CA1267952A - Electronic thermostat for heating and cooling system - Google Patents

Abstract

ABSTRACT

A thermostat for use in a building having a furnace and an air conditioning system adapted to generate energizing signals for only one of the furnace and the air conditioner at a given time. An operator programs the thermostat with a series of desired temperatures over a repetitive heating or cooling cycle by a manual data entry device. The thermostat includes a clock that interrogates the program to generate the desired temperature for the present time which is compared to the measured temperature within the building to generate control signals for the furnace.
A desired heating temperature is equal to the desired heating temperature programmed for the present time. A
desired cooling temperature is equal to a first predetermined temperature if said desired heating temperature programmed for the present time is in a first range of temperatures. The desired cooling temperature is equal to a second predetermined temperature, higher than the first, if the desired heating temperature programmed for current time is in a second range of temperatures, lower than the first range. The thermostat can be placed in and of three different modes, the third one having control signals generated for either the furnace or the air conditioner based upon the measured temperature and the respective desired temperature signals.

Description

'" 1 This inven-tion rela-tes to thermosta-ts for controlling both heating and air conditioning systems for -the same enclosed area and, more particularly, to such -thermostats including means for preventing rapid oscillation of the controlled -temperature cluring periods of transition between low -temperatures which re~uire hea-ting and high -temperatures which req-uire air conditioning. This application is a division of Canadian Patent Applica-tion Serial No. 502,24~ filed February 19, 1986.
Many residences and commercial buildings in -temperate climates have both heating and air conditioning systems. The hea-ting sys-tems are generally used during -the winter and the air conditioning during -the summer, but frequen-tly during -the spring and fall both systems are utilized during different periods of the day. Thermostats previously provided for these dual function systems typically included means for storing a first temperature set point for the heating system which may be manually adjusted to a level of say, 68 degrees, and separate means for storing a set point for the cooling system which may be adjusted to a higher temperature, such as 74 degrees. If the two set points are adjusted too close to one another the system may oscilla-te between heating and cooling modes because the temperature may overshoot the heating set point during -the heating mode and undershoo-t the cooling se-t poin-t during the cooling mode. This undesirable oscilla-tion may also occur as a 126795~
. 2 resul-t of thermal exchange between adjacent zones in a mul-ti-z.one heat:ing system. If the se-t points are set -too widely apar-t, to minimi~e the possibility of oscilla-tion, the comfort o~ the occupan-ts, who would prefer a single constant temperature, is diminished.
These problems are exacerbated when multl-tempera-ture programmable thermostats of the type disclosed in U.S. Patent 4,172,555 are employed since the maximum -temperatures -that may be programmed during a heating cycle must be limited to avoid unintentional energiza-tion o:f the cooling system and vice-versa.
The present inventi.on is directed toward a -thermostat system Eor controlling both heati.ng and cool:ing sys-tems fo:r -the same area which allows ~6 .relatively olosely spacecl hea-ting and cooling se-t points without danger of oscillation.
According to -the invention, there is provided a thermostat for use in a building having means for heating and means for cooling the building and operative to generate an energizing signal for only one of sa:id heating means or cooling means at a given -time, the thermos-tat comprising:
. means for measuring -the ambient temperature within the building;
manual da-ta entry means;
means for storing a program of desired hea-ting -temperatures over a repeti-tive -time cycle, programmed by said manual data entry means;
a clock opera-tive to generate -time signals within . .... ~ -~l2679S~
. 3 said repe-titive time cycle;
means for generating a signal represen-tative of a desired hea-ting temperature and a desired cooling temperature at -the present time based upon the signals from the clock in the stored -temperature program, said desired heating temperature equal to said desired heating temperature programmed for the present time, said desired cooling tempera-ture equal to a first predetermined tempera-ture if said desired heating temperature programmed for the present time is in a first range of temperatures, and said desired cooling temperature equal to a second predetermined temperature, higher than said first prede-termined tempera-ture, if said desired hea-ting temperature programmed for the current time is in a second range of temperatures, lower than said first range of temperatures; and means for placing the thermostat in either a firs-t mode wherein control signals are generated onl~
for said heating means as a function of the difference between the measured temperature within the building and the desired heating temperature signal, a second mode wherein energizing signals are only generat~d for said cooling means as a function of the measured temperature and the desired cooling temperatures si.gnals, or a third mode wherein control signals are generated for either said heating means or said cooling means based upon the measured temperature and the respective desired heating and cooling temperature : . . . - , ~IL2~i795~
. l signals.
A preferred -thermostat, disclosed herein, includes a programmable memory for storing a single schedule of desired temperatures over repetitive time cycles such as one week. The thermos-tat includes a three posi-tion switch for placing the thermosta-t in-to any one of a heating mode, a cooling mode, or an automa-tic mode. Whe-ther -the s-tored schedule is used as a heating schedule or a cooling schedule depends upon -the position of -the mode switch during the programming of the memory. If the mode switch is in automa-tic or heating mode during programming, -the program is viewed as a heating cycle. Otherwise, -thè program is viewed as a cooling cycle. The -thermosta-t includes a clock that interro~ates -the memory causing it -to ou-tput a signal representing the desired -temperature a-t the time of interrogation. This signal is compared to a signal representing the measured temperature wi-thin -the building to generate an on/off control signal for the furnace.
If the -thermos-tat is programmed, for example, while in -the heating mode, the thermostat acts on -the stored program as a hea-ting schedule as long as -thé
mode switch remains in the heating posi-tion. In this sta-te, the -thermos-ta-t will generate energizing signals exclusively for -the furnace to raise the ambien-t ternperature toward the programmed tempera-ture. If -the switch is moved to the automatic mode, the thermostat will con-tinue to operate to energize the furnace until ,.. ' ~ ` '' : .

