CA1267953A - Electronic thermostat for heating and cooling system - Google Patents

Abstract

ABSTRACT

A thermostat for use in a building having a hot air furnace, an air conditioning system and a blower for circulating air within the building. The thermostat is connected to all three to generate energizing signals for them. An operator programs the thermostat with a series of desired temperatures over a repetitive heating or cooling cycle. A clock interrogates the program to generate the desired temperature signal for the present time which is compared to the measured temperature within the building to generate control signals for the furnace.
An energizing signal is generated for the furnace when the ambient temperature is below the desired temperature and below a predetermined minimum level.
An energizing signal is generated for the air conditioner and blower when the ambient temperature is above the present desired temperature and above a predetermined maximum level.

Description

~ .~67~53 This invention relates to thermostats L`or con-trolling both heating and air conditioning sys-telns for the same enclosed area and, more particularly, to such -thermos-tats including means for preventing rapid oscillation of the controlled temperature cluring periods of transition between low tempera-tures wl~icl require heating and high temperatures which require air conditioning. This application is a division of Canadian Patent Applica-tion Serial No. 502,244 filed February 19, 1986.
Many residences and corlllnercial buildings itl temperate climates have both hea-ting and .l;r conditioning systems. The heating systems n~e generally used during the ~inter and the ai~
oonditioning during the summer, but frequently duL~ g the spring and fall both systems are utilized clu~ g different periods of the day. 'I'hermostats previously provided for these dual function systems typically included means for storing a first temperature x~t point for the heating system l~hich may be manually adjusted to a level of say, 68 degrees, and separate means for storing a set point for the cooling syst,c which may be adjusted to a higher -tempera-ture, such as 74 degrees. If the two set points are adjus-ted tc)o close to one another the sys-tem may oscilla-te betl~eell heating and cooling modes because the temperature may overshoot the heating set point duL~ing the hea-ting mode and undershoot the cooling set pOillt during the coolirlg

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mode. this undesirable oscillutiotl ma,Y also occur ;~-: a result of thermal exchange betweell adjacent zones i.ll a multi-zone heating s~stem. If tlle set po.int.s are .~i -too widely apart, to minimi7le the possibilit~- of oscillation, the comfort of the occupants, ~ho l~o~.lld prefer a single constant temperat~lre, is diminished.
These problems are e~acerbated ~ erl multitemperature programmable thermostats of the t~pe disclosed in U.S. Patent 4,1~2,5~5 are employed Sil,-~C
the maximum temperatures that ma~ be programmed ~t a heating cycle must be limited to avoid unintentio~ ].
energization of the cooling system an~l vice-versa.
The present inverltiol1 is directed toT~ard a thermostat system f or control:L:ing both heating alld cooling systems for the same area which allows : relatively closely spaced heating and cooling set points without danger of oscillation. According to the invention, there is provide(l a thermosta-t for use ; in a building having a hot air furnace, an air conditioner system and a blo~er for circulating air within the building, the thermostat being connected to -the furnace, air conditioner, and a blower to generate energizing signals for them, the thermostat including means for measuring the ambient tcmperature within ~lle building; means for storing a program of desire(l temperatures for the building over a repetitive t i me cycle, a clock operative to generate signals representative of the presenl time; means for generating the desired temperatllre signal for i lle ~2~7953

3 --present time based upon the output of -the clock and lh( condition of the program; means for comparil1g l;he ambient temperature to the present desired temperatule;
means for genera-ting an energi~ing signal for the furnace when the ambient temperature is below the desired temperature and below a predetermined minimum level; and means for generating an energizing signal for the air conditioner and blower when the ambiellt temperature is above the present desired temperature and above a predetermined maximum level.
One version of the thermostat disclosed herein includes a programmable memory for storing a single schedule of desired -tempelatures over repetitive time cycles such as one weel~. Tlle t}lermostat incllldc~s a three position switch for placing tlle thermosta-t intc~
any one of a heating mode, a cooling mode, or an automatic modeO Whether the stored schedule is used as a heating schedule or a cooling schedule depends ul~or the position of the mode switch during the programmillg of the memory. If the mode switch ;s in automatic C)I`
heating mode during programming, the program is viel~ed as a heating cycle. Otherwise, the program is vicl~ed as a cooling cycle. The thermostat includes a clock that interrogates the memory causing it to output :~
signal representing the desired temperature at the t:iule of interrogation. This signal is compared to a si~
representing the measured temperature within t;he building to generate an on/o~f control signal for Ille furnace.

