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Heat Pumps
By definition, a heat pump is a machine which moves heat. Heat exists in all
air at all temperatures down to "absolute zero" (-460º F).
In the winter, a heat pump draws heat from the outdoor air and circulates
it through ducts into your home. During the summer, it reverses the process
and draws heat from your interior air and releases it outdoors. It also dehumidifies
the indoor air as it cools it.
Benefits of a Heat Pump System
Because a heat pump does not burn fuel, it is safer and cleaner to run than
a gas powered furnace.
A heat pump provides a more uniform temperature throughout a building. It
does not produce a sudden blast of hot air as traditional furnaces do each
time they kick on.
In the heat mode, heat pumps do not dry out the air the way traditional heaters
do. The higher humidity maintained by heat pumps during cold weather provides
for a healthier environment.
Heat pumps are more efficient and cost less to run than electric furnaces.
Because heat pumps are used year round (for cooling as well as heating needs),
they cost less per hour of use (cost of purchase and installation divided
by total number of hours used per year) than do individual heating and cooling
systems, which each sit idle for a good part of the year.
The Heat Pump as an Air Conditioner
The heat pump serves as an air conditioner by absorbing heat from indoor air
and pumping it outdoors. The heat pump contains an indoor coil which, in turn,
contains a very cold liquid refrigerant. As indoor air passes over the indoor
coil, the refrigerant-cooled coil absorbs heat from the air and so quickly
cools that air. The cooled air cannot hold as much moisture as it did at a
higher temperature. The excess moisture condenses on the outside of the coil,
resulting in the dehumidification of the air. The cooled, dehumidified air
is then forced (by a fan) into the duct system which, in turn, circulates
it throughout the building.
At the same time, the absorption of heat by the refrigerant turns the refrigerant
from a liquid into a vapor. A compressor pumps the heat laden vapor through
a vapor line to an outdoor coil which discharges the heat extracted from the
indoor air. As the heat is discharged, the vapor is cooled and changes back
into a liquid refrigerant. The refrigerant is then pumped back through a liquid
line to the indoor coil and the cycle is repeated.
In addition to serving as an air conditioner, the heat pump contains a reversal valve which reverses the flow of refrigerant and thus allows the heat pump to serve as a heater during cold weather.
The Heat Pump as a Heater
The heat pump serves as a heater by absorbing heat from outdoor air and pumping
it indoors. All air, even cold winter air, contains a certain amount of heat.
As the outdoor air passes over the outdoor coil, heat from that air is absorbed
by the refrigerant contained inside the coil. This absorption of heat changes
the refrigerant from a low-temperature liquid to a low-temperature, low-pressure
vapor. The vapor then passes through a compressor where it is compressed into
a high pressure, high-temperature vapor. The hot vapor then circulates into
the indoor coil. As indoor air passes over the indoor coil, it absorbs heat
from the coil. The warmed air is then redistributed through the duct system.
The Need for a Supplemental Heater
As explained above, as outdoor air passes over the outdoor coil, its heat
is absorbed by the refrigerant contained inside that coil. The temperature
of the outdoor air passing over the outdoor coil is reduced by about 10º
F (or 5.56º). This means that even if the outdoor temperature is above
freezing (say, 35º to 40º F), the air closest to the outdoor coil
will be reduced to below freezing (32º F or 0º). This reduction
in temperature will cause the moisture contained in that air to freeze and
to form frost on the surface of the outdoor coil. When the coil is iced over,
it must be defrosted. Methods for defrosting the coil vary among manufacturers.
The most popular method uses a reversal of the refrigerant flow. As in the
system described above (The Heat Pump as an Air Conditioner), warm air is
absorbed by the refrigerant as it passes through the indoor coil. This raises
the temperature of the refrigerant and turns it from a liquid into a vapor.
As the hot vapor passes, in turn, through the outdoor coil, it defrosts it.
Because the heat pump is operating in a cooling mode (drawing heat from indoors
and pumping it outdoors) in order to defrost the outdoor coil, it must be
supplemented by a secondary heater. Once all the frost on the outside coil
has melted, the defrost controls cause the reversal valve to switch over and
the unit returns to its heating mode.
Efficiency Ratings
Heat pumps are assigned two efficiency ratings, a SEER rating based on a unit's
cooling efficiency and a HSPF rating based on a unit's heating efficiency.
If you care about saving money on your electric bills and protecting the environment,
ask your air conditioner contractor about heat pumps which have been awarded
the EPA's coveted Energy Star label .
