Information
-
Patent Grant
-
6310330
-
Patent Number
6,310,330
-
Date Filed
Wednesday, April 12, 200024 years ago
-
Date Issued
Tuesday, October 30, 200122 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Breedlove; Jill M.
- Stoner; Douglas E.
-
CPC
-
US Classifications
Field of Search
US
- 219 501
- 219 505
- 219 491
- 219 493
- 219 490
- 337 102
- 337 41
- 337 42
-
International Classifications
-
Abstract
An improved system and method for controlling the temperature of a resistance heater in a heating, ventilation and air-conditioning (“HVAC”) system is disclosed. The control circuit and method includes three separate thermostats per heater leg and are designed to trip at three, sequential pre-selected temperatures to address certain issues associated with prior art designs for thermal overload protection. One of the thermostats is of an automatically resettable type wherein the remaining two are of a “one-shot” design and will remain open until there is human intervention.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a heater power and control circuit for use in a heating, ventilation and air-conditioning (“HVAC”) system. In particular, the present invention relates to an improved system and method for controlling the temperature of a resistance heater in an HVAC system.
Although thermal overload protection has been used for many years with HVAC resistance heaters to sense and control when the heater temperature exceeds a predetermined level, there is still room for improvement in the design and operation of such circuits. In particular, as will be explained further below, prior art designs for sensing and controlling the temperature of resistance heaters in HVAC systems may not address certain issues encountered in situations of thermal overload. Accordingly, it would be desirable to be able to provide an improved heater power and control circuit for use in an HVAC system. It would also be desirable to be able to provide an improved method for controlling the temperature of a resistance heater in an HVAC system.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved circuit and method for controlling the temperature of a resistance heater in an HVAC system, which may be composed of one or more heater elements connected in parallel. The circuit comprises: (A) a power circuit including [i] a dual thermostat having a first and second thermostat in series with each other, [ii] a first switch coupled to a first contactor for controlling the operation of the switch, and [iii] a resistance heater element coupled in series between the dual thermostat and the switch, wherein the first thermostat cycles open and closed automatically and is set to trip at a first temperature T
1
, and the second thermostat is a manual thermostat and is set to trip at a second temperature T
2
, where T
2
is higher than T
1
; and (B) a control circuit including [i] a third thermostat coupled in series with the first contactor wherein energizing and de-energizing the first contactor causes the first switch to close and open, respectively, and wherein the third thermostat is a manual thermostat and is set to trip at a third temperature T
3
, where T
3
is higher than T
2
, and wherein when the temperature exceeds T
3
, the third thermostat trips, thereby de-energizing the first contactor, causing the first switch to open in the power circuit and preventing current from being supplied to one end of the resistance heater element. When the third thermostat trips, it also opens the circuits for any other heater elements connected in parallel and the blower motor that forces air over the heater elements. As a result of this design, continual oscillation in the opening and closing of the first thermostat as a result of oscillation in the temperature above and below T
1
, but less than T
3
, is prevented by said second thermostat having a trip temperature T
2
above T
1
but less than T
3
. Further, the circuit operating the blower motor is controlled by a thermostat that controls all of the heater elements, instead of less than all of those elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like-reference numerals refer to like-parts throughout, and in which:
FIGS. 1A and 1B
are electrical schematic diagrams of prior art circuits for controlling the temperature of a resistance heater in an HVAC system; and
FIG. 2
is an electrical schematic diagram of a preferred embodiment of the HVAC heater power and control circuit in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Thermal overload protection has been used for many years with HVAC resistance heaters to sense and control when the heater temperature exceeds a predetermined level.
FIGS. 1A and 1B
illustrate two such prior art techniques for providing overload protection.
