Hot Market For Heat Pumps
What is a heat pump?
If you’ve ever stood outside where an air conditioner vents, you know
that air conditioners have to do something with the heat they
remove from inside. An AC system moves heat from a cool space
indoors to the hot air outdoors. A heat pump does the same thing;
the only difference is that it moves the heat from the cool air
outdoors to a warm space indoors. In 1997, 6.8% of residential
heat demand in the U.S. was supplied by heat pumps. The only country
with greater use of home heat pumps was Japan. (See sidebar on
heating in other countries.)
The best way to envision how a heat pump works is to think of
a refrigerator. One set of coils is located in the refrigerator,
the other outside the refrigerator. The “pump” in
“heat pump” refers to the pump that moves a liquid,
known as the working liquid or refrigerant. The “heat”
is embodied in the working liquid as stored energy. The coils
inside the refrigerator have enough surface area to expose the
liquid to excess heat, which causes it to expand. The pump also
causes additional expansion, forcing the liquid to absorb more
heat. The liquid is then forced through the coils on the exterior
of the refrigerator, where it is compressed so that it gives off
heat. The same liquid then repeats the cycle. Instead of moving
heat out of a room, like an air conditioner, the process moves
heat out of the insulated refrigerator. So the overall effect
is that heat is being pumped from one location to another.
In the summer, an air conditioner makes your house like a very
large refrigerator, only not as cold. In the winter, a heat pump
can do the reverse, taking heat out of the exterior environment
and transferring it inside. Even though it seems cold outside,
the heat pump can capture some of the heat energy that is stored
in the air. Air at 0°F still contains nearly 85% of the heat
that it contains at 70°F. Because a heat pump is only moving
heat instead of making new heat, it is much more efficient than
direct heating (like electric baseboard heaters), easily able
to transfer two to three times as much heat as can be directly
generated with the same amount of electricity.
Practical applications of heat pumps
A heat pump that can both heat and cool is called a split system. It transfers heat outside during summer like an air conditioner, and operates in reverse in winter, pumping heat in from the air.
In most cases, a heat pump supplements another heating system
and enables it to operate more efficiently. One can be added to
an existing home to improve heating efficiency. It may also eliminate
the need for central air conditioning. Some systems also provide
heating for hot water and clothes dryers.
Factors that can effect the life-cycle efficiency of a heat
Local method of electricity generation
Type of heat pump (ground vs. air source)
Size of the heat pump
Quality of work during installation
Energy efficiency of home's layout, insulation and ducts
Safeguards against damage to coils from construction, gardening equipment, etc.
Heat pumps have different levels of popularity in different areas
depending on outside temperature, the cost of electricity versus
fossil fuel that might otherwise be used for heating, and the
type of heat distribution system that is commonly used. However,
they make the most financial sense in climates where both heating
and cooling are needed. They also make the most sense when electricity
prices relative to fossil fuel prices are low, because a heat
pump typically runs on electricity. Though heat pumps are always
more efficient than direct heating, they are only more cost-effective
in homes that are well insulated, as the capital cost will not
be recovered if the heat leaks out too quickly.
Where temperatures drop below 40°F in winter, an air source heat pump will need to be operated with a supplementary heating system, such as a furnace. However, this may change with more energy efficient houses and newer models of heat pumps. Air source heat pumps, where the exterior unit transfer heat from the air, are the most commonly used types of heat pumps in houses, and the most feasible for an installation in an existing home, though more efficient ground-source models may become more common in the future. (See sidebar on ground source heat pumps.)
Choosing a heat pump
Heat pumps are not available on the retail market for do-it-yourselfers.
Since each home requires a custom installation, a certified technician
should help to determine the best system for the home, and should
be hired to install the heat pump. The capacity of the heat pump
you choose will depend on the specifics of the house, climate,
and existing heating system. Components such as the thermostat
and ducts will also affect the pump's efficiency. Your home thermostat
needs to control both the heat pump and supplementary heating
system, so it may need to be replaced with the installation.