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~2~7~353 If the tllermostat is ~)rogla~ ned, Eor exnn~
while in the heating mode, the thermostat acts on the stored program as a heating schedule as long as Ille mode switch remains in the heating position. In l;~is state, the thermostat will generate energizing signals exclusively for the furnace -to raise the ambien-t temperature toward to the progra~lmed tempera-ture. If the switch is moved to the automatic mode, the thermostat will continue to operate energizing tlle furnace until the ambient temperature exceeds both t;lle temperature stored in the program and a predetermi~led maximum temperature. ~hen the ambient temperature exceeds this predetermined level, the thermostat automatically energize the air collditioning unit, alol,~
with any associated blower fan, to maintain tlle temperature at or about the predetermined ma~i nl~
temperature. If the ambient temperature thereafter falls and remains below the ma~imum temperature, -lhe thermostat returns to its normal operating state wherein the furnace is energized to maintain t}le desired temperature. To prevent oscillation of tlle 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 pre-determined time, e.g., one half hour, and vice-versa.
When the cooling season begins, the unit may be reprogrammed with a cooling cycle b~7 placing tlle mode switch in the cooling mode and reprogramming the thermostat. When -the mode switch is then returrlecl l.o automatic, the system will follow the programmf.~d cooling cycle unless the ambient tempera-ture (~I ol~S
below -the desired cooling ternperature and below a predetermined minimum temperatllre. If this occurs ";lle furnace will be energized to raise the temperatu 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 mode is determined relative to the set point value stored for the particular time withou-t regard to predetermined maximum or minimum temperatures. In this embodiment, the ambient temperature is maintained at OI`
abou-t -the ~et point value at all times during the day.
One unit disclosed herein may be progra-nmed with both a full heating time/temperature program and a :~ full cooling timejtemperature program. In the heat o]
oool mode, the appropriate time/temperature progra wil.l be opera-tive. In the auto mode, only one Or tllc programs will be operative a-t any given time and will.
control either the heating or the cooling in a norma].
manner. For example, when the unit is u~ing the : heating schedule, if the ambient temperature e~ceer1s the cooling set point stored for that period and tlle furnace has not been energizecl for a predeterm.illc~1 period of time, the unit will switch over to tlle cooling mode. A switch in the reverse direction occur under complementary condi.tions.

- 6 - 1~6~953 Other advantages and applications l~ill t)e made apparent by the following detailed description of preferred embodiments of the invention.
In the drawings, Figure 1 is a block diagram illustrating tlle operation of the main control routine of the thermostat control of the present invention.
Figure 2 is a block diagram of the opera-tlon of the automatic heat/cool control for a thermosta-t of the present invention.
Figure 3, which appears on the same sheet as Figure 1, is a block diagram illustrating t,l~e temperature adjus-tment sequence of the -thermostat of the present invention.
Figure 4 is a block diagram illustrating l,lle cool and auto heat modes of a second embodimen-t of the presen-t lnvention.
Figure 5 is a block diagram illustrating the auto cool and auto heat modes of a third embodiment, of the present invention.
The present thermostat provides for automat,ic switching between heating and cooling modes of operation ~o that a comfortable temperature ;s maintained inside a building at all times. Ill a preferred embodiment of the thermostat, a single heating or cooling schedule is programmed into tlle thermostat using any conventional means. For example, the thermostat may be programmed as described in IJ.S.
Patent No. 4,172,555. Using the programming procedure _ 7 _ ~267953 described in that patent, the primary operating mo(le ol the thermostat is first selec-ted. For example, if the unit is programmed during the winter, the heating mo(le is the primary mode of operation for the thermostat.
The thermostat is placed in the heating mode, for example, by moving a switch.
After the operating mode is selected, desired temperatures are programmed into the thermos-tat. Tlle operator enters temperature se-t poin-ts for various times during the day. Once the thermostat has bern programmed for a repetitive cycle, e.g., one week, it can then be used to heat the house accordin~ to the programmed schedule.
Figure l illustrates -the basic operation oE
the thermostat after the programming sequence has been accomplished. The operation 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 at or about the programmed set point. Thus, if -the thermostat remains in heating mode, the air conditioner will not ~)e energized even when the ambien-t temperature rises above the set point temperature.
Automatic control of the system is effectecl by placing the thermostat in automatic mode. Wherl the thermostat is placed in automatic mode, the temperalllre ~l2~79~;~

