Demand defrost control method and apparatus

A demand defrost controller for a heat pump. The controller compares the temperature of an outdoor heat exchange coil with an enable temperature. A timer is activated when the coil temperature is less than the enable temperature. When the timer senses the compressor has run for a predetermined time the controller checks outdoor and coil temperatures to determine if a defrost cycle of the heat pump should be conducted. A defrost cycle is achieved by reversing refrigerant flow in the heat pump system for a predetermined period or until the outdoor heat exchange coil has been heated to a termination temperature.

TECHNICAL FIELD 
The present invention relates to a control system for periodically 
defrosting a heat pump. When a heat pump heats a building interior, 
refrigerant passing through an outside heat exchanger gathers heat from 
outside the building, and delivers that heat to a heat exchanger inside 
the building. The outdoor heat exchanger typically includes a tubular coil 
of highly conductive metal. In the heating mode, an expansion valve 
delivers refrigerant to the outside heat exchanger coil where the 
refrigerant is heated and expands as it is vaporized. As the outside 
temperature approaches freezing, frost or ice can form on the outside heat 
exchange coil. This reduces the heat pump's efficiency and requires 
periodic defrosting of the outside coil. One defrost method is to reverse 
refrigerant flow and pump hot refrigerant from a heat pump compressor 
through the outside coil to thaw the ice on the coil's outside surface. 
BACKGROUND ART 
Different prior art procedures for detecting and controlling the formation 
of frost or ice on a heat pump outdoor heat exchange coil have been 
performed with varying degrees of success. These procedures include 
cyclical deicing, sensing air pressure drop across the outdoor coil, 
sensing temperature differences between the air and the outdoor coil, 
photo-optical responses from the frost (reflectivity), capacitance change 
due to the frost build up as well as tactile change due to ice formation 
on the coil. While some of these methods directly sense the formation of 
frost or ice, others use secondary effects, such as air pressure drop or 
thermodynamic and heat transfer changes in the system for initiation 
and/or termination of a deicing cycle. 
One prior art proposal for defrosting makes use of a power factor change of 
an outdoor fan motor as ice builds up on the outdoor coil. The ice impedes 
air flow and changes the loading on the fan motor. This system is 
dependent on motor selection for the fan. 
Photo-optical systems have been used which are positioned to view heat 
exchange fins or tubes on outdoor heat exchange coils and detect the 
presence of ice by observing changes in reflectivity of a light source. 
The ability to detect hoar frost and/or glare ice and differentiate the 
thickness of the ice build-up have been problems for these systems. 
Measuring the capacitance of the frost has been tried with minimal success, 
due to the variability of ice, sensitivity of the signal, and critical 
placement of metal plates between which the frost build up occurs. 
Fluidic sensors use "Coanda principles", in which air is passed thru one 
leg of a flow path and diverted to a second leg when a blockage signal is 
received. These sensors experience problems associated with dust and dirt 
clogging the filters protecting the small passages used in the fluidic 
sensor. 
Still other methods employ tactile means of detecting the presence of ice, 
or employ the freezing effects of ice to increase friction and loading on 
a movable lever mechanism. These systems can only be employed on certain 
coil designs and adjustability has been a problem. 
Other systems use electromechanically-operated timing devices to start a 
defrost cycle. They either reverse the refrigerant flow through the 
outdoor coil, turn on heaters, or blow hot gas over the coil. 
These timing systems are simple and reliable. They do not, however, defrost 
"on demand" and therefore utilize energy for defrosting when there may not 
be a need to deice. Since it has been shown that a light hoar frost may 
even improve the effectiveness of some heat transfer surfaces the timed 
defrost systems appear to be undesirable. 
Use of temperature responsive devices in combination with a clock-operated 
timer makes the defrosting "permissive". One example of this type of 
process is to initiate a defrost cycle only when outdoor temperatures fall 
below 32.degree. F. 
Electromechanical timing devices can generally also be programmed for both 
frequency and duration of the deice cycle. A degree of selectability is 
desirable to accommodate both variations in climate and idiosyncracies of 
individual heat pumps. 
Integration of temperature responsive elements with a clock driven 
mechanism offers both cost effectiveness and ease of installation and 
servicing of the devices. These systems, when properly programmed, will 
perform reasonably well under most climatic conditions and offer energy 
savings over the inflexible cyclical defrost procedures. 
Defrost systems capable of sensing two temperatures (the outdoor ambient 
and the outdoor coil temperature) can provide a signal when the insulating 
effect of frost on the coil causes the air and outdoor coil surface 
temperature difference to increase to a predetermined value. Such systems 
provide reasonable performance when properly installed and adjusted. They 
provide a form of "demand" defrost which is more energy conserving than 
cyclic heat pump defrost controls. 
The effectiveness of defrost systems using the temperature difference 
between outdoor air and the outdoor heat exchange coil is decreased at low 
temperatures. At low temperatures the heat transfer capacity of the heat 
pump is decreased and a fully frosted heat exchange coil doesn't deviate 
as greatly from outdoor air temperature. To activate defrosting at low 
temperatures, the threshold temperature difference between coil and air 
temperature must be smaller. Furthermore, the temperature difference 
between an unfrosted coil and a fully frosted coil is reduced markedly 
from differentials encountered at higher outdoor air temperatures. This 
can lead to false defrosting if the coil temperature fluctuates for 
reasons other than a frosted coil. 
