Mini-split heat pumps, also called ductless heat pumps, are becoming more common in North America. Under the right conditions, these systems offer a variety of advantages to homeowners, but those advantages should be carefully weighed against the drawbacks to determine whether or not these systems are a good option for your home.
An example of particular concern is any situation where a homeowner is considering replacement of a boiler with mini-split heat pumps, while expecting the same heating performance that they’ve enjoyed with their existing system.
Mini-split systems have, in part, gained popularity because they’re able to provide heat and air conditioning to a space and are easy to install in a small area, such as an addition or a sunroom. In addition, these systems are ductless, meaning they don’t require a network of ductwork behind walls, under floors, or in ceilings in order for them to operate. They hang on the upper part of walls or in the ceiling and only require power connections and refrigerant lines, which are fed between wall joists and connect to an outside condenser. They can be an effective way to add air conditioning to a home which is heated by a boiler and does not have ductwork. The challenge comes when these systems are asked to provide heating as well. In warmer parts of the country, these systems can adequately satisfy the heating load for most of the year. In times of the year that more serious cold sets in, or in areas of the country that experience a “traditional winter”, the limitations of these systems become evident.
Low-temp performance: electric heat
In heating mode, these mini-split heat pumps, which run entirely on electric power, extract heat from outdoor air, and through a refrigerant cycle, deliver that heat to indoor spaces.
As such, the unit’s ability to efficiently provide heat to the home is greatly dependent on outdoor air temperature. Unlike a boiler, which maintains the same efficiency level regardless of outdoor conditions, the efficiency of a heat pump in heating mode falls dramatically as the outdoor temperature drops.
Many mini-split manufacturers claim the ability to provide heat to a home down to 0°F or lower outdoor ambient temperatures. While this may be true, there comes a point where the unit is not operating much more efficiently than an electric baseboard system might. This can result in surprisingly-high electric bills. A ductless heat pump in North Carolina, for example, is likely to provide heat much more efficiently than one in upstate New York, New England, the Great Lake states, etc.
The question to consider here is: How cold do my winters get, and how long is the heating season in my area?
Also, a properly installed boiler, which is not affected by outdoor temperatures, will maintain setpoint temperatures dependably. There are instances where mini-split heat pumps are not able to maintain setpoint. Part of this is due to how it works, but a significant part is evident in what it is actually designed to do. For that, we have to talk about basic physics; hot air rises and cold air falls. Being mounted either at the top of a wall or in the ceiling, a mini-split will do a great job at cooling, as the AC function will cascade cooling air down into a room. Conversely, by pumping warm air through the same system in an attempt to heat a living space, a mini-split is attempting to defy physics. Yes, you will feel warmth in the upper part of the room, but your ankles and feet won’t be enjoying the same experience. As a sidenote, homes with ductwork and furnace/air conditioning systems also make this tradeoff. In areas that experience cold winters, the ducts port air into floor vents. Heat rises! These systems are actually designed with winter in mind.
A good question to ask yourself: Do I prefer to keep my home very warm in the winter with a smooth and even distribution of heat, or am I willing to accept the possibility that I may need to sacrifice some level of indoor comfort if I replace my boiler with a mini-split system?
Boiler systems are known to be the longest-lasting type of heating system available. Mini-split heat pumps do not have the same reputation. When used for both heating and cooling, they generally have a shorter lifecycle. A large factor is that these units will likely be running in both heating and cooling modes for most of the year.
Beyond that, every component in a mini-split heat pump system is proprietary, manufactured by a single company and specifically intended for that system only. Like most electrical devices, these systems are rendered obsolete in a few short years.
Because of this, these systems can’t typically be partially replaced, repaired, or upgraded. Rather, the entire system needs to be replaced at considerable expense.
Boiler systems, on the other hand, are forward-backward compatible, mostly without proprietary components. The boiler, pump, pipe, controls and valves are all independent components that can be repaired, replaced or upgraded at any time.
Ask yourself: Am I willing to accept that mini-split heat pumps don’t last as long as boilers, and that repair or partial replacement options may be few or non-existent in the event of a failure?
Presence of an outdoor unit
Conventional boiler systems have a distinct advantage over ductless heat pumps in that there are no outdoor components to the system. In a ductless heat pump system, the condensing unit (or units) must be placed outside.
This is an aesthetic consideration; the homeowner and neighbors can see and often hear the unit.
Ask yourself: Will seeing and hearing a condensing unit in the yard bother me?
The outdoor condensing unit must be kept clear of landscape shrubbery, leaves, dust and debris, requiring regular outdoor maintenance to ensure maximum operating efficiency.
A final consideration on outdoor units comes during heating season, when the unit is most needed. Condensers are prone to icing over while in heating mode, depending on outdoor conditions. Because of this, they often require an electric base pan heater. When this electric element is installed, it draws a good deal of power to keep the condenser coil free of ice.
To compound the issue of winter ice accumulation, mini-split condensers must be mounted high enough to remain clear of the anticipated snow load. If they are not, the homeowner must remember to shovel snow away from the unit to ensure proper air circulation.
Ask yourself: Am I willing to keep the condensing unit clear of debris and snow?
Domestic hot water production
The majority of boiler installations in North America serve as the source of hot water in addition to space heat. This can be accomplished with an internal coil, an indirect-fired water heater, or with a secondary heat exchanger in the case of a combi boiler application.
If you currently have a boiler system that provides your hot water and are considering replacing the boiler with mini-split heat pumps, your contractor will also need to install a water heater.
Ask yourself: Am I willing to install a new water heater in addition to a mini-split system?
Aesthetically speaking, ductless systems have a few drawbacks. The indoor portion of the system, called an evaporator, is most often hung on the wall. To properly condition a space, they typically must be installed in areas that are very visible.
Many homeowners find these units visually obnoxious, detracting from the rooms in which they are installed.
Ask yourself: Will I be satisfied with the indoor environment of my home with units mounted on the wall?
It should also be mentioned that evaporators contain a powerful fan to distribute conditioned air throughout the living space. They are relatively quiet, but certainly create sound. Moreover, occupants can feel air movement. When the fan is running at high speed, it can be distracting. In addition to the fans, the evaporators themselves make sounds as they are cycling. These sounds are comparable to those that window/wall air conditioners make.
Ask yourself: Will the possibility of having strong indoor air currents and the sounds of the evaporator cycling bother anyone in my household?
Using mini-split heat pumps can be an effective way to heat an indoor space, under proper conditions, in certain climates, and when designed and installed correctly. They do not provide equivalent comfort, longevity, efficiency or value to that of a boiler system, especially when an entire home relies on the ductless system for heating. This is especially true in cooler climates, where winter conditions can drop below freezing and remain there for any length of time. Mini-split systems are most effective when used for air conditioning, and they can be an attractive option for those who are looking to add a cooling component to homes that do not have existing ductwork.
As with all HVAC systems, be sure to hire a licensed professional if you are considering a ductless system installation, and carefully discuss the level of comfort and performance you expect from your new system.