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the ambient temperature exceeds both the -temperature stored in the program and a predetermined maximum temperature. When the ambient temperature exceeds -this predetermined level, the thermostat will automaticalIy energize the air conditioning unit, along with any associa-ted blower fan, to maintain -the temperature at or about the predetermined maximum tempe:ra-ture. If the ambient temperature thereafter falls an~ remains below the maximum temperature, the -thermostat re-turns to its normal operating state wherein the furnace is energized to maintain the desired tempera-ture. To prevent oscillation of the system between heating and cooling, a -time delay is incorporated which prevents -the energization of the air conditioner and blower fan if the furnace has been energized within a predetermir1ed : time, e.g., one half hour, and vice-versa.
When the cooling season begins, the unit may be reprogrammed with a cooling cycle by placing the mode switch in -the cooling mode and reprogramming the thermostat. When the mode switch is then returned to automatic, the system will follow the programmed cooling cycle unless the ambient temperature drops below the desired cooling te.mperature and below a predetermined minimum temperature. If this occurs, the furnace will be energized to raise -the temperature, subject to the time delay described above.
In one version of the thermostat disclosed herein, the energization of the furnace and air conditioner when the thermostat is in the automatic ~LZ~i7~s~

mode is de-termined relative -to the set poin-t value stored for the particular time without regard to predetermined maximum or minimum temperatures. In this embodiment, the ambient temperature is maintained at or about -the set point value a-t all times during the day.
One unit, disclosed herein, may be programmed wi-th both a full heating -time/temperature program and a full cooling time/temperature program. In the heat or cool mode, -the appropriate time/temperature program will be operative. In the auto mode, only one of the programs will be operative at any given time and will control either the heating or the cooling in a normal manner. For example, when -the unit is using the heatlng schedule, i~ the ambient temperature exoeeds the cooling set point stored for tha-t period and the furnace has not been energized for a predetermined period of time, the unit will swi-tch over to the cooling mode. A switch in the reverse direction will occur under complementary conditions.
Other advantages and applications will be made apparent by -the following detailed description of preferred embodiments of the invention.
In the drawings, Figure 1 is a block diagram illustrating the operation of the main control routine of the thermostat control of the present inven-tion.
Figure 2 is a block diagram of the opera-tion of the automatic heat/cool control for a thermostat of the present invention.