Many heat pump expansion valves meter refrigerant to the outdoor coil 
depending on the heating demands sensed inside the building. These valves 
commonly include an expansion valve member driven between fully opened and 
closed positions by an electric motor and drive train which, in turn, are 
operated in response to sensed conditions. When the expansion valve first 
opens the valve member can oscillate as the valve drive and condition 
sensing devices seek a stable, appropriate setting. This "hunting" 
behaviour of the valve member causes the outdoor heat exchange coil 
temperature to oscillate. If the oscillatory variations in coil 
temperature are large enough, the difference between sensed outdoor air 
temperature and sensed coil temperature become sufficiently great to 
indicate a defrost is necessary. This is caused by a temporarily unstable 
expansion valve and not by a frosted outdoor coil. 
Expansion valve instability can cause the coil temperature to oscillate by 
more than 5 degrees Fahrenheit. One solution to this temporary instability 
problem has been to increase the temperature differential threshold level 
required to begin defrosting the coil so that these fluctuations will not 
initiate a defrost. This solution has made the systems particularly 
insensitive to needs to defrost at low outdoor temperatures and, in 
addition, when the system refrigerant charge becomes low the system will 
not be defrosted. 
DISCLOSURE OF THE INVENTION 
The present invention provides a new and improved method and apparatus for 
defrosting a heat pump wherein the defrosting cycle is initiated by 
sensing the difference between outdoor air and heat exchanger temperature, 
comparing that sensed temperature difference with a value determined as a 
function of sensed outdoor air temperature, and initiating a defrost if 
the sensed temperature difference bears a predetermined relationship to 
the value. 
In accordance with a preferred embodiment of the invention, a programmable 
controller monitors the temperature of the outdoor heat exchanger as well 
as outdoor air temperature. This information and use of a number of 
parameters, which are either factory programmed or set by the user, to 
establish a calculated comparison value determines whether a defrost cycle 
should be initiated. 
An important feature of the invention resides in the provision of an 
outdoor air temperature responsive timed defrost control which operates in 
concert with the differential temperature responsive defrost control. The 
timed defrost control provides a variable "lockout" time during which 
defrosting can not be initiated. The lockout time is an accumulation of 
the time the system compressor has run. The lockout time required for 
defrosting increases in duration as outdoor air temperature is reduced. 
The differential temperature responsive defrost control is prevented from 
initiating a defrost cycle until a requisite lockout time has been 
accumulated. 
The prior art problem of sensing temperature differences between the air 
and the outdoor heat exchanger at low outdoor air temperatures is thus 
addressed by the timed defrost control. The heat exchanger is defrosted 
after the heat pump compressor has run for a predetermined lockout time 
which, like the sensed temperature difference criteria is varied as a 
function of outdoor temperature. Since the lockout time is increased at 
low outdoor temperatures defrosting caused by false heat exchanger 
temperature sensing is prevented, at least for the predetermined lockout 
time. At higher outdoor air temperatures the lockout time is less and the 
differential temperature control predominates in determining whether the 
outdoor heat exchanger needs to be defrosted. 
Another important feature of the invention resides in operation of a heat 
pump system so that initiation of a defrost cycle is precluded so long as 
the outdoor heat exchanger temperature is too great to justify defrosting. 
In the preferred embodiment if the outdoor heat exchanger is warmer than a 
predetermined "enable" temperature, there is no danger that the heat 
exchanger will frost over and therefore no defrost cycle is necessary. 
When the outdoor heat exchanger temperature drops below the enable 
temperature, however, the possibility that the heat exchanger must be 
defrosted is examined. This enable temperature is one of the parameters 
that can be factory adjusted to control the defrost cycle. Other 
parameters define the lockout time and temperature difference defrost 
criteria. 
Practice of the invention results in three outdoor air temperature defrost 
control zones. At relatively high outdoor air temperatures the lockout 
times are short and the temperature difference between the outdoor air and 
the outdoor heat exchanger becomes the dominant defrost control. At low 
outdoor air temperatures the temperature differential needed to enable 
defrost is low so that the dominant defrost determining factor is the 
compressor lockout time. Thus, at low temperatures the defrost control is 
principally a timed function. 
At intermediate outdoor air temperatures either the timed defrost control 
or the differential temperature defrost control can predominate in 
controlling the defrost cycle. After the compressor has run for a period 
approaching the lockout time, an outdoor heat exchanger may or may not 
have frosted over to cause the sensed temperature difference to enable 
initiation of a defrost cycle. 
Use of a programmable controller to monitor the status of the outdoor heat 
exchanger and control a defrost cycling of the heat pump adds flexibility 
to the heat pump system. Different sets of parameters can be programmed 
into the controller to accommodate different heat pumps and their 
operation. By way of example, the enable temperature can be changed for 
different heat pump systems since a heat exchanger temperature monitoring 
sensor is located at different locations for different heat pumps. Use of 
a programmable controller also allows the lockout time and temperature 
difference balance to be adjusted differently for different heat pumps as 
well as different user needs. The temperature difference balance is 
adjusted by the selection of several constants that define a defrost 
control threshold which is examined once the lockout time has expired. 
