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.

Installation Considerations

The indoor unit’s style typically depends upon the wall space available, whether the installation is for a retrofit application or a new home, and your aesthetic. For example, a floor- or ceiling-mounted indoor unit might be preferred due to aesthetics, or the available width on a wall may dictate the model that will fit in that space.

Indoor units are connected to their outdoor units via electrical wiring and two insulated copper lines. The wiring provides electricity to the indoor unit from the outdoor unit and provides communication between the two units. An additional line drains condensate that is captured by the indoor unit when running in cooling or dehumidification mode and transports it to the building’s exterior.

The practical details for routing these lines may affect the style and placement of indoor unit that is used. Wall-mounted units tend to have the highest rated capacity and can be simpler to install when mounted on an exterior wall or on a wall with a closet or chase on the opposite side. These configurations allow easy access to the refrigeration fittings and often permit the lines to be surface-mounted within a chase. When refrigeration lines are concealed in building partitions, the installation process is more complex.

Floor-mounted units share a physical size similar to that of modern radiators. When installing these units, the refrigeration piping, wiring, and condensate can be routed through the wall or through the floor. In rooms with limited upper wall space or depending on your aesthetic, these models may be preferable to wall-mounted units.

If you’d prefer less-visible indoor units, ceiling cassettes and units with limited ducting may be used. One particular model of indoor unit serves as a picture frame to disguise the presence of a heat pump. Some of these units may have reduced performance, installation limitations, or may be significantly more expensive.

Most outdoor units look similar to one another, with a key difference being that outdoor units with larger capacities tend to be physically larger. When used in areas with little to no snowfall, outdoor units can be installed on a slab. In cold climates, outdoor units are typically installed on a wall bracket or a ground stand to elevate them above the snow line and to provide adequate clearance for draining during defrost mode.

When determining the location of the outdoor unit, there are a few considerations. If the unit is located under a drip edge at a roof eave, a rain cap should be installed to direct any rain or snow melt to the front of the unit. If this moisture splashes onto the back of the unit, it will likely be pulled into the heat exchanger, where it could freeze. Additionally, there needs to be sufficient airflow for the fan. If the unit is mounted with a shrub in front of it, this will decrease the output by restricting the amount of air that can be used for extracting or expelling heat. The electrical system in the outdoor unit is considered a source of combustion and must have adequate clearances from propane tanks and gas regulators.

Multizone Systems

There are several approaches to integrating a heat pump into a home. The most budget-minded solution is to install a single-zone unit in the most heavily used living space. If more conditioned zones are needed, multiple single-zone units may be used, but another option is a multizone heat pump.

Multizone MSHP systems connect a single outdoor unit to several separate indoor units. Each indoor unit has its own controls. Indoor unit styles can be mixed and matched. For example, one zone of the MSHP might utilize a wall-mounted indoor unit, while another zone could utilize a floor-mounted or ducted unit.

Using a multizone system reduces the electrical installation work compared to multiple single-zone units. The trade-off is that the cold-weather output of one multizone MSHP is typically less than that of multiple single-zone MSHPs. For example, the heat output of two Fujitsu 12,000 Btu/hour single-zone heat pumps at 5°F is 75% higher than the output a single Fujitsu 24,000 Btu/hour multi-zone heat pump with two indoor units attached.

A key characteristic of MSHPs is that as the outdoor ambient temperature drops, so does the amount of heat that the MSHP can extract from the outdoor air. The drop in efficiency varies by manufacturer and model. Multizone MSHPs often see a larger drop in efficiency when compared to comparable single-zone units.

While multizone units suffer some efficiency challenges relative to single-zone units in climates with long stretches of freezing weather, they can also have significant benefits. In homes with lots of smaller rooms that are heated and cooled with electricity, having numerous outdoor units scattered along the exterior of the home can be unsightly. In addition, some single-zone units may be oversized for the heating demands of a small room. With careful planning, these small rooms can be served by a single outdoor unit to provide the same level of comfort with less equipment, less visual impact, and less cost.