In
FIG. 1A
, heater circuit
10
includes three heater legs
11
,
12
and
13
, all The present invention provides an improved circuit and method for controlling the temperature of a resistance heater in an HVAC system, which may be composed of one or more heater elements connected in parallel. The circuit comprises: (A) a power circuit including [i] a dual thermostat having a first and second thermostat in series with each other, [ii] a first switch coupled to a first contactor for controlling the operation of the switch, and [iii] a resistance heater element coupled in series between the dual thermostat and the switch, wherein the first thermostat cycles open and closed automatically and is set to trip at a first temperature T
1
, and the second thermostat is a manual thermostat and is set to trip at a second temperature T
2
, where T
2
is higher than T
1
; and (B) a control circuit including [i] a third thermostat coupled in series with the first contactor wherein energizing and de-energizing the first contactor causes the first switch to close and open, respectively, and wherein the third thermostat is a manual thermostat and is set to trip at a third temperature T
3
, where T
3
is higher than T
2
, and wherein when the temperature exceeds T
3
, the third thermostat trips, thereby de-energizing the first contactor, causing the first switch to open in the power circuit and preventing current from being supplied to one end of the resistance heater element. When the third thermostat trips, it also opens the circuits for any other heater elements connected in parallel and the blower motor that forces air over the heater elements. As a result of this design, continual oscillation in the opening and closing of the first thermostat as a result of oscillation in the temperature above and below T
1
, but less than T
3
, is prevented by said second thermostat having a trip temperature T
2
above T
1
but less than T
3
. Further, the circuit operating the blower motor is controlled by a thermostat that controls all of the heater elements, instead of less than all of those elements coupled in parallel. Each heater leg (
11
,
12
,
13
) includes a thermostat (
14
,
15
,
16
), a resistance heater (
17
,
18
,
19
) and a switch (
20
,
21
,
22
), all coupled in series. Heater circuit
10
is connected to a power source
6
,
8
(typically 120 to 240 volts A.C.) through circuit breakers (not shown). Switch
20
is controlled by single-pole contactor
26
that includes a solenoid that is activated and de-activated through a separate control circuit (not illustrated). Similarly, switches
21
and
22
are controlled by two-pole contactor
23
that includes a solenoid that is activated and de-activated through a separate control circuit (not illustrated). Contactors
26
and
23
are connected to the “heat” side of a thermostat (not illustrated). Blower motor
25
, used to circulate hot air (or cold air when the HVAC unit is set to provide air conditioning), is turned on and off through, on the one hand, normally open switch
22
and normally closed switch
5
or, on the other hand, or normally open switch
24
. Two-pole contactor
6
, which is coupled to the “cool” side of a thermostat (not illustrated), controls switches
5
and
24
. Thus, blower motor
25
is turned on either when switch
24
is closed via contactor
6
or switches
22
and
5
are closed via contactors
23
and
6
, respectively. In the prior art circuit of
FIG. 1A
, the control circuit includes a manually resettable thermostat (i.e., a thermal cut-off) that is set to trip whenever the temperature adjacent heaters exceeds a pre-determined value T
CC
. When it does, the contactor is de-energized and it opens its respective pair of switches, requiring a technician to manually “reset” or replace the thermostat so as to allow the solenoids in contactors
23
and
26
to be energized which, in turn, closes switches
20
,
21
and
22
. Thermostats
14
,
15
and
16
in heater circuit
10
are automatically resettable thermostats and are set to trip whenever the temperature adjacent heater
17
,
18
and
19
, respectively, exceeds a pre-determined value T
PC
. After tripping, when the temperature cools down below T
PC
, the switches in thermostats
14
,
15
and
16
automatically close allowing power to be supplied to their associated heaters. Typically, the tripping temperature T
PC
of thermostats
14
,
15
and
16
in power circuit
10
are set to a temperature value lower than the tripping temperature T
CC
of the manually resettable thermostat in the control circuit associated with switches
20
,
21
and
22
. Accordingly, under conditions where the temperature adjacent a heater
17
,
18
or
19
oscillates above and below T
PC
(but lower than T
CC
), the switch in its associated thermostat
14
,
15
or
16
will also oscillate open and closed, respectively.