The preferred programmable controller is a microprocessor executing an 
operating system and control program that responds automatically to sensed 
conditions. One interrupt on the microprocessor is coupled to a test input 
to allow the user to conduct a test of the defrost cycle. Whenever this 
interrupt is activated, the microprocessor enters a defrost cycle to allow 
the defrost cycle to be monitored and evaluated. 
Internal timers are driven by a second microprocessor interrupt coupled to 
an a.c. signal. These timers perform the lockout delay and other timing 
dependent functions. 
An automatic defrost cycle option is provided to initiate a defrost after a 
certain amount of compressor run time even through the temperature 
criteria for defrosting have not been satisfied. When the heat pump is in 
a heating mode, flow reversal of refrigerant through the system at 
periodic intervals (every 6 hours of compressor run time, for example) is 
recommended by many heat pump manufacturers to recirculate lubricating oil 
and thereby increase the operating life of the heat pump. This flow 
reversal also cleans the inner surface of the outdoor heat exchanger and 
thereby increases heat transfer efficiency. 
From the above it is appreciated that one object of the invention is an 
efficient and flexible demand defrost control that adjusts heat pump 
defrosting based upon sensed outdoor air and heat exchanger temperatures. 
This and other objects, advantages, and features of the invention will 
become better understood by reviewing the accompanying detailed 
description of a preferred embodiment of the invention which is described 
in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Turning now to the drawings, FIG. 1 illustrates a heat pump unit 10 for 
heating or cooling the inside of a building. The heat pump system 10 
includes an indoor heat exchanger 12, an outdoor heat exchanger 14, and an 
expansion valve 16 coupled between the heat exchangers. Refrigerant is 
circulated through the system by a refrigerant compressor 20 with the 
refrigerant flow direction controlled by a flow reversing valve 18. The 
heat pump system 10 also includes electric resistance heaters 22 (called 
strip heaters) which are energized to heat the building whenever the heat 
pump system is not effective. The compressor 20 and strip heaters 22 are 
cycled on and off in response to control signals from a thermostat control 
unit 24. The unit 24 has a sensor responsive to indoor air temperature for 
producing an error signal having a value which depends upon the difference 
between sensed air temperature and a preselected set point temperature. 
In the preferred embodiment of the invention the thermostat unit 24 
includes a manually actuated "change over" switch (not illustrated). The 
change over switch is operated to a "cooling" position to position the 
reversing valve 18 so that the heat pump system cools the indoor air in 
response to cooling control signals from the thermostat 24. When the 
change over switch is in its "heating" position the valve 18 is positioned 
to direct refrigerant flow in the system for heating the indoor air and 
operation of the strip heaters is enabled. The heat pump and the strip 
heaters are operated under control of the thermostat unit 24 to heat the 
indoor air according to the sensed indoor air temperature. 
The process of heating and cooling by a heat pump system is well known and 
will only be briefly summarized. In either the heating or cooling mode of 
operation, the compressor 20 receives gaseous refrigerant that has 
absorbed heat from the environment of one heat exchanger. The gaseous 
refrigerant is compressed by the compressor and discharged, at high 
pressure and relatively high temperature, to the other heat exchanger. 
Heat is transferred from the high pressure refrigerant to the environment 
of the other heat exchanger and the refrigerant condenses in the heat 
exchanger. The condensed refrigerant passes through the expansion valve 16 
into the first heat exchanger where the refrigerant gains heat, is 
evaporated and returns to the compressor intake. 
Typical heat pump units of the sort referred to here are constructed using 
heat exchangers formed by tubular coils of highly conductive metal through 
which the refrigerant flows. Ambient air is directed across the coils to 
produce conductive heat transfer. The heat exchangers are thus referred to 
as coils, although they could take other forms if desirable. 
When the heat pump 10 operates as a air-conditioning unit the valve 18 is 
positioned to direct refrigerant flow so that the indoor coil 12 absorbs 
heat from the indoor air and the coil 14 gives off heat to the outdoor 
air. The thermostat 24 energizes the compressor 20 in response to sensed 
indoor air temperature above the thermostat setting and terminates 
compressor operation when the sensed indoor air temperature reaches the 
set point temperature. 
When the heat pump 10 is operating as a heating unit, refrigerant is 
discharged from the compressor through the valve 18 to the indoor coil 12. 
The compressed gaseous refrigerant condenses in the coil 12 giving up heat 
to the indoor air. Fans (not shown) blow indoor air across the coil 12 and 
facilitate heat transfer from the coil to the air. 
As the refrigerant gives up its heat content it condenses and passes 
through the expansion valve 16. The low pressure liquid refrigerant 
expands as it passes into the outdoor coil 14. The refrigerant in the 
outdoor heat exchange coil absorbs heat from the outdoor air and 
evaporates. The gaseous refrigerant then passes through the valve 18 back 
to the compressor intake. 
