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The use of ductless heat pump HVAC systems has broadened considerably since they were introduced to the U.S. market decades ago, but their technology isn’t familiar to all technicians or in all parts of the country.

Let’s say it’s 15°F outside and the ductless mini-split system you’ve been called to service is leaving your residential customer cold. Or maybe it’s 95°F and humid, and the same type of system, in cooling mode, isn’t keeping up.

What’s next?

The ACHR NEWS recently asked experts at some of the major manufacturers of ductless equipment to weigh in on this question. Here are some of the tips and issues they mentioned:

Check Error Codes

When something is amiss, modern ductless systems will typically display error codes that give a technician a head start in troubleshooting and correcting any problems.

OUTDOOR UNIT: A Mitsubishi Electric Trane HVAC U.S. heat pump, the outdoor unit for a ductless HVAC system. (Courtesy of METUS)

Is there a communication fault between the indoor and outdoor units? Something wrong with a fan motor? A bad sensor in the system?

The error code will offer some clues and the manufacturer’s website or product manual will have guidance on the interpreting codes and taking next steps.

“Communication errors or temperature sensor errors are the most common ones,” said Sean Patterson, team leader of the technical services department at METUS, or Mitsubishi Electric Trane HVAC U.S.

Most mini-split systems, Patterson said, are constantly communicating, and a communication error means the system wouldn’t be working in the first place.

“And then it’s checking wiring and (circuit) boards, board communication,” he said.

Maintenance Issues

Poor maintenance is often the culprit when ductless systems aren’t working properly, said John Pratt, ductless product specialist at Daikin Comfort Technologies. A dirty filter, dirty blower wheels, dirty coils, and debris in the drain pan are the most common issues, he said.

“Those would classify as maintenance items and all of those can reduce performance,” Pratt said. Customers, he said, should put their ductless systems on a program of regular cleaning and maintenance.

“Maintenance is a big factor,” Patterson said. “And yeah, we see a lot of stuff that just isn’t properly maintained, so it doesn’t perform.” That’s “pretty much” the case across all types of HVAC systems, he added.

Check that Charge

Absent an error code or filthy critical components, check that refrigerant — and make sure it’s the proper charge for the system’s specific brand and size.

“If the system is running and not electrically failed, you can almost bet the bank that the refrigerant charge is incorrect,” said Terry Frisenda, national sales manager for construction application at LG Electronics.

REFRIGERANT TECHNOLOGY: An LG Electronics Smart Inverter heat pump outside a shipping container home. (Courtesy of LG Electronics)

“The No. 1 call is improper charge that’s caused by the No. 1 failure: a leaking flare fitting,” Frisenda added.

“The only way to charge a mini split with a variable-speed compressor is to recover the charge, weigh it, see how low it is,” said Patterson. “Find the leak and weigh the refrigerant back in” after fixing the leak.

But, added Frisenda, the refrigerant in a ductless system can also be affected by other factors, including improper insulation of the refrigerant lines, or the lack of insulation altogether.

For example, Frisenda said, if a line is not properly insulated and the piping allowed to “float” within its insulation, condensation can build up around the line when the system is in cooling mode.

“And that water, because it’s cold, would change the state of the refrigerant,” Frisenda said.

Refrigerant lines must be completely, and separately, insulated, he said — even the parts that go through a wall in a chase.

In addition, repeated attempts to top off or change refrigerant, without regard to the correct charge for the system, can result in an imbalance.

“Sometimes that’s successful for a moment until it leaks out again,” Frisenda said. “At some point ... the composition of that refrigerant is so out of whack, the system has no idea what’s going on,” he added.

Lined Up Right?

Ductless manufacturers, Frisenda said, typically have specifications for the refrigerant piping diameter, and for the minimum and maximum lineset lengths, that should be used with a given system. Deviations from those specs will result in refrigerant performance issues.

Sometimes, Frisenda said, improper line sizing is the result of new indoor and outdoor units being connected to the linesets that were used with the equipment being replaced. That’s “never OK,” he said.

“If the line size is incorrect and the minimum or the maximum pipe length is not correct, nothing else will be correct,” he said.

Size Matters

Improper system sizing or equipment that’s not right for the climate in which it’s installed — a heat pump that performs well during January in Alabama but not in Maine — can also be behind the poor performance of a ductless system, experts said.

“You’ll see in the Northeast, somebody does not put a low-ambient heat pump in and they have problems when it’s 15° outside, or they size the equipment incorrectly, and then it becomes a problem when it’s already installed,” said Patterson.

Or, said Pratt, someone may have designed the system to work with backup heat, but the home doesn’t have backup heat.

“So they didn’t size it for the coldest day of the year,” Pratt said.

An undersized system, Pratt said, will fail to keep up with heating and cooling demands. An oversized system will overshoot the desired temperature and will generate excess humidity when in cooling mode.

Large temperature setbacks, which homeowners with gas-fired furnaces commonly do in order to save energy when they are sleeping or not at home, are not recommended for ductless systems, Pratt cautioned.

“Sometimes maybe the system was sized correctly but there’s an education piece that needs to take place,” he said. “Sometimes it’s just a matter of explaining to them, when it’s cold out they can’t do more than a five-degree setback.”

Frisenda, who has worked on ductless systems for more than 30 years, said that during the early history of mini splits in the U.S., some in the industry developed a mindset that the equipment was disposable and not worth fixing. That’s just not the case, he said, and the availability of technical support and replacements parts for ductless systems has increased over those years.

“This is serviceable, repairable, engineered product. Given the opportunity to work, it does exactly that,” Frisenda said. “There’s a reason the rest of the world uses this technology to heat and cool.”