Central systems use the ducts of the house to provide heating and A.C. One thing to be aware of is that they will have an achieved energy efficiency ratio 10-20% lower than the nameplate rating due to the duct-related losses.
Heating efficiency for air-source electric heat pumps is indicated by the heating season performance factor (HSPF), which is the total space heating required during the heating season, expressed in Btu, divided by the total electrical energy consumed by the heat pump system during the same season, expressed in watt-hours.
Cooling efficiency is indicated by the seasonal energy efficiency ratio (SEER), which is the total heat removed from the conditioned space during the annual cooling season, expressed in Btu, divided by the total electrical energy consumed by the heat pump during the same season, expressed in watt-hours.
The HSPF rates both the efficiency of the compressor and the electric-resistance elements. The most efficient heat pumps have an HSPF of between 8 and 11.
The HSPF and SEER is related to the coefficient of performance for a heat pump for heating and cooling, which measures the ratio of heat energy delivered or removed to electrical energy supplied. The HSPF and SEER can be converted to a seasonally-averaged COP by converting both the BTU heat output or removal and the electrical input to a common energy unit. Since 1 BTU = 1055 J, and 1 watt-hour = 3600 J, the seasonally-averaged COP is given by: Avg COP = Heat transferred / electrical energy supplied = (1055 J/BTU) / (3600 J/watt-hour) = 0.2928104 HSPF or SEER.
So a heat pump with an HSPF of 9 would transfer 2.635 times as much energy than it used. If it had a SEER of 16 then it can transfer 4.7 times as much energy than it used.
A very important point is that HSPF ratings are made with an electric coil back up. When a gas furnace is used to heat below 0 Celsius the c.o.p values will be higher, and the savings will be much greater. At temperatures over 0 Celsius a good heat pump will have c.o.p’s between 3 and 6 as it gets warmer- the heat pump manufacturer will have this exact info for their different models.
Used with an electric coil backup the most efficient air source heat pumps will be similar in cost to run compared to a high efficient furnace while the cost of gas is at present one third of the cost of electricity. That same heat pump used with a gas furnace for low temperatures and the cost should be about half that of a high efficient gas furnace.
The main advantages of mini splits are their small size and flexibility for zoning or heating and cooling individual rooms. Many models can have as many as four indoor air-handling units (for four zones or rooms) connected to one outdoor unit. Each of the zones has its own thermostat, so you only need to condition occupied spaces. This will save energy and money.
Ductless mini-split systems are easier to install than some other types of space conditioning systems. For example, the hook-up between the outdoor and indoor units generally requires only a three-inch hole through a wall for the conduit. Most manufacturers of this type of system can provide a variety of lengths of connecting conduits, and, if necessary, you can locate the outdoor unit as far away as 50 feet from the indoor evaporator. This makes it possible to cool rooms on the front side of a house, but locate the compressor in a more advantageous or inconspicuous place on the outside of the building. Mini splits also have no ducts, so they avoid the energy losses associated with the ductwork of central forced air systems.
In comparison to other add-on systems, mini splits offer more interior design flexibility. The indoor air handlers can be suspended from a ceiling, mounted flush into a drop ceiling, or hung on a wall. Floor-standing models are also available. Most indoor units are about seven inches deep and have sleek, high tech-looking jackets. Many also offer a remote control to make it easier to turn the system on and off when it’s positioned high on a wall or suspended from a ceiling.