control sequence begins at step 8. The set poi~lt~
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. I~ sol a control temperature value is set equal to -the programmed temperature at step 11 and the operating mode is set to the mode in which the thermostat was programmed, at step 13. After these values have been set, or if the programmed temperature did not change at step 10, control passes to step 12. At step 12, the actual temperature in the building is compared with the con-trol temperature which was set either in step 11, as described above, or by the au-to heat/cool sequellce which is de4cribed in detail below. If the actu~l temperature is at the control temperature, -the thermostat turns off the furnace and air conditioner at step 14 and returns to the beginning of the program, step 10. If, however, the actual temperature is hi~her or lower than the desired temperature, the thermostat performs the auto heat/cool sequence described in detail below. When the au-to heat/cool sequence has been completed1 the thermostat control program returns to step 10 and continues its usual sequence described above.
Referring now to Figure 2, a preferr((l embodiment of the auto heat/cool sequence is illustrated beginning at step 20. At step 22 tlle system checks whether a sufficient amount of t;me, 1~t)9~3 _ 9 _ preferably a half hour, has elapsed since the heat/cool sequence was last -executed. This time delay prevents the thermos-tat system from rapidly switching be-tween 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, control will return to step 10 in Figure 1.
If sufficient time has elapsed since the last execution of the heat/cool sequence, the auto heat~cool sequence continues at step 24. Step 24 checks whether tlle manual override has been activated by the operator Or the thermostat. The manual override may consist, for example, of a switch on the thermos-tat which may be set to permit manual control of -the temperature in the building. If the manual override is on, the auto heat/cool sequence will not be invoked. Instead, the temperature is adjusted, as described below with reference to Figure 3, and control returns to step 10 in Figure 1.
If 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 not required for proper operation oE the system, but is - included in the preferred embodiments. In -the preferred embodiments of -the invention, the absolute maximum is set at 840F. IP the actual temperature is ~ ~i7~S3 above 84~F, the control passes to step 28 and a coo.li.r mode of operation begins, as will be described below.
If the actual temperature is below the absolute maximum value, control passes to step 30 where the actual temperature is compared to an abso]u-te minimum temperature. As with the absolute maximum temperature, the actual temperature is checked against the absolute minimum temperature in order to ensure 10 . that the tempera-ture of the building does not become dangerously low. Again, this check is not required for proper operation of the system but is included in the preferred embodiments for safety. In the preferred embodiments, the absolute minimum temperature i9 set at 540F. If the actual temperature is below the absolute minimum temperature, control passes to step 32 where the thermostat is forced into a heating mode. If -the temperature in the building is above the absolute minimum temperature, control i5 passed to step 34 where the normal auto heat/cool sequence begins.
At step 34, the system checks whether the system is presently in the cooling mode, l.e., whether the present operating mode, set either in step 13 lFigure 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 temperatllre that was programmed by the operator during -th initi~l ~2~ 53 . . \

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 furnace and air conditioning unit so that the proper temperature level is achieved.
If the actual temperature is greater than the control temperature, the air conditioner must be activated. Since the system is already in the cooling mode, as determined in ~tep 34, the syctem merely ~ branches to the adjust temperature routine at step 37 - and returns to the main routine. The operation of the temperature adjustment sequelloe is described below with reference to Figure 3.
If, however, the actual temperature in -the building is less than the control temperature, at step 36, energizing the air conditioning unit will simply lower the actual temperature further. Thus, the system must determine whether to switch from cooling mode to heating mode and begin raising the ambient temperature usin~ 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 ambient ; temperature is less than or equal to a predetermined minimum temperature. In step 38, then, when the actual temperature is less than the control temperature, tl-e actual temperature is compared to the predetermined minimum temperature which may be set, for example, at 700. If the actual temperature is above this minimum, ~2~9~

