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Select an Energy Efficient Heating & Cooling System Step 8

Reducing the energy required for heating and cooling is essential to meeting the net zero goal. An air-source heat pump is the most energy efficient heating and cooling system currently available. It takes heat out of the air, even when temperatures fall below zero, and moves it as hot refrigerant through a small pipe to one or more indoor units that deliver heat into the house. In the summer, an air-source heat pump removes heat from the home, even when temperatures are above 100˚F, and delivers cold air to the indoors.

Mini-splt Heat Pumps

Air-source mini-split heat pumps are highly efficient. Compared to large central air handlers, they can be sized more appropriately for homes with very low heating and cooling requirements, which makes them a good match for many zero energy homes. In addition to saving energy, they cost less to install, are usually ductless, and create only two small penetrations in the building shell.

Mini-split Heat Pumps

Energy Efficiency

Choose a highly efficient model with a heating season performance factor (HSPF) rating of 10.5 or higher and a seasonal energy efficiency rating (SEER) in the mid to high 20s. There are many mini-splits on the market that meet this target. If you’re building in a colder climate, be sure to select a model that will perform well down to -10° F and that they are sized correctly for the specific heating and cooling needs of the home.

Mini-splits owe much of their efficiency to variable speed operation. They run very slowly when the heating need is low and then speed up when the need is high resulting in very stable room temperatures. Most conventional heating systems (including furnaces, central heat pumps and hydronic systems) can operate at only one level and turn on and off in an attempt to keep the living space at a constant temperature.

Distribution

Mini-splits deliver heat or cold, depending on the season, to an indoor unit that usually hangs on a wall or can be recessed into a ceiling. In a smaller home with a more open floor plan only one or two indoor units may be needed to distribute heat or cold to the entire house. In larger homes, where heating and cooling diverse spaces are necessary, a multi-zone system with a single outdoor unit connected to two or more indoor units may be appropriate. While most mini-split systems are ductless, a ducted system is also available that uses relatively small, short ducts to reach different rooms or may share ducting with the ventilation system. Since warm air rises and cool air falls, there will be a special challenge with two-story homes. A multi-zone mini-split allows one unit to be installed upstairs and one downstairs to cover both heating and cooling needs more effectively.

Cold Climates

In cold climates, be sure to select a cold climate rated heat pump, such as the Mitsubishi HyperHeat. They work well even when the temperatures are -10°F, provided they are sized properly. In very cold climates a simple backup heating system, such as one or more electric resistance wall heaters, can be installed in the bathrooms or areas not well served by the mini-splits. These heaters will be used rarely or not at all, but they provide inexpensive insurance in case of extreme weather or heat pump failure.

Warm Climates

High SEER mini-splits cool much more efficiently and are more quiet than standard air conditioners. That makes them especially suitable for warmer climates.

Proper Sizing

Mini-splits work especially well in small- and medium-sized, energy efficient homes. Heating professionals may not be familiar with these highly efficient homes. So it’s critical that the system be designed using industry protocols developed by the Air-conditioning Contractors Association (ACCA) including Manual J. While your energy model does not substitute for a Manual J calculation, it will show the proper capacity in BTUs per hour at the design temperature and serves as a good second opinion. A large discrepancy between Manual J and the energy model would be cause for investigation.

The lowest expected outdoor temperature is called the “design temperature” of the building. Conventional heating and cooling equipment is commonly sized for this worst case temperature. There are usually only a few hours of the year that reach the design temperature and therefore require full capacity. Recent research has shown that it’s important not to oversize a mini-split system, as is commonly done with other types of heating equipment. Oversizing can lead to excessive “cycling” when the system turns on and off much too frequently. This happens when the outdoor winter temperature is warmer and the equipment capacity is too great. The thermostat is rapidly satisfied and the unit shuts off, only to come on again very soon. Excessive cycling will reduce operating efficiency and shorten equipment life. Within normal temperature ranges, properly-sized equipment will run for longer periods at slower speeds providing for more even temperatures and more efficient operation.

It’s okay to match the mini-split size to the expected design heat load without oversizing for additional capacity. Small electric heaters can be installed as backup heat for the mini-split on rare days of unusually extreme cold or in case of emergency. Read The Mystery of Misapplied Mini-Splits for more detailed information about proper design for mini-split systems.