The outdoor coil 14 is an energy absorber since the atmospheric air heats 
(and vaporizes) the refrigerant passing through the coil 14. Since the 
refrigerant in the outdoor coil is at a lower temperature than the 
atmospheric air atmospheric moisture tends to condense onto the outdoor 
coil. When the coil temperature is at or below freezing temperature the 
outdoor coil accumulates frost or ice over its outside surface. The 
accumulation of frost or ice impedes heat transfer from atmospheric air 
into the refrigerant thus reducing the effectiveness of the heat pump 
system. 
According to the present invention conditions leading to the need for 
defrosting the outdoor coil are monitored so that the outdoor coil can be 
defrosted periodically when needed. The outdoor heat exchange coil 14 is 
deiced or defrosted by reversing the flow of refrigerant through the heat 
pump 10 for a relatively short period of time so that hot refrigerant from 
the compressor is directed by the valve 18 to the outdoor coil 14. The 
flow of hot gaseous refrigerant heats the coil 14 and melts accumulated 
frost or ice on the coil's outside surface. 
When the coil is defrosted, the valve 18 reverses the system refrigerant 
flow direction again so that the heat pump resumes its heating function 
with renewed effectiveness. 
The defrosting cycle of the heat pump system 10 is initiated and terminated 
by a demand defrost controller 30 in response to sensed conditions 
indicative of the need for performance of a defrosting cycle. 
The controller 30 provides three interactive defrost cycle controls. The 
preferred controller 30 only enables initiation of a defrost cycle when: 
(1) the outdoor coil temperature is low enough to warrant defrosting; and 
(2) when a timed defrost control enables defrosting; and (3) when a 
differential temperature responsive demand defrost control enables 
defrosting. It has been found that outdoor coils do not accumulate frost 
or ice when the measured coil temperatures exceed certain levels (which, 
in certain cases, may be below freezing). By definition, defrosting is not 
necessary at such coil temperatures. The controller 30 operates to enable 
a defrosting cycle only when the sensed coil temperature is below a 
predetermined value. 
The controller 30 also functions as a timed defrost control by accumulating 
the amount of time the compressor 20 runs and enabling a defrost cycle to 
be initiated when sufficient compressor run time is accumulated. The 
preferred controller 30 operates to vary the amount of the accumulated run 
time necessary to enable a defrost cycle depending on sensed outdoor air 
temperatures. 
The differential temperature demand defrost control function is provided by 
the controller 30 so that, when the first two defrosting criteria are 
satisfied, the defrost cycle is only initiated when outdoor air and coil 
temperatures differ sufficiently to indicate a frosting condition. In this 
regard the controller 30 compares the sensed outdoor coil and outdoor air 
temperature differential and compares that differential with a value which 
varies as a function of outdoor air temperature. When the measured 
differential and the calculated value bear a predetermined relationship 
the controller 30 initiates a defrost cycle. The outdoor coil and outdoor 
air temperatures are monitored by temperature sensors 32, 34, 
respectively, which generate control inputs to the controller 30. An 
additional input to the controller 30 is generated when the compressor 20 
is running so that the timed defrost function control can be realized. 
These three controller inputs provide sufficient information for the 
controller 30 to determine when to defrost the outside coil 14. 
Demand Defrost Function 
FIG. 4 is a graph showing sensed outdoor coil temperatures plotted against 
outdoor air (or "ambient") temperatures for a heat pump unit operating in 
its heating mode. The graph of FIG. 4 shows plots for a clear (i.e., 
unfrosted) heat exchange coil and for a "frosted" heat exchange coil. 
These plots are based on identical heat pump units operating under 
identical circumstances. The data show that the temperature difference 
between the heat exchanger coil 14 and outdoor air is smaller for a clear 
coil than for a coil covered with ice. At 30.degree. Fahrenheit, for 
example, the temperature difference between atmospheric air and a coil 
covered with ice is approximately 20.degree. F. At lower outdoor air 
temperatures (5-10.degree. F.) the temperature difference between a coil 
covered with ice and outdoor air decreases to about 5-10.degree. F. The 
disclosed demand defrost control operates primarily in response to sensed 
temperature difference at relatively high outdoor air temperatures and 
primarily on the timed defrost basis at low outdoor air temperatures where 
the small temperature differences between the coil and air may be 
difficult to use as an accurate defrost indicator. 
Three outdoor air temperature based zones of control are generally defined. 
At relatively high outdoor air temperatures if after a relatively short 
compressor run period the sensed coil and air temperatures are below the 
line designated Defrost Control Line in FIG. 4 the heat exchange coil 14 
is defrosted. This Defrost Control Line is derived from a control equation 
relating coil and air temperature differences to air temperature. The 
slope and offset of the Defrost Control Line are determined by three 
constants which are set to customer specifications. In this first control 
zone the temperature differences are relatively large and can be 
accurately sensed. 
At low temperatures (5-10.degree. F.) the sensed coil and air temperatures 
fall below the FIG. 4 Defrost Control Line. Even the clear coil 
temperatures fall below this line. Thus, at low temperatures, when a 
compressor lockout time has elapsed, a defrost will be initiated since the 
temperature difference criteria will be satisfied. To avoid too frequent 
defrosting the compressor lockout time is increased at low temperatures. 
