Heat Pumps for space heating and air conditioning
Heat pumps extract heat from the outside (usually from the air, although some extract it from the ground) and pump it into a building to provide heat. They reverse that cycle to provide air conditioning. See the Building Electrification FAQ for a more detailed explanation of the science of how a heat pump works)
Heat pumps are typically 3-4 times more efficient than the highest efficiency gas furnace or an electric baseboard system. This means that they are far better for the climate and air quality than either electric baseboard or gas systems.
In new construction, a heat pump system is cheaper to install then separate gas heating and electric air conditioning systems.
In existing buildings when adding to or replacing heat in an existing buildings, a heat pump will usually cost more than an electric baseboard or gas system. However, an increasing number of utilities and local governments are offering rebates to reduce the installation cost of a heat pump below that of a gas system.
When adding air conditioning to a previously heat only system, it will generally cost less to install a single heat pump for both heating and cooling, than to install an air conditioner and separate electric or gas heating system.
Heat pumps cost far less to operate than electric baseboard systems. Currently, heat pump systems do generally cost slightly more to operate than gas systems in most parts of the country. Will that remain? Gas prices, are volatile and hard to predict, but likely to rise faster than electricity as the fracking boom plays out and the cost of solar & wind electricity continue to fall. Of course a solar electric system on your rooftop can reduce or even eliminate the operating cost for your heat pump.
Most, but not all, heat pumps are “split systems.” An outside compressor unit extracts the heat and one or more inside condenser units distribute the heat or cooling. The inside units are connected to the outdoor unit by a pipe carrying refrigerant. There are a variety of types of heat pump systems that are appropriate for different situations
Replacing a central air furnace
Ducted Air Source Heat Pump (also called a “central” or “unitary” air source heat pump).
A single outdoor compressor is connected to a single indoor condenser unit that is connected to the existing house ductwork. If the ductwork goes through an unconditioned crawlspace or attic, it is important to first seal the ductwork well to avoid losses.
Replacing a radiant floor boiler
Air-to-Water Heat Pump (also called a “hydronic” air source heat pump).
A single outdoor compressor unit is connected to a single indoor condenser unit that heats the water for the radiant floor water distribution.
Outside heat pump compressor unit
Adding a small addition or new standalone space of one to three adjoining rooms (e.g. Accessory Dwelling Unit or ADU)
Mini-Split (or Single Zone) Heat Pump
One one outdoor compressor connected with a refrigerant line to a single indoor condenser unit with a fan (called a “fan coil”) to distribute the heat into the space.
A Packaged Terminal (through the wall) Heat Pump
Both the outdoor compressor and indoor condenser are packaged together into one unit, like a small air conditioner, mounted in a cutout in an outside wall. A packaged system may be less expensive and simple to install but requires careful air sealing not to lose heat around the opening.
Sometimes referred to as a "PTAC" for "packaged terminal air conditioner" although it both heats and cools.
The indoor unit on a mini-split system requires running a refrigerant line through walls and/or above ceilings, but provides more flexibility in the location of the indoor unit.
Ceiling and wall mounted fan coil units
Through the wall packaged unit
All new construction, OR replacing a wall or floor furnace or electric baseboards. No existing central duct system OR where the existing distribution system also needs replacement
Ductless Multi-Zone Air Source Heat Pump (also called a “multi-split” heat pump).
One or more outdoor compressors are connected by refrigerant pipes to multiple indoor units. These indoor units (called “fan coil units” or “heads”) blow air out to distribute the heat into the space. Each fan coil unit can be individually controlled depending on the heat (or cool) needed in each room or zone. Fan coil units may be mounted on a wall or ceiling or may be hidden behind a flush mounted register in a wall or ceiling space.
Fan coil unit that mounts flush to the ceiling
Very cold climates
Ground source heat pumps and variable speed systems (also called “variable refrigerant flow”)
(Info on these options for maximum efficiency in colder climates is coming soon)
Combined heat & hot water system with storage (plus cooling options)
At least one manufacturer (Harvest Thermal) is producing a combined system that provides both space heat and hot water. The system cuts utility costs and carbon footprint by using the hot water storage tank for space heating also. It does this by using the cheaper (and lower carbon) electricity available during the day from your utility to store up extra heat in the tank to use for night time space heating needs when electric rates are more expensive. Got solar? It will use the free electricity generated by your PV system in the day to provide you with heat at night.
It also uses CO2 as its refrigerant, which has a much lower climate impact in case of leakage than the gases normally used in heat pumps.
The current system offering is designed for central air duct systems.
It also offers night time cooling through the duct system, like a whole house fan. If daytime AC is desired, it currently uses a separate integrated heat pump system for the cooling.
How We Fully Electrified Our Very Old House, Dave Margulius DM Workshop, Dec 30, 2021. Description of a conversion of a Berkeley CA residence from gas to heat pump for both space heat and water heat.