Efficient Heating with Minisplit Heat Pumps

Intermediate

Inside this Article

Outdoor portion of mini-split heat pump.
Indoor portion of mini-split heat pump.
This wall-mounted indoor air handler is equipped with a pump that is used when the condensate cannot drain by gravity.
A ceiling-mounted indoor air handler can be slightly less efficient, but is less obtrusive than a wall-mounted unit.
A stand-mounted outdoor unit, raised above typical snow levels. The cap prevents snowmelt from accumulating on the coil.
A floor-mounted indoor air handler looks like a space heater, but with heat-pump efficiency.
A wall-mounted outdoor heat pump allows placement flexibility and simplifies installation. Care should be taken to avoid installing outdoor units near bedroom windows.
Air filters help improve indoor air quality. They are easy to remove and clean with soap and water or a soft vacuum brush.
Heat pump controls can be on-the-unit or wall-mounted thermostats, handheld remote controls, or smartphone apps.

Low-maintenance, high-efficiency minisplit heat pumps provide significant energy savings for both heating and cooling.

Unlike resistance heat, which uses electric elements to generate heat, a minisplit heat pump (MSHP) moves heat from one location to another using refrigerant, a compressor, heat exchangers, and an expansion valve. During the summer, an MSHP moves heat from inside the building to the outside. During the heating season, the unit operates in reverse, capturing heat from the outside air and moving it into the home. Since the heat source for these units is air, they are commonly referred to as air-source (or air-to-air) heat pumps.

These systems are referred to as “split” systems because they use two units—an outdoor unit (the condenser) and an indoor unit (the evaporator)—to transfer heat. They are referred to as “minisplits” to differentiate them from larger heat-pump systems, such as commercial HVAC systems that utilize large roof-mounted units and often ductwork.

The idea that heat can be extracted from outdoor winter air can be a bit perplexing, but it is this same refrigeration principle that allows a fridge or freezer to use room-temperature air to cool its inside. As long as the temperature of the refrigerant is below the outdoor air temperature, it will be able to absorb heat. This is accomplished by blowing the warmer outdoor air past a heat exchanger containing the refrigerant. The refrigerant changes to a gas as it is heated, and its pressure and temperature are increased by a compressor. This superheated gas is then transported to a heat exchanger in the indoor unit. A fan in the unit circulates indoor air past the heat exchanger to heat the inside of the home. As the heat is released, the refrigerant condenses and is returned to the outdoor unit. An expansion valve reduces the pressure of the liquid at the outdoor unit, and the refrigerant becomes a gas again as it absorbs heat from the outdoor air.

With electric resistance, each kilowatt-hour consumed generates 1 kWh of heat (3,412 Btu). An MSHP can collect, move, and release 1.5 to 4 kWh of heat for each kWh of electricity consumed, depending upon the unit’s efficiency and the outdoor and indoor temperatures. Compared to other conventional heating appliances, an MSHP system can reduce heating costs significantly.

Since an MSHP can pump heat out of a building, it can also provide air conditioning. With a better distribution system and more efficient components, MSHPs provide better comfort than a traditional window unit at less than half of the operating cost. For example, an Energy Star window-type air conditioner may provide 12 kWh of cooling capacity per kWh of electricity consumed by the unit. An equivalently sized heat pump might provide 25 kWh of cooling per kWh consumed.

Applications

Most MSHP systems are ductless, making them a versatile option for retrofits. Heated or cooled air is distributed via a fan in the indoor unit, which may be mounted on a wall, on a floor, or in the ceiling. Since airflow is resistant to constrictions, such as doorways, these units have the greatest impact in open spaces that permit broad distribution. In homes that are heavily partitioned, multiple indoor units may be needed to provide comfort throughout the home. For some applications, small ducted indoor units may be used.

Because ductless MSHPs are point sources of heating and cooling, this often makes them best suited to reducing the energy demands of a central heating system rather than replacing the system entirely. In a new, well-insulated and sealed home, many designers combine heat pumps with strategically located electric resistance heat in bathrooms and other critical areas to avoid the need for a central heating system.

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Comments (5)

Greg Smith_0_0's picture

Mini splits are a much better option on the backup panel than a central HVAC unit. I always cringe when someone send me a load profile with a 5T AC unit, or two... You can get so much more bang for your buck from your storage system if you are smart about what loads you run when the grid goes down.
"But Greg, I GOTTA HAVE MY KOI POND PUMP AND MY GLASS KILN ON THE BACKUP PANEL!!!"

Marc Fontana's picture

Thanks for the article . A few questions : What is the average cost for a small 1-zone unit? Do they require a 240v circuit ? What maintenance is there on the outdoor unit ? Can installing a MSHP system be a DYI project ?

vwoodruff@insourcerenewables.com's picture

Marc, the average cost for the equipment for a single-zone mini-split heat pump is around $2k. The cost including labor typically is in the $2,500-4,000 range depending upon the size of the unit and the difficulty of the installation. Though there are 120V models, their performance tends to be dwarfed by the 240V models. Maintenance on the outdoor unit includes cleaning the coil periodically - annually clearing leaf debris, for instance, and a low pressure coil cleaning wash every few years. With the high performing units, it is highly recommended to have a refrigeration professional perform at least that portion of the installation. The greenhouse gas potential from losing refrigerant from these units is not to be ignored.

Ronald Gilliland's picture

Has anyone measured the power draw on their mini-split even when the unit is powered off? I have a Fujitsu Halcyon 9RLS3 and have found that there is a continuous draw of 100 watts after using the control panel to fully power the unit off. That seems a shockingly high power draw for a device intended to be highly efficient. During months when in Pennsylvania climate when there is no need to use the unit this parasitic load adds up to a large power usage. I now keep the breaker off to avoid this except for the times when need for unit is clear.

Vince Caruso's picture

Nice article. Some comments on instalation could include self install units now available, some may be moved to new home or other rooms many times as they have valves in the precharged hose's connectors. Just screw them on and turn on the valve. This could be a real cost savings for DIY'er. Great technology for solar powered homes.

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