At intermediate temperatures (15-20.degree. F.) both elapsed compressor run 
time and temperature difference contribute to the defrost control 
decision. In some instances the coil will be frosted (as defined by the 
Defrost Control Line) when the elapsed compressor run time condition is 
met. In other instances the lockout time will expire and the coil is not 
yet frosted so the controller 30 waits for the sensed temperatures to fall 
below the Defrost Control Line. Since the Defrost Control Line determines 
these zones of control and since the slope and offset of this line are set 
by the adjustable constants programmed into the controller 30 the zones 
are also adjustable depending on customer needs. 
The Demand Defrost Controller 30 
FIGS. 2A and 2B depict a detailed schematic of the demand defrost 
controller 30. The controller 30 includes a model 47C210 microprocessor 36 
commercially available from Toshiba. This microprocessor 36 operates at a 
clock frequency of 3.58 megahertz and has an internal memory for storing 
an operating system as well as control parameters and therefore needs no 
support peripheral devices in the way of RAM and ROM circuits. 
Power is applied to the control 30 by a 24 volt 60 hertz a.c. input signal 
(FIG. 2A that energizes a precision zener diode 35 which in combination 
with a resistor and capacitor produce a filtered, regulated 12 volt d.c. 
signal. A diac 37 in parallel with the zener diode 35 limits voltage 
reaching the zener diode 37 to less than 60 volts. Two operational 
amplifiers 38a, 38b are energized by this 12 volt signal. A first 
operational amplifier 38a provides a regulated 5.6 volt d.c. signal to 
energize the microprocessor 36. The second operational amplifier 38b 
activates a reset input 39 to the microprocessor when the control 30 is 
initially energized. The receipt of a signal at the reset input 39 causes 
the microprocessor 36 to begin execution of its operating system. 
Temperature Sensors 
The control 30 monitors heat exchanger coil and ambient temperatures at 
periodic intervals. The two temperature sensors 32, 34 (FIGS. 1 and 2A) 
are coupled to two comparator amplifiers 40, 42 (FIG. 2B) having outputs 
connected to the microprocessor 36. The outdoor coil sensor 32 monitors 
the temperature of the outdoor coil 14 and is physically attached to that 
coil. The sensor 34 monitors outdoor air temperature. The sensor 32 
includes three resistors 44, 45, 46. Two resistors 44, 45 have fixed 
resistances and the third resistor 46 is a precision thermistor whose 
resistance varies with temperature. The combination of the three resistors 
44, 45, 46 forms a potentiometer whose voltage varies with temperature. As 
the temperature of the thermistor resistor 46 rises, its resistance lowers 
as does the parallel combination of the thermistor resistor 46 and the 
resistor 45. The voltage on an output 32a from the sensor 32 is directly 
related to the temperature of the heat exchange coil 14. In a similar way 
three resistors 44', 45', 46' define the sensor 34 for measuring air 
temperature by providing a voltage at an output 34a. 
The comparator amplifier 40 (FIG. 2B) has two inputs, one of which is 
coupled to the output 32a from the sensor 32. A second input to the 
comparator 40 is generated by a voltage divider 50 which includes an array 
of resistors which are selectively coupled in parallel arrangements under 
control of the microprocessor 36. 
When a pin designated R73 on the microprocessor 36 goes low, a transister 
Q1 is energized and two resistors Ra, Rb coupled to a collector junction 
of the transistor Q1 define a reference voltage Vref at a noninverting (+) 
input to the comparator 40. The status of eight additional microprocessor 
pins R50, R51, R52, R53, R60, R61, R62, R63 are turned on or off to vary 
the reference voltage Vref. These pins can function as a current source 
due to a pull-up resistor configuration integral within the microprocessor 
36. By selective energization of these pins, the microprocessor can select 
one of 256 (2.sup.8) reference voltages for the voltage divider 50. 
The microprocessor monitors (at pin R71) the output status of this 
comparator 40 as the reference voltage is adjusted. A change in state is 
correlated with a resistor configuration used to generate the reference 
input to the comparator 40. In this way, the output potential of the 
sensor 32 is sensed and converted via a look-up table to a temperature. 
The combination of the voltage divider 50 and comparator 40 defines an 
analog-to-digital (A/D) converter that converts the analog output from the 
sensor 32 to a digital value sensed at the comparator output. 
In a similar fashion, the reference voltage from the voltage divider 50 is 
varied by the microprocessor 36 as it monitors the output of the 
comparator 42 coupled to the sensor 34 for monitoring ambient temperature 
in close proximity to the outdoor heat exchange coil 14. 
To help avoid an erroneous defrost cycle initiation due to coil temperature 
fluctuations (as the expansion valve 16 hunts for a proper setting for 
example), the coil temperature T.sub.c sensed by the sensor 32 is averaged 
with seven previous readings and stored in memory. This average reading is 
used in testing to determine if defrosting is needed. When the compressor 
20 is not running, no coil temperature readings are sensed but previously 
sensed average coil temperatures are stored. When the compressor 20 next 
cycles on and the coil temperature is again sensed it is averaged into the 
stored temperature so that first reading (which tends to be inaccurate if 
the system has not stabilized at compressor start-up) is low weighted. 