1~67952 -~ 7 Figure 3, which appears on -the same sheet as Figure 1, is a block diagram illustrating the temperature adjustment sequence of the thermostat of the presen-t invention.
Figure 4 is a block diagram illustrating the cool and auto heat modes of a second embodiment of the present invention.
Figure 5 is a block diagram illustrating -the auto cool and auto heat modes of a third embodiment of the presen-t invention.
The present thermosta-t provides for automatic switching between heating and cooling modes of operation so -that a comfortable tempera-ture is maintained inside a building a-t all times. In a pre-ferred ernbodiment of the thermosta-t, a single heating or cooling schedule is programmed into the -thermos-tat using any conven-tional means. For example, the thermostat may be programmed as described in U.S.
Patent No. 4,172,~55. Using the programming procedure described in that patent, the primary operating mode of the thermostat is first selected. For example, if -the unit is programmed during the winter, the heating mode is the primary mode of opera-tion for the thermos-tat.
The thermostat is placed in the heating mode, for example, by moving a switch.
A:~ter the operating mode is selected, desired temperatures are programmed in-to the thermostat. The operator enters temperature set points for various times during the day. Once the thermostat has been ~2~i79~X

programmed for a repetitive cycle, e.g., one week, it can then be used to heat the house according to the programmed schedule.
Figure l illustrates the basic opera-tion of the -thermostat after the programming sequence has been accomplished. The opera-tion of the -thermostat may be implemented in hardware or, preferably, by a microprocessor under program control. If -the -thermostat remains in the operating mode in which it was programmed, e.g., heating mode, it will activate the furnace (or air conditioner) exclusively to maintain -the ambient temperature a-t or about the programmed set point. Thus, if the thermostat remains in heating mode, -the air condi-tioner will not be energized even when the ambient temperature rises above the set point temperature.
Automatic control of the system is effected by placing -the thermosta-t in automatic mode. When the thermostat is placed in automatic mode, the temperature control sequence begins at step 8. The set points stored in the time/temperature program are read out of the memory by a real -time clock (not shown) at step lO
to determine whether the temperature has been programmed to change at the present time. If so, a control temperature value is set equal to the programmed temperature at step 11 and the operating mode is se-t -to the mode in which the thermostat was programmed, at step 13. After these values have been set, or if the programmed tempera-ture did not change at 5~
, ~ g step 10, control passes to step 12. ~t step 12, -the ac-tual -temperature in -the building is compared with the control tempera-ture which was set either in s-tep 11, as described above, or by the au-to heat/cool sequence which is described in de-tail below. If the actual temperature is at the con-trol temperature, the -thermostat turns off the furnace and air condi-tioner at step 14 and returns to the beginning of the program, s-tep 10. If, however, the actual -tempera-ture is higher or lower than the desired temperature, the -thermostat performs -the auto heat/cool sequence described in detail below. When the auto heat~cool sequence has been completed, the thermostat control program returns to step lO and continues its usual sequence described 16 above.
Referring now to Figure 2, a preferred embodiment of the auto heat/cool sequence is illustrated beginning at step 20. A-t step 22 the system checks whether a sufficient amount of time, preferably a half hour, has elapsed since the heat/cool sequence was last executed.
This time delay prevents the thermostat system from rapidly switching between heating and cooling modes when -the ambient temperature falls slightly below or rises slightly above -the control temperature. If sufficient time has not passed at step 22, con-trol will re-turn to step 10 in Figure 1. If sufficien-t -time has elapsed since the last execution of the heat/cool sequence, the au-to heat/cool sequence continues a-t step 24. Step 24 checks whether the manual override has i7952 been ac-tivated by the operator of the thermostat. The manual override may consist, for example, of a switch on the thermostat which may be set to permit manual control of the temperature in the bu;lding. I the manual override is on, -the auto heat/cool sequence will no-t be invoked. Instead, the tempera-ture is adjusted, as described below with reference to Figure 3, and control returns to step 10 in Figure 1.
I- the manual override is not on at step 24, control passes to step 26 where the system checks whether the actual -temperature is above an absolute maximum. The purpose of the check in this step is to ensure that the temperature in the building does not rise to a dangerously hot level. This safeguard is no-t required for proper operation of the system, but is included in the preferr-ed embodiments. In the preferred embodiments of the invention, the absolute maximum is set at 8~oF. If the actual temperature is above 840F, the control passes to step 28 and a cooling mode o operation begins, as will be described below.
If the ac-tual temperature is below the absolute maximum value, control passes to step 30 where the actual temperature is compared to an absolute minimum temperature. As with the absolute maximum temperature, the actual temperature is checked against -the absolute minimum temperature in order to ensure -that -the temperature of the building does not become dangerously low. Again, -this check is no-t required for proper operation of the system but is included in the ~;~6~95X