Ground Source Heat Pumps
Ground source heat pumps, also known as geothermal or ground-coupled
heat pumps, are a more efficient alternative to air source heat
pumps and other home heating systems. These may become more
easily available for homes in the near future, however they
are much more suited to new construction than to installation
in an existing home. These systems transfer heat from the ground
in winter, and to the ground in summer. The higher efficiency
is achieved in winter, because ground temperatures are more
constant and tend to be warmer than air temperatures. Currently,
ground source heat pumps are more widely used in areas with
very cold winter climates. Installations require more exterior
space and a greater degree of customization than for air source
Ground source systems have a number of advantages over air
source systems. They are generally less expensive to operate.
They reduce the need for auxiliary heat because more heat is
available in the ground than in the air. They also last longer:
there is less wear on the compressor because ground temperature
has significantly less variation than air temperature. In addition,
there is no need to defrost outdoor coils as there is with air
Ground source systems have significant disadvantages, which
make them most feasible in new construction. The installation
cost is higher than for air source systems. A unique design
is needed for each site. It requires a geothermal assessment
and must take into account the moisture, soil temperatures and
heat conductivity at the site. The design must ensure that the
soil near it does not freeze when heat is extracted. Systems
may require extensive digging or drilling for placement of the
exterior coils. For a home system, these costs may offset the
savings in operating cost during the system's usable life. In
most areas, there are also fewer installers capable of doing
the work. Finally, some jurisdictions require special permits.
Ground source heat pumps may make more sense in commercial
and institutional buildings, where heating and cooling needs
are large enough to make the initial investment worthwhile,
and where engineering consultants are more likely to already
be involved in the design of building mechanical systems. However,
these systems offer a great deal of promise for energy efficient
homes and may gain market footing in the future. Buyers should
beware of situations where exterior coils are installed under
a building's concrete floor, making repair very expensive.
Water source heat pumps can be used where a well or body of
water is close to a building. However, in addition to the disadvantages
of ground source heat pumps, these systems may be subject to
additional local environmental regulations. Altering water temperature
is a way of altering water quality, affecting the aquatic habitat.
In some systems, heat could be transferred into or removed from
water that is put in a separate location.
Sizing a heat pump is important. A system that has too much capacity
will turn on and off more, functioning less efficiently and wearing
down more quickly than one the right size. A system that is too
small may require more supplemental heat, increasing your utility
bills. Some climates have a greater need for heating in winter
than for cooling in summer. In these cases the best choice may
be a smaller unit, so summer cooling operates efficiently, along
with added insulation to increase winter efficiency.
Because each installation is different, the industry does not
generally make component prices available. Installation of a system
in an existing home, including parts, will cost anywhere from
$3500 to $8000, with the high end of the range representing the
most efficient systems. This is substantially more than the cost
of replacing an existing furnace (generally under $3000 for systems
that have a longer life-span than a heat pump). Of course, the
operating cost of the heat pump will be lower due to the higher
efficiency. Cost savings cannot be estimated accurately without
knowing the specifics of the climate and house. Operating savings
for heating and cooling must be calculated separately.
Homebuyers should beware that new homes equipped with heat pumps
often have relatively low efficiency systems. Additionally, in
very cold climates, the exterior unit should have a mechanism
to defrost the coils periodically.
Heat pumps include a refrigerant, such as Freon (R-22) or the
more environmentally sound Puron (R-410A). Recharging and replacement
of the refrigerant may be needed during the life of the heat pump,
depending on wear and proper installation. Each heat pump is made
for use with a specific refrigerant, so you should not buy an
older model expecting to be able to run it with a less environmentally
damaging refrigerant when the old refrigerant needs replacing.
Table 1 shows a small sample of units that are available. Each
model is available in a number of capacities. To make comparisons
fair, only three ton capacity models are shown; this is a typical
capacity for an 1800 square foot, energy-efficient, 2-story house
in the Seattle area. Capacity is generally measured in tons, a
holdover from when refrigeration was accomplished with home ice
boxes. One ton of capacity is equal to the refrigerating power
of one ton of ice melting over the course of 24 hours. This is
equal to about 13 MJ/hr, so a three ton capacity unit can move
at the maximum 3 x 13 = 29 MJ of heat in each hour.
Make and Model
Bryant 698B NX 036000
Bryant 661C NX036000
Trane XR 12
Table 1 - Sample of Available Split System
The heat pumps in the table are all split system, air source
heat pumps, meaning that they pump heat both to and from outdoor
air, offering cooling in the summer as well as heating in the
Home Heating in other Countries
Central heating with a duct system for air distribution is
the most common type of home heating in the U.S. Forced air
heating systems like these work well with heat pumps, because
the air only needs to be heated to a relatively low temperature.