, ~ .

the auto heat ~equence is not invoked. Instead, the control temperature and operating mode simply remain at their original programmed values in steps 40 and ~2, or, if the control temperature and operating mode were changed, are reset back to their original values.
control 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 must switch from cooling to heating mode. if the unit was originally programmed in hea-ting mode, step 32, the original programmed values are simply restored for both control temperature and operating in steps 40 and 42. In this manner, the system reverts to heating mode, the programmed mode, and the furnace may be energized in the adjust temperature rou-tine described below.
If the unit was not programmed in the heating mode, the system must automatically switch from cooling to heating mode~. One difficulty encountered in making this switch is that, in the embodiment disclosed in Figure 2, only a single ~et of temperature values are stored. Consequently, the system cannot simply look-up the proper heatin~ temperature. Instead, it must derive the temperature based on the original temperature programmed by the user. In the embodiment illustrated in Figure 2, the heating temperature is derived by dividing the cooling temperatures into two temperature ranges. The first range is called the "occupied"

~26~353 ~.. ~ .

I temperature range and comprises cooling temperatures which the user would choose if -the building was occupied, for example, cooling temperatures in the range of 620 -to 74. 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~or temperatures within the first range, the system assumes that the heating temperature ~hould be set at a relatively warm temperature to maintain the comfort of the occupant~. On the other hand, if the thermosta-t 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 ~tep 44, if the programmed temperature is in the first, occupied temperature range, control passes to step 46 where the control temperature, i.e. the temperature used to control the operation of the furnace or air conditioner, is se-t -to 70 de~rees. If, however, the programmed temperature is within the second, unoccupied range of values, the control temperature is set at 64 degrees in step 48.
After the control -temperature has been set, the sys tem is placed in the heating mode at step 50 and the ambient temperature is adjusted, step ~2. Control -then passes to step 10 in Figure 1.
A similar sequence of steps is fol.lowed when the present operating mode of the sys-tem is the heatin~