preferred embodiments for safety. In the preferred embodlments, -the absolute minimum tempera-ture is set at 540F. If the actual temperature is below the absolute minimum -temperature, oontrol passes to step 32 where the thermostat is forced into a heating mode. If the tempera-ture in the building is above -the absolute minimum temperature, control is passed to step 3~ where -the normal auto heat/cool sequence begins.
At step 3~, the system checks whether the system is presently in the cooling mode, i.e., whether the present operating mode, set either in step 13 ~Figure 1) or during the auto heat/cool sequence, is the cooling mode. If -the operating mode is the cooling mode, control passes to step 36 where the actual temperature is compared to the control temperature.
The control temperature is normally -the temperature that was programmed by the operator during th initial programming operation. However, as described below, the control temperature is some-times a temperature set during the auto heat/cool sequence. The control temperature is used to control the operation of the fllInace and air conditioning unit so that -the proper tempera-ture level is achieved.
If the actual temperature is greater than the control temperature, the air conditioner must be ac-tivated. Since the sys-tem is already in the cooling mode, as determined in step 3~, the system merely branches to the adjust temperature routine at s-tep 37 and returns to the main routine. The operation of the ~2si~9~2 tempera-ture adjustment sequence is described bel~w with reference to Figure 3.
If, however, the actual tempera-ture in -the building is less than the control temperature, at step 36, energizing the air conditioning uni-t will simply lower the actual temperature further. Thus, the system must determine whether to swi-tch from cooling mode to heating mode and begin raising the ambient temperature using the furnace. In the preferred embodiment of Figure 2, the switch from a programmed cooling mode to an automatic heating mode is made only if the ambien-t temperature is less than or equal to a predetermined minimum temperature. In s-tep 38, then, when -the actual tempera-ture is less than the control temperature, the actual temperature is compared to the predetermined minimum temperature which may be set, for example, at 70O~ I the actual temperature is above this minimum, the auto heat sequence is not invoked. Instead, the control temperature and operating mode simply remain at their original programmed values in steps 40 and 42, or, if the control temperature and operating mode were changed, are reset back to their original values.
con-trol -then passes, through -the adjust temperature sequence, back to the main routine.
On the other hand, when the actual temperature is below the predetermined minimum -temperature, the system mus-t swi-tch from cooling to heating mode. if the unit was originally programmed in hea-ting mode, step 32, the original programmed values 795~

are simply restored for both control temperature and operating in steps 40 and ~2. In this manner, the sys-tem reverts -to heating mode, the programmed mode, and the fu:rnace may be energi~ed in the adjust tempera-ture routine described below.
If the unit was not programmed in the heating mode, the system must automatically switch from cooling to heating modes. One difficulty encountered in making this switch is that, in the embodiment disclosed in Figure 2, only a single set of temperature values are stored. Consequently, the system cannot simply lool~-up the proper heating temperature. Instead, it must derive the temperature based on the original temperature programmed by the user. In the embodiment illus-trated in Figure 2, -the heating -temperature is derived by dividing the cooling temperatures into two temperature ranges. The first range is called the "occupied"
-temperature range and comprises cooling temperatures : which the user would choose i.f the building was occupied, for example, cooling temperatures in the range of 620 to 7~o. Similarly, the second range of temperatures is called the "unoccupied" temperature range and comprises temperatures greater than the occupied temperatures, e.g. 760 and above. Thus, if the thermostat was programmed in the cooling mode for temperatures within the first range, the system assumes -that -the heating temperature should be set at a relatively warm tempera-ture to maintain the com~ort of the occupan-ts. On the other hand, if the thermostat ~2~i795~

