we are getting into the heating season this month we would
like to discuss how heat pumps work.
heat pump is essentially a reverse cycle air conditioner.
It can move heat from indoors to outdoors, or it can do
the opposite, and extract heat from the outdoor environment
to heat the interior. With a traditional split system furnace
and AC, the indoor unit is referred to as the evaporator
coil and the outdoor unit as the condensing coil.
With heat pumps, the function of each coil depends on the
operation mode of the system, heating or cooling. When the
system is in cooling mode, the functions are the same as
with a straight cooling split system. In heating mode, the
indoor coil becomes the condenser and the outdoor coil is
the evaporator. Therefore, with heat pumps the coils are
simply referred to as indoor and outdoor.
most common type of heat pump is one that transfers heat
from air to air. Other types of heat pumps are water source
and ground source. The outdoor coil of a water source heat
pump could be in the bottom of a lake or pool. This article
will focus on the most common type of heat pump, air to
air. Heat pumps include additional equipment that is not
found in cooling only split system units. The outdoor unit
contains an accumulator and a reversing valve
which are not found in split system AC units. The indoor
air-handler of a heat pump is often accompanied by electric
strip heaters. Heat pumps also contain a filter-drier to
prevent contaminates from flushing through the system.
reversing valve is the key component that determines the
mode of operation of a heat pump. It contains a piston or
slide that is activated by a solenoid, and four pipes. The
direction of refrigerant from the compressor does not change.
The high pressure line from the compressor enters the reversing
valve, which determines whether the indoor or the outdoor
coil will act as the condenser. In one position, the valve
will direct the high pressure line to the outdoor unit,
and in another position, it will direct the high pressure
line to the indoor unit. The pipe in the center is the suction
line that returns to the compressor, and depending on the
position of the valve it is connected to the pipe from either
the indoor or outdoor coil.
pumps have a suction line accumulator to prevent liquid
refrigerant from entering the compressor. In heating mode,
the outdoor coil (functioning as an evaporator) might not
be able to fully vaporize the refrigerant. The accumulator
collects the liquid refrigerant and protects the compressor.
It is a large cylinder that can be found near the compressor.
Because these components must be matched with the particular
compressor type and size of the system, a traditional split-system
condenser cannot successfully be field-modified to convert
it to a heat pump.
heat pump inspection involves the same procedures as AC
inspections, except that the heat pump need only be tested
in one mode - if it works in heating mode it will work in
cooling mode. If the inspection takes place on a cold day,
it should be tested in heating mode, and on a warm day,
in cooling mode. The only item that would be tested by operating
it in both modes during an inspection would be the reversing
valve itself. When a heat pump is tested in heating mode,
the sensible temperature of the air from the registers might
feel cool, because it will be less than body temperature.
The air temperature might be in the 80's or low 90's. Though
it feels cool to the touch, air at such temperatures will
warm the house.
the outdoor temperature drops lower, two other factors come
into play with heat pumps. The balance point of a
heat pump is the point where the outdoor temperature has
dropped so low that the heat pump cannot supply the conditioned
space with sufficient heat. At that time, supplemental strip
heaters are activated to supply electric-resistance heat
indoors. Some heat pumps are designed with a gas or oil
furnace as the backup heating mode, though strip heaters
are much more common. Strip heaters are sometimes arranged
so as to come on in stages, depending on the conditions
at the time of use.
extreme cold weather or a large difference between the thermostat
setting and the room temperature, all the strip heaters
will activate. Heat pumps also might experience icing of
the outdoor coil during the heating mode. When this occurs,
the heat pump goes into defrost mode by reversing
direction. During such time, the system is trying to provide
cool air to the interior, so the supplemental electric resistance
heaters will activate to maintain a warm air flow.
pump thermostats will typically have an "emergency
heat" setting. In such a mode, the electric strip heaters
will be turned on and override the other settings of the
system. During emergency heat mode, the air from the registers
will feel warmer than in ordinary heating mode. When testing
a heat pump in heating mode, inspectors should be careful
not to turn the thermostat too high, or they may activate
the emergency heat and not be able to accurately test the
heat pump function.
pumps are far more economical than pure electric resistance
heat. Electric heat generates 3.413 Btu's per watt-hour. The
efficiency of heat pumps is measured in terms of the coefficient
of performance (COP) and will be at least twice that
of electric resistance heat.
all heat pumps include a crankcase heater to prevent excess
migration of oil and/or liquid refrigerant to the compressor.
If the unit has not been turned on for the required amount
of time - typically 12 to 24 hours - the compressor could
be damaged. Inspectors encountering a heat pump where the
breaker or switch is turned off should not operate the system.
heat pumps will have an air handler and a duct system. However,
heat pumps can also be self contained units mounted in a
wall (such as those found in hotels) or they could contain
a wall-mounted indoor coil and blower with a separate outdoor
coil and refrigerant tubing between the two (commonly marketed
as "ductless" units).
Casey & Douglas Hansen
Kaplan Professional Schools
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