How to use a mini

Jul 24, 2023

Q: I am one of a growing number of Americans with a new mini-split heat pump. I have no idea how to maintain it or use it efficiently. I’ve talked with many other owners and we are all largely in the dark. We’ve received conflicting advice, such as whether we should change the temperature when we leave the house. Is it more efficient to keep the temperature the same all the time?

A: You’re not alone, and there is a good reason you might be confused. A mini-split heat pump, also known as a ductless heat pump, delivers warm or cool air directly to specific rooms. It gets part of its efficiency from incorporating heat pump technology, which has long been used for air conditioning and some heating in homes with duct systems. In both ductless and ducted systems, heat pumps use a refrigerant that pulls heat from one place and transfers it to another — from inside to outside in the summer and from cold outdoor air, which still has considerable heat in it, to the indoors in winter.

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But mini-split systems also gain a lot of efficiency by skipping the ducts that central heating and air conditioning systems use. Instead, there is an indoor unit — or several, if the system is set up with zones — where the air moves in or out. “A central air system has a very big, powerful motor to push air through the whole house,” said Jennifer Amann, a senior fellow in the buildings program at the American Council for an Energy-Efficient Economy (ACEEE), a nonprofit research organization focused on reducing energy waste. Mini-splits have much smaller fans that operate most efficiently when they run at low speeds. They can do that and deliver comfortable temperatures only if the temperature settings remain fairly constant.

“For years, we’ve told people to use setbacks,” Amann said. With a ducted system, turning down the thermostat in winter or up a few degrees in summer when the house will be empty or you go to bed really would save energy. But with a mini-split, adjusting the temperature setting frequently means the fan needs to move a lot more air, and faster, to get the temperature to a comfortable level when you return or get up in the morning. This wastes energy, and it makes living with a mini-split less pleasant. No one likes having to sit under a blasting fan while the room adjusts to a comfortable temperature. “Set and forget” is the best way to use a mini-split, Amann said.

But what temperature should you set? Mini-split owners often complain that they’ve had to set the temperature higher in winter and lower in summer than what they used with a central, ducted system, Amann said. Even so, it’s wrong to conclude that the mini-split isn’t as efficient. The real explanation: In a ducted system, the thermostat is usually on an interior wall, where it will sense the temperature where people are most likely to be. With a mini-split system, unless you invest in a separate thermostat, the sensor (or sensors, for systems with multiple zones) is where the air comes out. And that is likely to be on or near the ceiling, where the air is hottest. Setting a mini-split to 68 degrees won’t result in the same level of comfort that you got with a central system’s thermostat set to 68 degrees, even if the system is moving an identical amount of heat inside or out. “Experiment,” Amann said. Don’t feel guilty because you want the temperature higher in winter or cooler in summer than it was with your old system. “Once you find what makes you comfortable, then set it and forget it,” she said.

Amann says she is a relatively new owner of a mini-split system. It’s been “really quite a learning experience,” she said, with a lot of experimenting to figure out the best temperature settings for various rooms. But once she figured out what made her family comfortable and she stopped fiddling, everyone has been happy, she said.

That said, there are times when temperature settings should still be adjusted, such as when no one will be home for more than a week, or when a teen goes to college and a room in a zoned system will be unoccupied for months. Reading the instructions for your unit can help you fine-tune options, including the direction the fan blows air and an energy-saving mode appropriate for when rooms are unoccupied or people are sleeping, if you really want a different temperature then.

“Five lessons from my first year with mini-split heat pumps,” a blog post by Mariel Wolfson, an editor and writer for the ACEEE, has tips about what the remote control enables you to change. The terminology on various brands’ remotes may differ, but the same general functions should be available in all systems. Some manufacturers offer added options. Mitsubishi, for example, offers an Econo Cool setting for hot weather that “swings” the air flow like an oscillating fan does, which helps you feel cooler even at a warmer temperature. The Econo Cool setting automatically adjusts the compressor output to take advantage of that fact.

As for maintenance, one critical task is to clean the filter on the indoor unit frequently. Filters on ducted systems need to be cleaned or changed regularly, but with ductless systems, it’s even more important, Amann said. “If the filter gets clogged, it can really lower the efficiency of the fan and its ability to circulate air.”

Wolfson called in a pro to clean her system so she could learn how to do it. Amann suggested watching YouTube or asking the installer for advice. “It’s relatively straightforward,” she said.

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