was programmed at a -temperature in the second range, the system will control the heating temperature at a lower value to conserve.energy.
Thus, in step 44, if the programmed teMperature is in the first, occupied temperature xange, con-trol passes to step 46 where the control temperature, i.e.
-the temperature used -to control the operation of the furnace or air conditioner, is set to 70 degrees. If, however, the programmed temperatule is within -the ~0 second, unoccupied range of values, the control temperature is set at 64 degrees in s-tep 48. After the control temperature has been set, the system is placed in the heating mode at step 50 and the aMbient temperature is adjusted, step 52. Control -then passes to step 10 in Figure 1.
A similar sequence of steps is followed when the present operating mode of the system i5 the heating ; ~ mode. In tha-t case, at step 34, control passes to step 54 where the :aotual temperature is compared to the control -temperature. As decided above, the control temperature is either the temperature programmed in by the user o~ the thermosta-t or it is the temperature selected by the auto heat/cool sequence. If -the actual temperature is less than or equal to the con-trol -temperature, the furnace is activated at step 38 and the temperature is adjusted toward the con-trol temperature. The operating sequence then continues at step 10 in Fi~ure 1.
If, however, the actual temperature is grea-ter ~L2~i79~
" .

than the control temperature, step 54, -the actual temperature is compared to a predetermined maximum temperature at step 56. As with the predeterminecl minimum temperature described above, the predetermined ma~imum temperature allows the system to decide whether i-t should switch from the present heating mode to a cooling mode. In the preferred embodiment illustrated in Figure 2, the switch will occur only when the ambient temperature is a-t or above the predetermined maximum temperature, 74 ln the illustrated embodiment. If the ambient temperature is below 74 in s-tep 56, the system simply rese-ts the control temperature and the operating mode to their original programllled values, in steps 40 and 42 and returns to the main routine, in s-tep 37, via the adJust temperature sequence.
When the -temperature in the building is at or above 740 in step 56, the system has determined that the building is too warm and the air conditioning unit should be activated. Therefore, at step 28, i-f the thermostat was originally programmed in the cooling mode, the system need only restore the original control temperature and operating mode values to their programmed values at steps 40 and 42. The air conditioner is then activated in the adjust temperature routine, described below, and con-trol returns to step lO in Figure 1.
If the thermostat was not initially programmed in the cooling mode, s-tep 28, -the system will be forced ~z~ s~
`` lfi into the automatic cooling mode of operation and will select an appropriate cooling temperature depending upon whether the programmed tempe~ature is ln an occupied -temperature range or ln an unoccupied -tempera-ture range. Thus, if the thermostat was programmed in heating mode at a temperature in an occupied temperature ran~e, e.g. 660 to 780J the sys-tem branches from step 58 to step 60 where the control temperature is set to a relatively cool temperature, e.g. 7~o. If, on the other hand, the programmed temperature is in an unoccupied -tempera-ture range, e.g.
below 660, -the con-trol temperature for air condi-t.ioning operation is set at a h.igher value in step 62, Thi 5 permits energy savings when -the when the building is unoccupied. A~ter the control tempera-ture has been set to -the appropriate temperature in either step 60 or step 62, the system is placed in the cooling mode, step 64, and the air condi-tioner is activated by the adjust temperature routine at step 52. Control is then returned to -the main rou-tine in Figure l.
Turning now to Figure 3, the thermostat, af-ter selecting the appropriate control temperature and operating mode, performs the adjust temperature routine. The routine begins at step 70 by determining whether the system is to operate in heating or cooling mode. If the operating mode is cooling, the ac-tual temperature is compared to the control temperature in step 72. When -the actual -temperature is above the control tempera-ture, the air condi-tioner and fan must ~679~;~