Heat pumps are most common in the U.S. and Japan because forced
air systems dominate in both countries. U.S. systems generally
heat an entire house, while Japanese systems generally heat
In Europe, in contrast, radiant heat is more common. Hot water
is passed through a radiator, which can be a baseboard unit
or the type of unit common in older apartment buildings in the
U.S. This requires high water temperatures, about 160 °F, for
which a heat pump is usually not efficient.
Heated water can also be passed through tubing in floors or
walls, providing more surface area and therefore enabling heating
with water at a lower temperature. Low temperature systems are
becoming more common, and heat pumps will make more sense with
these systems. Additionally, with smaller rooms found in new
homes in many urban areas, the trend is toward radiant heat
systems that reduce energy requirements. The future may be in
combining a ground source heat pump with low-temperature, high
surface area, radiant heat.
For each model we report the summer and winter average coefficients
of performance "COP." These are based on ratings provided by the
manufacturer and should be regarded as the best expected performance.
The COP is the ratio of energy embodied in the heat moved, to
the energy in the electricity used to run the heat pump. So any
COP greater than 1.0 means that the heat pump moves more heat
than the same amount of electricity could produce in direct generation.
So you can see from the table that most of these heat pumps move
between two and three times the heat that could be generated directly
with the electricity!
Summer COPs are much higher than winter COPs. In other words,
typical split-system installations move more heat in their cooling
mode than in their heating mode, for the same amount of electricity.
This is because in the summer the heat pump might be moving heat
from a typical indoor environment of 80°F to an outdoor environment
of say 95°F, or a difference of 15°F. In the winter, the unit
might be moving heat from an outdoor environment of say 35°F to
an indoor environment of 70°F, or a difference of 35°F. In most
United States climates the outdoor-to-indoor temperature differential
is greater in the winter than in the summer, and it is more difficult
for the heat pump to move the heat across it (imagining pushing
a ball up a bigger hill). Hence the heat pump is less efficient
in the winter.
This table only represents a small sample of products available.
Air source heat pump manufacturers include Bryant, Trane, Carrier,
Rheem, Lennox and York, among others. Each manufacturer makes
high, medium, and low efficiency models at a variety of capacities.
Efficiency varies with external temperature. Maximum, rather than
average, operating efficiencies are shown.
Homeowners considering an installation should be aware that some
models make substantial noise, but most manufacturers have engineered
less noisy models. The particulars of the installation can also
make a difference.
 International Energy Agency, Heat Pump Centre Newsletter, Volume
15, No. 3, 1997
 Installers and technicians who work with or purchase refrigerants
must be certified by an organization that is approved by the U.S.
Environmental Protection Agency. Therefore, heat pump installers
should be certified by one or several of the following: heat pump
manufacturers, the Refrigeration Service Engineers Society (RSES);
the Air Conditioning Contractors of America (ACCA); the Mechanical
Service Contractors of America; local chapters of the National
Association of Plumbing-Heating-Cooling Contractors; and the United
Association of Plumbers and Pipefitters. There is also a voluntary
national certification program for heat pump installers run by
the North American Technical Excellence (NATE) Inc. In addition,
the International Ground Source Heat Pump Association (IGSHPA)
provides voluntary accreditation to ground source heat pump installers.
 1800 square feet would today be considered a compact house for
a family and would likely have three or four bedrooms. For perspective,
older houses in urban neighborhoods are often 1,000 square feet
or less, and newer suburban homes may be 4,000 square feet or
 ILEA reports most energy values in megajoules (MJ). A megajoule
is enough energy to bring about 3 quarts of room-temperature water
to boiling, or to run a 1500 watt hair driver for 11 minutes.
 The COPs shown in the table are derived from the industry-standard
measures heating seasonal performance factor (HSPF) and seasonal
energy efficiency ratio (SEER).
HSPF is the total heat output indoors during the heating season,
in British thermal units (Btu), divided by the total energy consumed
during that time, in watt-hours (Wh). It includes energy for supplemental
heating. Average weather data are used in representing the heating
season. SEER is a similar measurement of cooling efficiency over
the entire cooling season: it is the total heat removed from the
indoors during the cooling season in Btu, divided by the total
energy consumed during that time, in Wh.