Ductless heat pumps - or mini-split heat pumps - are excellent for zoning, that is, to heat and cool just a part of the house, or just a room or a couple of rooms at a time. But they can also be used advantageously to heat small homes, or very well insulated buildings, even in colder climates.
They are, from an energy and environmental standpoint, the most interesting type of heat pumps in very energy efficient homes. Till recently, they were not a good choice in climates with very large periods of freezing temperatures, but that's not the case anymore.
That may sound a bit strange and contrary to what many people think, especially in North America, where this type of heat pumps is largely undervalued, and where contractors often discourage their installation.
But there are some strong evidences in support of this view, especially the role that they may have in well insulated homes.
Ductless heat pumps and zoning
Typically, ductless heat pumps can have up to four air handlers, for four rooms or parts of the house, each one with its own thermostat.
Like central air-source heat pump units, ductless heat pumps have an indoor air-handling unit and an outdoor compressor and condenser. The difference is that they use a conduit - housing the power cable, and the suction and refrigerant tubing - to link the outdoor and indoor units (instead of ductwork).
Because they do not rely on ductwork, they haven't the energy loss of ducted systems. And since they are designed to heat or to cool specific rooms or groups of rooms (zoning), they can provide lower running costs and significant energy savings.
Ductless heat pumps vs. central heat pumps
Ductless heat pumps are relatively inexpensive ($1.500-$5.000, installed) when compared to geothermal heat pumps ( costs typically above $40.000) or common central air-source heat pumps, with installed costs close to $10.000.
They can't provide the output of geothermal heat pumps and other central heating systems, but the fact that they are relatively inexpensive and able to provide heat and cool air to several rooms without major energy loss, make them a good choice in energy efficient homes.
ductless heat pumps & Low Temperatures
Like other air-source heat pumps, traditional ductless heat pumps lose their efficiency when temperatures fall below 40-35ºF/5-2ºC.
Traditional mini-splits have to resort to electric-resistance elements at such temperatures, which increases running costs.
Anyway, there are now new mini-splits able to perform greatly in colder climates.
Mini-split heat pumps for colder climates
There are now new air-source heat pumps able to respond very effectively to freezing temperatures, that is, down to -10ºF or even -13ºF.
This tremendously important innovation is being led by Japanese companies (Mitsubishi, Daikin, Fujitsu) in the last few years, and you should consider this heat pumps if you are building a new energy-efficient home, in a cold climate. They may not be fully operational at temperatures as low as say -20ºF, but they are a really great innovation.
Since the heat pump output drops as the outdoor temperature also drops, the equipment should be sized accordingly, that is, taking into account your climate and the temperatures in your region. To avoid an over-sized unit, consider using simple electric resistance heating, a wall-vented ductless gas furnace or another source to make up the difference during abnormal freezing temperatures.
Mount the outdoor compressor unit at least 2 feet above the ground, and protect it properly, by using a cover or a roof (without restricting airflow). Keep the snow and cold winds off and away, as much as possible.
Most manufacturers provide warranties of one year for indoor units, and 3-6 years for compressors...
It's easy to install a ductless heat pump. The indoor elements come in various designs (floor-standing, ceiling-suspending, wall-hanging models) that are connected to the outdoor unit through the conduits and small holes in the walls.
Each indoor air unit (with it's own thermostat) is connected to the same outdoor unit, but since the conduits can vary in their lengths, the outdoor units can be located (and hidden) on the most convenient site.
Be careful anyway. The equipment should be carefully selected, according to the size of the rooms or parts of the house where it is going to operate. Oversized units and incorrectly located air-handlers can cause temperature swings and improper control of humidity.