Air-Source Heat Pumps (ASHP)

Types of Air-Source Heat Pumps

Ducted versus ductless air-source heat pumps

Air-source heat pumps are versatile. They can use three types of systems to distribute heating and cooling:

Ducted

Ducted systems have an outdoor unit (similar to a central air conditioner), which is connected to an indoor air-handling unit that connects to the home’s ductwork. Ducted systems can work well for homes that already have ducts or where the homeowner is planning to install ducts. A version of ducted systems known as “compact-ducted” uses much smaller air handlers that usually serve two to four rooms.

Ductless

Ductless systems (including “mini-splits”) have an outdoor unit which is connected to one or more indoor units (or “heads”) by small copper refrigerant pipes. Each head typically serves one room or area of a house. Ductless heads can be mounted on a wall, mounted to the floor, or embedded in the ceiling. Ductless systems are a great option for houses that have no existing ductwork.

Mix of Ducted and Ductless

Homes can be outfitted with a combination of ducted and ductless systems for a custom configuration that meets a home’s needs. This approach can be especially useful when building an addition; if the original part of a home already has ductwork, and the ductwork can’t easily be extended to the addition, homeowners can choose to install a ductless system in the addition, and also consider a ducted system for the rest of the home.

Single-zone versus multi-zone air-source heat pumps

Single-zone systems pair one outdoor unit with one indoor unit. That indoor unit can either be a ducted system that would heat multiple rooms in a home, or a ductless indoor unit (or “head”), that would heat one zone in a home (typically one room or an open living space).

Multi-zone systems feature an outdoor unit paired with a combination of indoor heads to create multiple indoor zones. Multi-zone outdoor units can be connected to serve a mix of ducted and/or ductless indoor units. You can also achieve multiple zones in your home by installing two or more single-zone systems.

Indoor units

There are different types of indoor units:

Wall-mounted
Wall-mounted units are typically about three feet wide and one foot tall. Ideally, they should be installed at least six inches below the ceiling to allow for air circulation. Installing these units on an exterior wall simplifies the connection to the outdoor unit and to the drain for the condensate water that results from dehumidification in the summer. Consider the rooms in which you would like to place the units and where on the wall they would be installed. These can be used for ducted and ductless systems.

Ceiling-mounted
These indoor units are popular with homeowners who want to conserve wall space. Because heat rises, ceiling-mounted units are most efficient in rooms with ceiling heights of 8 feet or less.

Floor-mounted
These units work efficiently when they have access to unobstructed air flow, similar to traditional radiators. Floor units are around two feet tall, between two and three feet wide, and 8 inches deep. If considering floor units, make sure you have an unobstructed floor space. Floor units can be a great option for heating, as heat rises naturally. If you are removing your old cast-iron radiators as part of your project, you could install these floor-mounted units where the old radiators used to be. These can be used for ductless systems.

Air vents
Air vents connect to an indoor air-handling unit that connects to ductwork in your home. Some ducted heat pumps can be installed in conjunction with a new or existing furnace to provide heating and cooling for most of the year before switching to the furnace during the coldest parts of the winter.

Outdoor units

Each ASHP outdoor unit takes up a few square feet of space. Exact sizing depends on the capacity of the heat pump and how many outdoor units you install. Outdoor units for central or multi-head heat pumps typically have a footprint that is twice as large as conventional air conditioning-only units; they are sometimes slimmer but taller than air-conditioning only units. Single zone outdoor units take up less space, but you may need more of them to heat your whole home.

Optional integration with existing heating system

While cold-climate air-source heat pumps are capable of providing 100% of a home’s heating needs, homeowners may opt to keep their existing heating system in place. In these cases, the operation of new air-source heat pumps can be integrated with the existing system. Depending on the heat pump configuration, there may be multiple thermostats, or “integrated controls” (i.e., one thermostat to handle both devices), that manage both the heat pump(s) and the existing heating system. This will help minimize the use of your existing system while maximizing the use of your heat pump to get maximum savings and comfort. Homeowners should talk to an installer about which configuration is right for them. If the existing heating system is left in place, homeowners should make sure that the installer leaves clear instructions about how to operate the two heating systems together.