Cooling Efficiency
The SEER (Seasonal Energy Efficiency Ratio) rating is used to identify the
cooling efficiency of both traditional air conditioners and heat pumps. The
SEER rating indicates how efficiently the unit utilizes electricity: the higher
the rating, the less electricity the unit requires to cool a given area. Heat
pumps manufactured today have a SEER rating from 10.0 to about 17.0.
Heating Efficiency
The HSPF (Heating Seasonal Performance Factor) rating is used to identify
the heating efficiency of heat pumps: the higher the rating, the less electricity
the heat pump uses to heat a given area. Today's heat pumps are generally
rated between 6.8 and 10.0 HSPF.
Initial Cost versus Long Term Expense
Generally speaking, heat pumps with the highest SEER and HSPF ratings are
more expensive to purchase than their lower rated counterparts. However, because
they utilize less electricity, they can actually save you money in the long
run. If you are planning to sell your residence in the near future, you may
not wish to invest in a unit with a high rating. However, if you plan to be
in your home for a while, it may be more cost effective to purchase a more
high efficiency unit.
Comfort Features
Some heat pumps come with additional features that provide greater comfort.
Two-speed units can run on low-speed (using about 50% of the energy) 80% of
the time. Consequently, they use fewer on/off cycles and produce fewer drafts.
Likewise, they produce much smaller temperature swings: only two or three
degrees rather than the four degree temperature swing commonly experienced
with single-speed units. Finally, the improved air circulation provided by
a two-speed unit helps to prevent air "stratification": warm air
rising to the ceiling and cold air settling near the floor.
Cooling Mode
A heat pump is essentially an air conditioner with a few additions. A heat
pump has a reversing valve, two metering devices and two bypass valves. This
allows the unit to provide both A/C and Heat. The diagram above shows a HP
(heat pump) in cool mode. The cycle goes like this;
The compressor (1) pumps the refrigerant to the reversing valve (2).
The reversing valve directs the flow to the outside coil (condenser) where
the fan (3) cools and condenses the refrigerant to liquid.
The air flowing across the coil removes heat (4) from the refrigerant
The liquid refrigerant bypasses the first metering device and flows to the
second metering device (6) at the inside coil (evaporator) where it is metered.
Here it picks up heat energy from the air blowing (3) across the inside coil
(evaporator) and the air comes out cooler (7). This is the air that blows
into the home.
The refrigerant vapor (8) then travels back to the reversing valve (9) to
be directed to the compressor to start the cycle all over again (1).
Heating Mode
The diagram above shows the heat pump in heat mode. The difference in the two diagrams is the reversing valve (2) directs the compressed refrigerant to the inside coil first. This makes the inside coil the condenser and releases the heat energy (3-4). This heated air is ducted to the home. The outside coil is used to collect the heat energy (3-7). This now becomes the evaporator.
Both heating and A/C modes do exactly the same thing. They PUMP HEAT from one location to another. In these examples the heat in the air is moved out of or into the home.
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1: Outdoor coil extracts heat from
outdoor air.
2: Refrigerant gas carries heat to indoor unit.
3: Circulating indoor air picks up heat and carries it throughout the home.
In the winter, the refrigerant absorbs heat from outdoor air drawn across the outdoor coil. The refrigerant becomes hot but is made even hotter (in excess of 140 degrees F) by going through the compressor.
This hot gas travels through a copper tube to the indoor coil. The fan draws air through your return grille and pushes the air across the indoor coil. The hot gas transfers its heat to the air blown across the coil and into the duct system.
1: Indoor coil extracts heat from home's air.
2: Refrigerant gas carries heat to outdoor unit.
3: Outdoor coil transfers heat to outdoor air.
In the summer, your heat pump simply reverses the flow of refrigerant. Now the refrigerant absorbs heat from room air blown across the indoor coil. In this manner, heat and humidity are removed from the air, and cool, dry air is distributed throughout your home.
The absorbed heat is carried by the refrigerant through the copper tube to the outdoor unit. Here the refrigerant goes through the compressor, then moves through the outdoor coil, which transfers the absorbed heat to the outdoor air.
Learn How Your Heat Pump Works
Any appliance that takes heat from one area and moves it to another is a heat
pump.
Most heat pump installations involve what is called a split system. The outdoor
unit contains the compressor and a heat exchanger, called a coil. The indoor
unit contains another coil, a fan that blows air through your duct system,
grille, and electric heating elements.
The outdoor and indoor units are connected by copper tubes that move a gas refrigerant (such as Freon) between the indoor and outdoor coils. This refrigerant has the ability to absorb heat from the air, even at very low temperatures.