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mode. In that case, at step 34, control passes to s-tep 54 where the actual temperature is compared to the control temperature. As decided above, the control temperature is either the temperature programmed in by the user of the thermostat or it is the temperature selected by the auto heat/cool sequence. If the actual temperature is less than or equal to the control temperature, the furnace is activated at step 38 and the temperature is adjusted toward the control temperature. The operating sequence then continues at step lO in Figure l.
If, however, the actual temperature is greater than the control temperature, step 54, the actual temperature is compared to a predetermined maximum temperature at step 56. As with the predetermined minimum temperature described above, the predetermined maximum temperature allows the system to decide whether it should switch from the present heating mode to a cooling mode. In the preferred embodiment illustrated in Figure 2, the switch will 04cur only when the ambient temperature is at or above the predetermined maximum temperature, 740 in the illustrated embodiment. If the ambient temperature is below 740 in step 56, the system simply resets the 2~ control temperature and the operating mode to their ori~inal programmed values, in steps 40 and 42 and returns to the main routine, in step 37, via the adjust temperature sequence.
When the temperature in the building is at or 3L2~7953 above 740 in step 56, the system ha~ determined that the ~uilding is too warm and the air conditioning unit should be activated. Therefore, at step 28, if the thermostat wa~ 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 control returns to step 10 in Figure 1.
I~ the thermostat was not initially programmed in the cooling mode, step 28, the sy~tem will be forced into the automatio oooling mode of operation and will select an appropriate cooling temperature depending upon whether the programmed temperature is in an occupied temperature range or in an unoccupied temperature range. Thus, if the thermostat was programmed in heating mode at a temperature in an occupied temperature range, e.g. 660 to 780J the system branches from step 58 to step 60 - where the oontrol temperature is set to a relatively cool temperature, e.g. 740. If, on the other hand, the programmed temperature is in an unoccupied temperature range, e.g. below 660, the control tempera-ture for air conditioning operation is set at a higher value in step 62. This permits energy savings when the wllen the building is unoccupied. After the control temperature has been set to the appropriate temperature in either step 60 or s-tep 62, -the system is placed in the cooling - 16 ~2~ 3~3 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 routine in Fi~ure 1.
Turning now to Figure 3, the thermostat, after selecting the appropriate control temperature and operatin~ mode, performs the adjust temperature routine. The routine begins at step 70 by determinin~
whether the system is to operate in heating or cooling mode. IE the operating mode i5 cooling, the actuu.l.
temperature is compared to the control temperature ;n step 72. When the actual temperature is aho~e tlle control temperature, the air conditi.oner and farl must be activated to lower the ambient temperature, step 7~.
Once the air condi-tioner 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 bacls to the main routine (Fi~ure 1) at s-tep 76.
When heatin~ mode is selected by tlle automatic heat/oool sequence, -the adjust tc~ml~emsl~ re routille branches from step 70 to step 78. Frolll st.ep 78, the system branches to step 80, -to elle~gize the furnace, when the actual temperature is below the control temperature. Control -then returns to the main routine (Figure 1) at step 76.
h5 The thermostat will operate as described above unless the user places the thermostat 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 ~.2679~3 operation as it~ primary (programmedt mode. If manual override is selected, the temperatures entered by the user will be used and the auto heat/cool sequence will not be performed.
In an alternative embodiment o~ the invention, when the thermostat switches automatically from heating to cooling or vice-versa, the control temperature is set to a temperature relative to the programmed temperature rather than at a predetermined flxed value. For example, the system mi.gh-t add 20 to : the current programmed heating temperature when choosing a control temperature in auto cool mode~ This would be accomplished, as ~hown in Figure ~, by replacing steps 58, 60 and 62 with a single step 60a which simply sets the control temperature to the programmed temperature plus 20. Likewise, when changing ~rom a programmed cooling mode to an automatic - heating mode, the control temperature would be set 20 less than the programmed temperature, step 46a, 2~ replacing steps 44, 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 building temperature at the desired temperature throughout the day. The system avoids the problem of rapid oscillation between the heating and cooling modes because time inhibition is built into the system using the time-out check in step 22. Thus, in the thermostat of -the present invention, ~i7~3~3 , ~ "

the heating -temperature and cooling -temperature may be se-t to the same value without encountering unwanted oscillation of -the system between heating and cooling modes.
In a third embodiment of the invention the thermostat may store both a heating schedule and a cooling schedule ~uch that, when the system enters the auto cool or auto heat portion of the se~uence, the system derives its control temperature from a cooling or heating schedule which has been preprogrammed by the user. In this manner, the user has complete control over the heating and cooling temperatures in the building. The selection of the control temperature during an auto oool 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. A similar replacement, step 46b, is made for steps 44, 26 and 48 in the auto heat cycle.
It can thus be seen that an elec-tronic thermostat is presented which provides for automatic heating and cooling of a building while avoiding , unwanted oscillation of the temperature control system j between heating and cooling modes or operatiGn. It is understood that various modifications to the system described and illustrated above may be made by those skilled in the art wlthou-t departing from the spirit and scope of invention expressed in the following claims.

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Claims (2)

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 a hot air furnace, an air conditioner system and a blower for circulating air within the building, the thermostat being connected to the furnace, air conditioner, and a blower to generate energizing signals for them, the thermostat including means for measuring the ambient temperature within the building; means for storing a program of desired temperatures for the building over a repetitive time cycle, a clock operative to generate signals representative of the present time; means for generating the desired temperature signal for the present time based upon the output of the clock and the condition of the program; means for comparing the ambient temperature to the present desired temperature;
means for generating an energizing signal for the furnace when the ambient temperature is below the desired temperature and below a predetermined minimum level; and means for generating an energizing signal for the air conditioner and blower when the ambient temperature is above the present desired temperature and above a predetermined maximum level.

2. The thermostat of Claim 1 including means for inhibiting generation of a control signal for one of said furnace or said air conditioner for a predetermined period of time after generation of an energizing signal for the other of said furnace or air conditioner.