity expense in addition to the initial cost,” says Stu Taylor, technical support manager, Samsung, Whittier, CA. “People understand that those considerations are more important than a low up-front price.”

One of the reasons a mini-split can save energy is that it can be used to cool one zone of a building. In a house, for example, residents often gather in one place for the majority of the time.

However, Taylor adds that homeowners should not use a mini-split system the same as, say, a window air conditioner. “Truthfully, mini-splits are sometimes better to be left on when it’s really hot and humid out. You can put these on a function called the auto mode, which is very cost effective, and it will allow you to keep the room at a temperature in either cooling or in the dry mode, or in whatever mode it needs to maintain the temperature.”

Another benefit includes having the air handler section in the living space (as opposed to a central system, which usually relies on a thermostat placed in a hallway or some other place that gives an average temperature of the entire house). With a mini-split, the temperature sensor is in the living space, and there will only be a plus or minus 2 degrees F difference in each zone.

In addition, the evaporator coil, which is also in the living space, dries the air better, as it has a 35 degree temperature (30 degrees delta T,) rather than the standard 45 degrees (20 degrees delta T) on a central system evaporator. The dewpoint is achieved quickly, so the discharged air is drier and cooler. With less humidity in the air, occupants have a tendency to turn up their system setpoints by 2 to 4 degrees, resulting in even more energy savings.

Having a mini-split in an occupied space often brings up concerns about the level of noise. This isn’t an issue, because the evaporator coil features a cross-fan impeller designed with staggered vanes of different lengths. This modifies the sound generated by the air crossing the vanes. Some new designs have wall-mount units operating at under 30 dB at high speed.

As for the concern about condensing units being placed close to a structure, owners need not worry. “A lot of people are not aware of how unbelievably quiet the outdoor units are and how they can really be put close to areas that they’re in, how little they impact the area that they’re in,” notes Taylor.

Because owners are concerned about noise, contractors have a tendency to overextend the amount of line set between the indoor and outdoor units; that hurts the mini-split’s performance. Keep the indoor and outdoor units as close to each other as possible, says Taylor, because more energy will be saved that way. The system will run more efficiently.

NOT SIZED THE SAME

As can be expected, mini-split systems are not sized the same way (using cfm as the measurement) as a central a/c system. Of course, the first item that needs to be performed by the contractor is a proper heat load calculation. Where some contractors make a mistake in determining the size of a mini-split is by counting the cfm. Mini-splits average about 250 cfm per ton, rather than the 400 cfm per ton of standard central systems.

As a result, a common problem is oversizing. Interestingly enough, mini-split systems usually work better when they’re slightly undersized. “If our systems are slightly undersized they’ll do a much better job, not only because they’ll run longer but because they’re dealing with latent heat; the evaporator temperature is so low, they’ll do a great job of moisture extraction,” says Taylor.

Another issue is the location. A mini-split’s fan blows constantly, allowing the entire area to circulate, mixing the air and eliminating temperature stratification. For this reason, the system should be put in a place that’s going to provide a constant good sweep of the air, while not blowing air directly onto occupants or directly into a wall.

The evaporator coil features a cross-fan impeller designed with staggered vanes of different lengths, so the unit is quiet indoors. Outdoor units can be placed close to a structure because they are extremely quiet.
“You want to have at least 15 to 20 feet in front of the unit, so the air returning is indicative of the temperature in the entire room,” adds Taylor.

He notes that he receives many calls where end users, concerned about energy savings, wish to turn off the fan. “We tell them that the fan is an integral part of the system, allowing greater temperature accuracy and comfort. Compared to standard systems using 1/3- to 1/2-hp fan motors, mini-splits typically use 30-watt motors or less,” says Taylor.

Most manufacturers, like Samsung, are happy to work with contractors on proper sizing and installation. The benefit to learning this information is greater profitability.

Still another benefit is that the first cost of a mini-split is now becoming comparable to more traditional air conditioning equipment, said Taylor. Also consider that the rest of the world, which historically has much higher energy costs than the United States, has used this technology for the last 30 years. Maybe it’s worth taking the time to check out mini-splits a little more closely.

Sidebar: Ratings Don’t Tell The Whole Story

Contractors and building owners are often stuck on an equipment’s SEER. Unfortunately, that rating doesn’t work well for mini-split systems. For the last 20 years, the Department of Energy has mandated that mini-splits be classified as a unitary product, even though they have a colder evaporator and no duct loss.

“We’ve made everything at 10 SEER and 6.8 HSPF, so we already meet the current guidelines,” says Stu Taylor. “A lot of our systems do better than that, everywhere from just a little bit to a whole SEER rating more.”

The problem is that the SEER has always been the best rating for central systems, and it’s just not possible to measure a central system against a mini-split; it’s not a fair comparison, asserts Taylor.

“Until they come up with partial load factors or other ways of measurement, we’re staying at the minimum. How can you take the same-SEER unitary unit and put mini-splits against it? If everything were the same and you were just using it in one room, how can that be considered an accurate comparison?”

— Joanna Turpin

Publication date: 01/14/2002