In light of this design, when any of the switches in thermostat
14
,
15
or
16
“sticks” closed, such “sticking” would allow current to be supplied to its respective heater
17
,
18
or
19
even when the temperature exceeds T
PC
. Under these conditions, circuit
10
is designed so that switches
20
,
21
and
22
open when the temperature adjacent its associated heater exceeds temperature T
CC
. In general, this will prevent both sides of a given heater from being energized (if its associated switch opens) and will prevent further operation of the heater even if the temperature cools back down to lower than T
CC
since contactors
23
and
26
are controlled by a manually resettable thermostat in the control circuit. However, under those conditions where a short circuit may develop in a portion of a heater (e.g., at point
29
between ends
27
and
28
of heater
18
), the opening of its associated switch
21
will not necessarily prevent current from being supplied through at least a portion of heater
18
. This is because with switch
21
open and point
29
of heater
18
shorted to ground, the portion of heater
18
between post
29
and end
27
and
28
will still be supplied with current through thermostat
15
if it is “stuck” closed. Accordingly, under this failure mode, the design of circuit
10
is such that temperatures in excess of T
CC
are not necessarily prevented by the combination of the use of two thermostats associated with each heater leg of circuit
10
and a corresponding control circuit.
A further aspect of the above design concerns the operation of blower motor
25
. In particular, the purpose of blower motor
25
is to remove the heat generated by heaters
17
,
18
and
19
when they are operating normally. However, because of the design, the same contactor
23
that cuts off power to heater
19
also turns off blower motor
25
as well. That action, however, becomes counterproductive if any of the remaining two heaters
17
and
18
remain in an “on” position generating heat while blower motor
25
has been turned “off.” Similarly, if either of thermostat
14
or
15
is “stuck” closed and a portion of their respective heaters are shorted to ground, when associated switches
20
,
21
and
22
open due to the temperature exceeding T
CC
, blower motor
25
will be switched “off” even though a portion of a heater may still be generating heat. This results in a condition where heat is not being removed from the housing.
The prior art circuit
50
illustrated in
FIG. 1B
differs in design from that shown in
FIG. 1A
in two primary respects. Thermostats
34
,
35
and
36
(i.e., thermal cut-offs), instead of being automatically resettable, are manually resettable so that if the temperature adjacent its respective heater
37
,
38
or
39
exceeds a predetermined value T
PCB
, the thermostat would need to be manually reset or replaced in order for its internal switch to be able to be closed once again and supply current to its respective heater. In addition, contactors
46
and
43
, which are controlled by a separate control circuit (not shown), include an automatically resettable thermostat (instead of a manually resettable one as in connection with
FIG. 1A
) that is set to trip at a predetermined temperature value T
CCB
. When it does, and the temperature cools down to less than T
CCB
, switches
40
,
41
and
42
are automatically closed through contactors
46
and
43
. Typically, the tripping temperature T
CCB
of the automatically resettable thermostat in the control circuit is set to a temperature value lower than the tripping temperature T
PCB
of manually resettable thermostats
34
,
35
and
36
. Accordingly, under conditions where the temperature adjacent a heater oscillates above and below T
CCB
(but lower than T
PCB
), switches
40
,
41
and
42
(as well as the one present in the automatically resettable thermostat associated with the control circuit) will oscillate open and closed. As discussed above in connection with the prior art circuit of
FIG. 1A
, under conditions where a short circuit may develop in a portion of a heater (e.g., portion
49
of heater
38
between ends
47
and
48
), the opening of the switch associated with thermostat
35
will not necessarily prevent current from being supplied to the portion of heater between points
48
and
49
through switch
41
when it is “stuck” closed (or when the switch present in the automatically resettable thermostat associated with the control circuit is “stuck” as well). Accordingly, the design of circuit
50
is also such that temperatures in excess of T
PCB
are not necessarily prevented by the combination of the use of two thermostats in connection with each heater leg of power circuit
50
and a corresponding control circuit. In addition, as discussed above in connection with the blower motor
FIG. 1A
, the design of
FIG. 1B
is such that under certain conditions blower motor
45
could be turned off through switch
42
even though one of the heaters (i.e., heater
37
or
38
) is still generating heat resulting in a situation where the blower motor is not performing its intended function of removing heat from the heater housing.