Timing and Interrupts 
To control the frequency at which temperature outputs from the sensors are 
obtained as well as to time compressor run times, the microprocessor 36 
implements an internal timer function. An input pin R83 is coupled to the 
same 24 volt 60 hertz alternating current signal that is rectified and 
filtered to produce the 12 volt d.c. energizing signal. Sixty times a 
second the voltage at this input goes low and the microprocessor 36 
updates an internal timer. The microprocessor monitors the status of this 
internal timer and updates the temperatures at the sensors 32, 34 at 
regular intervals. 
A signal at microprocessor pin R80 from the compressor 20 activates one 
microprocessor interrupt. When the compressor 20 is not operating, the 
microprocessor 36 is in an idle state awaiting this interrupt and does not 
monitor the temperature at the sensors 32, 34. After receipt of this 
interrupt the microprocessor also begins to accumulate compressor run 
time. 
A second interrupt at a microprocessor pin R82 is coupled to a test input 
60 that can be selectively grounded. When a test switch 61 is manually 
closed the microprocessor 36 initiates a defrost cycle to facilitate 
diagnostic testing of the heat pump system. 
Demand Defrost Parameters 
During demand defrost monitoring the microprocessor 36 utilizes numeric 
constants that are either stored internally in the microprocessor or 
accessed from an external diode array 70 (FIG. 2B) coupled to the 
microprocessor. These numeric constants are discussed in more detail 
below. Briefly, a defrost enable temperature, defrost termination 
temperature, and three constants C1, C2 and C3 for evaluating the 
temperature difference between the coil and ambient are used to initiate 
and terminate the defrost cycle. On power-up of the microprocessor it is 
assumed that the diode array 70 is preprogrammed to contain this 
information. 
By energizing four output pins P20, P21, P22, P23 connected to the diode 
array 70, and monitoring the status of four diode array outputs at pins 
K0-K3, the microprocessor 36 determines the value of four constants 
programmed in the diode array 70. If an invalid diode array code is sensed 
the microprocessor 36 checks to determine what combination of jumper 
diodes 72-75 have been coupled from pin P13 to the four microprocessor 
inputs K0-K3. In the configuration depicted in FIG. 2B four diodes are in 
place. This configuration represents one of sixteen possible sets of 
constants stored in a microprocessor read only memory (see Table II 
below). 
Defrost and Optional Strip Heater Outputs 
To initiate a defrost the microprocessor 36 energizes output pin R40 which, 
in turn, causes energization of a defrost relay coil 82. The coil 82 is 
energized to turn on the compressor 20 and activate the reversing valve 18 
to route hot refrigerant through the outdoor heat exchange coil 14. In the 
illustrated embodiment the output pin R40 is coupled to a triac 80 having 
a gate 80a. When turned on by the microprocessor, the triac 80 energizes 
the defrost relay coil 82 and an associated light emitting diode 81 to 
indicate a defrost cycle is in progress. A diac 84 prevents transients 
from damaging the triac 80 by limiting the voltage across the triac to 
approximately 60 volts. 
Microprocessor output pins R41, R42 are optionally employed to activate two 
strip heater relay coils 90, 91 via associated triacs 92, 93. This 
optional circuitry is illustrated within broken lines in FIG. 2B. Light 
emitting diodes 94, 95 indicate when the strip heaters are turned on by 
the microprocessor 36. The strip heaters are turned on simultaneously or 
in staged fashion when the coil 14 is defrosted and the outdoor air 
temperature determined by the sensor 34 is below a strip heat initiation 
temperature or temperatures. 
The Microprocessor Operatinqg System 
On receipt of a reset signal the microprocessor 36 initializes 110 (FIG. 3) 
the numeric constants used by the microprocessor operating system while 
conducting its demand defrost function. This initialization is 
accomplished by determining the status of the diode array 70 or the 
configuration of the diodes 72-75 to determine which set of constants 
stored in microprocessor ROM memory should be used. The constants are 
transferred to a RAM area of the microprocessor and accessed as needed 
during the execution of the microprocessor operating system. 
Status indicators or flags are set 112 at a next stage of the demand 
defrost procedure. In addition, timers are initialized and the 
microprocessor interrupts are enabled. The microprocessor then enters an 
inactive state 114 until it receives an interrupt at input pin R80 
indicating the heat pump compressor 20 is running. In the present 
embodiment, when the compressor is not running no temperature sensor 
readings are obtained. When the compressor begins to run, the 
microprocessor initiates a two minute wait period for the heat pump system 
to stabilize. This stabilization wait period is accomplished in software 
and is available as a manufacturing option. At the end of this two minute 
wait period the microprocessor 36 waits 116 for the evaporator coil 
temperature to drop below an enable temperature. 
The operation of the next four states 116, 118, 120, 122 depicted in FIG. 3 
are summarize in four pseudo-code program listings. During execution of 
the computer code summarized in these pseudo-code listings microprocessor 
subroutines are executed to perform specialized functions such as monitor 
a sensor temperature, access a constant stored in memory, perform a 
comparison or calculation, etc. 
Listing 1 (below) is a pseudo-code listing of a program the microprocessor 
executes while waiting for the outdoor heat exchanger coil temperature to 
fall below the enable temperature. The enable temperature is one of the 
sets of parameters stored in the diode array 70 and alternately stored in 
the microprocessor. A sensed coil temperature above the enable temperature 
indicates frost will not form on the coil. 
