Heat pumps are one of the most efficient ways to heat a house - since they pump heat, rather than create it, they can paradoxically have an efficiency greater than 100% (a resistive electric heating element has an efficiency of 100%, and a fossil-fuel heating system less than 100% - because some heat is lost up the flue).
Traditionally in North America ducted air heat pumps have been used for cooling, and also for heating in winter. Typically a large unit is installed which serves the entire house through a duct system installed during construction. The warm and cold sides of the pump are contained in a single unit.
In recent years the ductless split heat pump has been introduced. The warm and cold sides of the pump are contained in separate units, connected by refrigerant lines. A mini-split unit can serve a single room, while a dual or triple split unit can serve two or three rooms with separate indoor units connected to a single outdoor unit. The compressor is located in the outdoor unit, while the indoor unit is relatively small and lightweight.
Since the refrigerant lines are of modest diameter, and the indoor units small, these systems can be retrofitted to an existing house relatively easily with minimal disruption.
(these instructions were written for the Senville Aura series; there are a number of similar units by other manufacturers such as Fujitsu and Lennox)
The first step is to determine the correct size of heat pump. For a given location, climate records will give an outdoor temperature range. For instance, on 97% of days the temperature will not fall below -10C or rise above +25C. Then, given a desired indoor temperature range, say +15C to +21C, it is possible to calculate maximum heat loss and gain for a particular room given the envelope area (walls, ceiling etc.) and R-value of each component such as walls, doors, windows. This may have already been done as part of the original building design. Alternatively, one might estimate it based on floor area and local construction practice.
The next step is to determine suitable locations for the components (indoor and outdoor units). For the Aura, the indoor unit weighs about 10kg and is designed to be mounted high on an exterior wall. The outdoor unit weighs about 40kg and can be mounted on a concrete slab, on a bracket hung from a wall, or on a metal stand. Consult the installation manual for recommended clearances. The outdoor unit should not be exposed to severe weather and should not be located where it can suck in debris, dirt, leaves etc. In operation, it will blow warm air (when cooling), or cold air (when heating), so should probably not be sited next to a patio etc. where people sit.
The two units are connected by refrigerant lines, and by an electrical cable, and can be at some distance from one another. For the Aura, the maximum allowed line length is 20m, and the maximum difference in height is 8m. In addition to the lines and cable, both units have a condensate drain, which should be routed outside using pipe with a consistent downward slope.
If no engineering drawings are available for the units, it may be necessary to make measurements yourself to determine the location of mounting brackets and service lines. For instance, the smaller Aura indoor units have brackets on an 18" pitch, while the outdoor units have brackets on a 21" pitch.
The outdoor unit is significantly noisier (54dBA) than the indoor unit (37dBA); it should probably not be near an open window.
The smaller Aura set requires nominal 230V split-phase AC at 9A, on a 15A breaker. Larger sets may require 20A or 25A service. The indoor unit(s) are powered from the outdoor unit. Since regular outlets are probably nominal 115V, and the heatpump should have a dedicated breaker, it is probably necessary to run new cable from a breaker panel to the outdoor unit location.
The Aura is shipped with refrigerant line stubs folded inside the back of the unit. There are cutouts on either side of the case, so the lines may be routed out the left side across the wall, through a hole in the wall behind the unit of the left, or the lines may be bent straight and routed directly through the wall behind the unit on the right. Alternatively they could, with care, be bent to the right. It is necessary after installation to connect the refrigerant lines, so the geometry must allow access (not locate the joins inside a wall, for instance).
If the indoor unit is mounted on wood-frame construction, at least one bracket should be secured to a wall stud. Ideally a backing plate should be used if there are not two studs available at suitable locations, though cavity-wall toggles to hold 5kg should be adequate.
All measurements and planning should be done before drilling a single hole - try not to have the coolant lines emerge behind a downpipe that cannot be moved, or sever a wall stud or water pipe.