be activated to lower the ambien-t temperature, step 74.
Once the air conditioner and fan are activated in step 74, or if the actual temperature is not abo~e the control temperature in step 72, control is passed back -to the main routine (Figure l) at step 76.
When heating mode is selected by the automatic heat/cool sequence, the adjust temperature rou-tine branches from step 70 to step 78. From s-tep 78, the system branches to step 80, to energize the ~urnace, when the actual temperature is below -the control temperature. Control then returns to the main rou-tine (Figure 1~ at step 76.
The -thermostat will operate as described above unless -the user places the thermos-tat in the manual override mode or reprograms the thermostat in the cooling mode. If the user reprograms the thermostat, the thermostat will use the new mode of operation as its primary (programmed) mode. If manual override is selected, the tempera-tures entered by the user will be used and the auto heat/cool sequence will not be performed.
In an alternative embodiment of -the invention, when the thermostat switches automatically from heating to cooling or vice-versa, the control temperature is

2~ set to a temperature relative to the programmed -temperature ra-ther than a-t a predetermined fixed value.
For example, the system might add 20 to the current programmed heating -temperature when choosing a control -temperature in auto cool mode. This 79~

would be accomplished, as shown in Figure ~, by replacing steps 58, 60 and ~2 with a single step 6~a which simply sets the control temperature to the programmerl temperature plus 20. Likewise, when changing from a programmed cooling mode to an automatic heating mode, the control -temperature would be set 20 less than the programmed temperature, step ~6a, replacing steps ~4, 46 and 48.
In this alternative embodiment, the automatically selected control temperature may even be set equal to the programmed temperature. The system would thus maintain the bui:lding temperature at the desired temperature throughout the day. The system avolds the problem of rapid oscilla-tion between the hea-ting and cooling modes because time inhibition is buil-t into the system using the -time-out check in step~22. Thus, in the thermostat of the presen-t invention, the heating -temperature and cooling temperature may be set to the same value without encoun-tering unwanted oscillation of the system be-tween heating and cooling modes.
~ n a third embodiment of the invention the thermostat may store both a heating schedule and a cooling schedule such that, when -the system enters the auto cool or auto heat portion of -the sequence, the system derives i-ts control temperature from a cooling or heating schedule which has been preprogrammed by the user. In this manner, the user has complete con-trol over -the heating and cooling tempera-tures in the building. The selecti.on of -the control -temperature during an auto cool cycle may be accomplished, as shown in Figure 5, by replacing steps 58, 60 and 62 with a single step 60b which sets the control -temperature set point. ~ similar replacement, step ~6b, is made for steps 4~, 26 and ~8 in the auto heat cycle.
It can thus be seen that an electronic thermostat is presen-ted which provides for automatic hea-ting and cooling of a building while avoiding unwanted ~0 oscillation of -the temperature control system between heating and cooling modes or operation. It is understood tha-t various modifica-tions to -the system described and illustrated above may be made by -those skilled in -the a.rt without depar-ting :erom the spirit and scope of invention expressed in the following claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A thermostat for use in a building having means for cooling the building and means for heating the building, the thermostat being connected to said cooling means and said heating means and operative to generate an energizing signal for only one of said heating means or cooling means at a given time, the thermostat comprising:
means for measuring the ambient temperature within the building;
manual data entry means;
means for storing a program of desired heating temperatures over a repetitive time cycle, programmed by said manual data entry means;
a clock operative to generate time signals within said repetitive time cycle;
means for generating a signal representative of a desired heating temperature and a desired cooling temperature at the present time based upon the signals from the clock in the stored temperature program, said desired heating temperature equal to said desired heating temperature programmed for the present time, said desired cooling temperature equal to a first predetermined temperature if said desired heating temperature programmed for the present time is in a first range of temperatures, and said desired cooling temperature equal to a second predetermined temperature, higher than said first predetermined temperature, if said desired heating temperature programmed for the current time is in a second range of temperatures, lower than said first range of temperatures; and means for placing the thermostat in either a first mode wherein control signals are generated only for said heating means as a function of the difference between the measured temperature within the building and the desired heating temperature signal, a second mode wherein energizing signals are only generated for said cooling means as a function of the measured temperature and the desired cooling temperatures signals, or a third mode wherein control signals are generated for either said heating means or said cooling means based upon the measured temperature and the respective desired heating and cooling temperature signals.