While waiting for the coil temperature to fall below the enable temperature 
the microprocessor 36 periodically senses the coil temperature T.sub.c and 
the ambient temperature T.sub.a. The coil temperature is sensed at regular 
one minute intervals and the ambient temperature is measured as often as 
possible. The frequency of the ambient temperature measurement varies 
between one and two minutes. A test is performed to determine if the 
sensed temperature indicates the sensors 32, 34 have malfunctioned. A 
sensor is defined to be malfunctioning if a scanning of the 256 possible 
resistance combinations provided by the resistor array 50 fails to produce 
a change in the outputs of the comparators 40, 42. A short or open circuit 
condition of the sensor will cause this to occur. If the ambient 
temperature sensor 34 is disconnected or electrically shorted, the 
microprocessor initiates a defrost cycle at regular 90 minute intervals of 
compressor run time rather than perform the demand defrost function. If 
the coil temperature sensor 32 is either disconnected or shorted, the 
microprocessor 36 stops transmitting defrost relay control signals. 
Referring to the Listing 1 summarization, one sees that the while loop 
defining the microprocessor wait state 116 is exited when: 
(a) the coil falls to a temperature at or below the enable temperature; or 
(b) a test input (interrupt 1) is received at microprocessor pin R82. 
If the test input is active the microprocessor initiates a defrost 
immediately and if the enable condition is satisfied the microprocessor 
progresses to a lockout condition wait state 118. 
Compressor Lockout Condition 
If the outdoor coil temperature drops below the enable temperature, the 
microprocessor begins accumulating compressor run time (including the 
optional two minute wait state mentioned previously) and compares the 
accumulated run time with a microprocessor calculated time value that 
depends upon ambient temperature. This value is referred to as the 
"lockout compressor run time" and assures that the colder the outdoor or 
ambient temperature, the greater the amount of accumulated compressor run 
time required before a defrost cycle is initiated. Thus defrosting is not 
conducted at too frequent intervals during periods when heating demands 
are greatest and frost buildup conditions are diminished. 
A pseudo-code listing for the microprocessor wait state 118 to determine 
when the lockout time has expired is presented below in Listing 2. 
While waiting for the lockout timer to time out, it is possible that the 
outdoor heat exchanger coil temperature has risen above the enable 
temperature. If the coil temperature rises above the enable temperature, 
the lockout time wait state 118 is exited and the microprocessor returns 
to the state 116 where it waits for the coil temperature to again fall 
below the enable temperature. When this happens, the accumulated lockout 
time is maintained and the lockout timer re-started from the accumulated 
time the timer had reached when the expansion coil temperature exceeded 
the enable temperature. 
If the compressor 20 stops running as the lockout time is accumulating, the 
accumulated lockout time is also stored. When the compressor again turns 
on, if the enable temperature condition is satisfied, the lockout 
compressor run time is again started where it left off. 
The last if-then test of the Listing 2 pseudo-code refers to a defrost 
cycle that is performed in the event the ambient temperature sensor 34 has 
malfunctioned. The microprocessor 36 reaches this ambient sensor if-then 
test only if (1) the coil sensor is functioning, (2) the defrost test 
switch 61 has not been activated, (3) the coil temperature is not above 
the enable temperature, and (4) the lockout time has timed out. 
If the ambient sensor 34 has malfunctioned the controller 30 converts to a 
strictly timed defrost at 90 minute intervals. Whenever the ambient sensor 
34 fails the microprocessor 36 executes a subroutine that sets the lockout 
time to 90 minutes. Thus, whenever the sensor 34 fails, criteria 4 is 
adjusted to achieve a 90 minute defrost cycle time. 
Table 1 below lists the compressor lockout run times for different ambient 
temperatures when the ambient sensor 34 is properly functioning. The 
contents of this table are stored in the microprocessor's ROM memory. 
Test for Frost Condition 
A next microprocessor wait state 120 (FIG. 3) test for a frost condition. 
This state is entered only if both the coil sensor 32 and ambient sensor 
34 are functioning and the compressor lockout run timer has timed out. In 
this wait state 120 the microprocessor tests for a difference between an 
outdoor coil temperature and the ambient temperature. A large enough 
difference between these two temperatures causes the microprocessor 36 to 
energize the defrost relay 82 to cause outdoor heat exchanger coil 
defrosting. Equation 1 below summarizes the test the microprocessor 36 
performs in determining whether a frost condition exists: 
EQU T.sub.a -T.sub.c =.DELTA.T.gtoreq.C.sub.1 (T.sub.a -C.sub.3)+C.sub.2 Eq. 
(1) 
When the measured difference, .DELTA.T, of this equation is equal to or 
greater than the calculated value based on ambient temperature, the 
microprocessor initiates a defrost cycle by activating the triac 80 that 
closes a defrost relay contact and actuates the reversing valve 18. 
At a threshold wherein the temperature difference is set equal to the right 
hand side of equation 1 and if C.sub.3 is zero, one has an equation which 
after rearranging is of the form: 
EQU T.sub.c =T.sub.a (1-C.sub.1)-C.sub.2 Eq. (2) 
This equation is the Defrost Control Line of FIG. 4. 
Listing 3 summarizes the steps the microprocessor performs while awaiting 
the frost condition to occur. 