To address the issues discussed above,
FIG. 2
illustrates a preferred embodiment of the HVAC heater power and control circuit of the present invention. As explained below, circuit
100
is designed to both supply electrical power to and control the operation of a resistance heater and blower motor in an HVAC system.
In particular, circuit
100
includes a power circuit portion
120
and a control circuit portion
130
. The power circuit
120
consists of heater legs
140
,
150
,
160
, blower motor
126
and an AC power supply
101
,
102
through circuit breakers
103
,
104
, all arranged in parallel. Each of the heater legs has a switch (
141
,
151
,
161
), a heating element (
142
,
152
,
162
) and dual thermostats (
143
,
153
,
163
), all arranged in series. Each of the respective switches for each heater is in turn connected to two-pole contactors (i.e., contactor
105
/
106
to switch
161
; and contactor
107
/
108
to switches
141
and
151
). When the two-pole contactors are energized or de-energized, they will cause the switches to respectively close or open, thus controlling the supply of power to the heating elements. Initially, each switch is open so no power is supplied to the heating elements.
In the event of an unsafe operating temperature, power supplied to a respective heating element can be interrupted by either of the two associated dual thermostats
143
,
153
,
163
. Each of the dual thermostats
143
,
153
,
163
have two independent, for example, bi-metal disks which basically operate as independent electrical switches. The first thermostat switch (
144
,
154
,
164
) is set to trip at a first temperature (e.g., T
1
=145° F.) and will automatically cycle the heater on and off as the thermostat temperature drops below or rises above the first trip temperature. The second thermostat switch (
145
,
155
,
165
) is of a “one-shot” design and is set to trip at a second temperature (e.g., T
2
=180° F.). It will remain open until there is human intervention (e.g., by manually resetting the thermostat, if it is the type that is manually resettable, or by replacing the component). Dual thermostats
143
,
153
,
163
are available from, for example, Therm-O-Disc, Inc. of Mansfield, Ohio (Model Type 75TF).
The aforementioned contactors
105
/
106
and
107
/
108
are controlled by a room thermostat in control circuit
130
. The room thermostat
133
is connected in series to secondary of transformer
109
whose primary is arranged in parallel with all the elements of the power circuit. Room thermostat
133
has two settings “Heat” (side
135
of thermostat
133
) and “Cool” (side
136
of thermostat
133
) and will provide power to the two-pole contactors depending on said settings. When room thermostat
133
is set to “Heat” and reaches a certain temperature level, the two-pole contactors
105
/
106
and
107
/
108
will be energized and will close the switches for each of the heating elements, thus energizing the three heaters. In the control circuit
130
, a thermostat
134
is connected in series with the two-pole contactors
105
/
106
and
107
/
108
and the heat side
135
of the room thermostat
133
. Thermostat
134
(physically located adjacent the heater assembly) is of a “one-shot” design and is set to open at a third trip temperature (e.g., T
3
=208° F.). It will remain open until there is human intervention (e.g., by manually resetting the thermostat, if it is the type that is manually resettable, or by replacing the component). When this third thermostat
134
causes a break in the circuit, power to the two-pole contactors
105
/
106
and
107
/
108
will be cut off causing the contactors to de-energize, thus opening the switches to the three heaters (
142
,
152
,
162
). Thermostat
134
is available from, for example, Therm-O-Disc, Inc. of Mansfield, Ohio (Model Type 60T with manual reset).
A blower motor
126
is also controlled by room thermostat
133
. The motor can receive power through switches
127
and
128
or through switch
129
depending on the setting of room thermostat
133
. Initially, switch
128
is closed while the other two switches are open. These latter switches are connected to two-pole contactors (switch
127
to contactor
105
/
106
and switch
129
to contactor
131
/
132
) which are associated with the control circuit. More specifically, the two-pole contactor
105
/
106
is connected to the “Heat” side
135
of room thermostat
133
while the two-pole contractor
1311132
is connected to “Cool” side
136
. When room thermostat
133
is set to the beat side and signals for more heat, the two-pole contractor
105
/
106
will be energized and thus close switch
127
and allow power to flow to blower motor
126
. The motor will remain energized and continue to maintain air-flow as long as the thermostat control circuit keeps the switch
127
closed. This arrangement maintains the air-flow even if the temperature were to exceed the first and indeed the second trip-point of the dual thermostats (e.g., T
1
=145° F. and T
2
=180° F.). This is intended to reduce risk of rapid localized overheating. When in heating mode, blower motor
126
will not be de-energized until the third thermal overload temperature limit associated with thermostat
134
is exceeded (e.g., T
3
=208° F.). When room thermostat
133
is switched to the “Cool” side
136
, the two-pole contractor
131
/
132
will energize and thus open switch
128
and close switch
129
. Thus, motor
126
will then receive power through the switch
129
to provide for cooling.