MSHP production during cold temperatures can be improved by selecting a model that utilizes a pan heater to reduce the unit’s defrost cycling. During a defrost cycle, the unit extracts heat from inside the home to melt any frost that has accumulated on the outdoor unit’s heat exchanger. The pan heater is an electric resistance heater that reduces the accumulation of frost on the outdoor unit and minimizes these cycles (see “Defrost Cycle” sidebar). These models can be rated for outdoor temperatures as low as -15°F. As might be expected, this feature requires more electricity than for a unit without the pan heater.

Common Features

In addition to heating and cooling modes, MSHPs  commonly have a dehumidification mode and a fan mode. Dehumidification mode is similar to cooling mode—the unit removes moisture from the air, which condenses on the indoor heat exchanger and is drained to the outside. Dehumidification mode is less precise than cooling mode, in that it runs at a low fan speed to constantly cycle the air through the unit and a cooling capacity that is a fraction of the unit’s maximum output.

The fan mode can be used to cycle the air in the room through the unit to provide a more even distribution of temperature in the room, without turning on the heat pump. This may help distribute heat from a wood heater in the winter, for example, or simply provide more comfortable air movement in the summer.

The indoor unit also contains filters that limit the amount of dust and other airborne contaminants that accumulate on the heat exchanger, and help purify the air. These filters need to be cleaned regularly and are easily vacuumed with a soft-brush attachment or rinsed with tap water. The filters may need to be cleaned only every few months in homes with good air quality. In homes with smokers, where candles are used regularly, or that have airborne contaminants like pet hair, the removable filters need to be cleaned more frequently. If they are not cleaned, the airflow will be reduced, reducing the heat output.

Controls

The indoor unit functions are controlled by a wireless remote, a wired control, or through a wireless network to provide local or distance control with a smartphone app. Unlike standard heating systems that use a room thermostat to tell the unit when to turn on and off, MSHPs measure the room’s air temperature with a sensor at the indoor unit’s air intake. Another sensor in the indoor unit measures the temperature of the conditioned output. These temperatures determine how the indoor unit is run and are compared to temperatures measured by the outdoor unit to determine the speeds of the compressor and fans. Even when there is no heating or cooling demand, the indoor fan runs at its lowest speed to monitor the room temperature, unless turned off entirely.

Additional Features

Some indoor units have occupancy sensors (i.e., motion sensors) that can reduce temperature when there is no movement in the room, modifying its target temperature by up to 8°F to save energy. For example, if the control is set at 70°F and there is no movement in the room for 20 minutes, the unit will gradually adjust its target heating temperature to 62°F until someone enters the room. At that point, the unit ramps up to 100% of its output to increase the room temperature.

Some units provide timer programming similar to a programmable thermostat. Other features may include:

• A sweep function that directs the airflow out of the unit up and down and left to right, similar to the behavior of an oscillating fan, to more evenly distribute heat.
• For homeowners bothered by the outdoor fan, a mode that reduces the capacity of the MSHP, thereby decreasing the amount of noise made by the outdoor unit.
• An infrared sensor that can sense areas of a room that are hotter or colder than others and can redirect the airflow to provide consistent comfort.

These features may drive the selection of a unit for a particular installation. For example, if an indoor unit is placed on the longer wall in a narrow room, the automated sweep function or infrared sensor may significantly improve the room’s heat distribution.

Determining the best MSHP for your application depends upon several factors. Heating and cooling loads will determine capacity and whether you need a cold climate unit with a pan heater. The layout of the home and your heating zones will dictate how many indoor units are needed. The layout and available space for the indoor units in each room; the installation details needed to connect the indoor unit to the outdoor unit; and aesthetic preferences may influence the type of indoor unit you choose.

Web Extras

• Efficiency Maine heat pump tips • bit.ly/MSHPUserTips
• Northeast Energy Efficiency Partnerships’ Air-Source Heat Pump • bit.ly/MSHPNEEP
• YouTube Heat Pump User Tips channel • bit.ly/MSHPYouTube
• Comparing Home Heating Cost calculator • bit.ly/CompareHeating

Copyright © 1987-2017 Home Power Inc. All Rights Reserved

https://www.homepower.com/articles/home-efficiency/equipment-products/efficient-heating-minisplit-heat-pumps