If the compressor stops running or the outdoor coil temperature exceeds the 
enable temperature, the microprocessor will stop monitoring for the frost 
condition. When the compressor turns on and the coil temperature again 
falls below the enable temperature the microprocessor again checks to see 
if the frost condition is satisfied. 
The 6 hour override option in Listing 3 refers to an automatic defrost 
conducted every 6 hours of compressor run time regardless of other defrost 
criteria. This option can be programmed into the microprocessor operating 
system. 
Defrost Cycle 
The defrost cycle is conducted by reversing refrigerant flow through the 
valve 18. The defrost cycle is conducted until either the coil temperature 
rises above a termination temperature (one of the numeric constants 
initialized at step 110 FIG. 3) or until the defrost cycle has lasted a 
specified time, for example, 15 minutes. The steps conducted by the 
microprocessor 36 during a defrost cycle are listed below in Listing 4. 
If the strip heaters 22 are activated by the controller 30 and the outdoor 
air temperature is below a threshold value, both heaters 90, 91 are turned 
on to combat the cooling effects of a defrost cycle. 
Once a defrost cycle is entered, the coil sensor 32 is the only temperature 
sensor which is monitored by the microprocessor 36 and the effects of the 
reversal of refrigerant through the heat pump are monitored at this 
sensor. 
The while loop that checks the status of a termination flag monitors the 
output from the comparator 40 at microprocessor pin R71. A low output from 
the comparator 40 indicates the coil temperature is greater than the 
termination temperature and the defrost cycle has been successfully 
conducted. The termination flag is also set if the defrost cycle is 
conducted for 15 minutes. 
At the conclusion of a defrost cycle when either the time condition or the 
temperature condition is satisfied, the microprocessor sets the 
termination flag, exits the while loop and jumps to step 112 of the FIG. 3 
state diagram where the flags or status indicators are reset. 
The Table 1 lockout times are stored in the microprocessor's ROM memory. 
The coil enable and defrost termination temperatures and numeric constants 
C.sub.1, C.sub.2, and C.sub.3 of equation 1 are either programmed in the 
diode array 70 or stored in the microprocessor 36. One illustrative 
factory set-up of the diode array sets these values for these numeric 
constants: Enable temperature 35 degrees F., Terminaton temperature 55 
degrees F., C.sub.1 =0.1, C.sub.2 =12, C.sub.3 =0. 
Table II (below) illustrates sixteen different options stored in 
microprocessor ROM which are selected if the diode array 70 is not 
configured. Note, the constant C3 is zero for all sixteen sets of control 
constants. Other choices for this constant, 15.degree. F. for example, 
have been successfully utilized in conducting the demand defrost control 
of the invention. 
Demand Defrost Control Options 
In the present embodiment of the system 10, the strip heaters 22 respond 
only to the thermostat control 24. In an alternate embodiment of the 
invention the demand defrost control also activates the strip heaters when 
a defrost cycle is initiated and the sensed outdoor temperature is below a 
threshold temperature. 
When the strip heaters 22 are controlled by the microprocessor, however, 
they can be actuated simultaneously when a single strip heater initiation 
temperature condition is sensed. Alternately, the microprocessor can 
monitor ambient temperature from the sensor 34 and energize the two strip 
heaters based upon different threshold values so one or both strip heaters 
are energized as ambient temperature conditions change. 
The strip heat control temperatures, if used, are input via thumb wheel 
selector switches connected to microprocessor pins P10, P11. Four switch 
contacts of a thumbwheel selector switch allow 16 different settings for 
this temperature. In one embodiment of the invention, the sixteen possible 
switch settings are used to adjust this temperature in equal increments 
from 20 to 30 degrees F. The microprocessor samples the status of pins P10 
by energizing pin P10 (Aux 1) and monitoring the input state of pins 
K0-K3. If a particular switch contact is closed, the microprocessor will 
sense a high input at an associated one of the input ports K0-K3. In a 
similar manner the status of a second switch connected to pin P11 controls 
an initiation temperature for a second of the strip heaters 22. As an 
alternate method when no switch inputs are used as the strip heat 
initiation temperature or temperatures are stored in the microprocessor 
(see Table II). 
A second option that is not presently implemented is to sense for an 
outdoor heat exchanger coil melting condition. Temperature sensing of both 
the coil and ambient air is suspended when the compressor is not running. 
Since power is being applied to the microprocessor whether the compressor 
is running or not, however, these temperatures could be sensed at all 
times. If during a compressor off period the coil temperature rises high 
enough, above a melting condition temperature, all status flags can be 
reset and in particular the compressor lockout time can be reset. 
A temperature calibration option may also be added. If the resistor 
elements forming the sensors 32, 34 exhibit variations from their nominal 
resistance values a correction factor can be programmed into the diode 
array 70 and sensed at pin P12. In this way slightly inaccurate sensed 
temperatures are modified with a correction factor. This correction factor 
is determined after factory fabrication and testing of the sensors 32, 34 
and is used to compensate minor inaccuracies in those sensors. 
While one embodiment of the present invention has been described with a 
degree of particularity, it is the intent that the invention include all 
alterations and modifications from the disclosed design falling within the 
spirit or scope of the appended claims.