The thermal overload protection that is provided in connection with circuit
100
works as follows. Under normal operating conditions when room thermostat
133
is set to energize heaters
142
,
152
,
162
, thermostats
143
,
153
and
163
serve to control the current through the heaters and cycle automatically at a trip temperature T
1
(e.g., T
1
=145° F.) associated with portions
144
,
154
and
164
of thermostats
143
,
153
and
163
, respectively. Under those conditions where the temperature exceeds T
2
(e.g., T
2
=180° F.), the other switch (i.e.,
145
,
155
,
165
) in thermostats
143
,
153
and
163
is designed for single operation and, if tripped, will remain open until there is human intervention (e.g., by manually resetting the thermostat, if it is the type that is manually resettable, or by replacing the component). In addition, under those conditions where the temperature exceeds T
3
(e.g., T
3
=208° F.), thermostat
134
in control circuit
130
will also trip, similarly requiring human intervention. When thermostat
134
trips, heater switches
141
,
151
and
161
not only open, but so does switch
127
causing blower motor
126
to shut off as well. Accordingly, as a result of the design, blower motor
126
remains “on” until all three elements
142
,
152
and
153
are switched “off.” Unlike the prior art, manually resettable thermal protections both below and at the maximum limit temperature substantially reduce the likelihood that the contactor will cycle and wear excessively. Such excessive cycling and wear could result in certain circumstances in situations where one of the heaters could be still generating heat while the blower motor has been turned “off.”
Thus, an improved heater power and control circuit for use in controlling the temperature of a resistance heater in an HVAC system has been disclosed. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented here for purposes of illustration and not of limitation, and that the present invention is limited only by the claims that follow.
Claims
- 1. A circuit for controlling the temperature of a resistance heater comprising:a power circuit including a dual thermostat having a first and second thermostat in series with each other, a first switch coupled to a first contactor for controlling the operation of the switch, and a resistance heater coupled in series between the dual thermostat and the switch, wherein the first thermostat cycles open and closed automatically and is set to trip at a first temperature T1 and the second thermostat is a manual thermostat and is set to trip at a second temperature T2, where T2 is higher than T1; and a control circuit including a third thermostat coupled in series with the first contactor wherein energizing and de-energizing the first contactor causes the first switch to close and open, respectively, and wherein the third thermostat is a one-shot thermostat and is set to trip at a third temperature T3, where T3 is higher than T2, and wherein when the temperature exceeds T3, the third thermostat trips, thereby de-energizing the first contactor, causing the first switch to open in the power circuit and preventing current from being supplied to one end of the resistance heater, whereby continual oscillation in the opening and closing of the first thermostat as a result of oscillation in the temperature above and below T1, but less than T3, is prevented by said second thermostat having a trip temperature above T1 but less than T3.
- 2. The circuit of claim 1 wherein the resistance heater is used in a heating, ventilation and air conditioning system.
- 3. The circuit of claim 1 wherein a blower motor is coupled in parallel across the dual thermostat and resistance heater.
- 4. The circuit of claim 3 wherein the blower motor is not powered off until the temperature exceeds T3.
- 5. The circuit of claim 1 wherein the first, second and third thermostats comprise bi-metal disc thermostats.
- 6. The circuit of claim 1 wherein the control circuit includes a transformer coupled in parallel across the power circuit.
- 7. The circuit of claim 6 further including a room thermostat coupled in series with the third thermostat and first contactor.
- 8. The circuit of claim 7 wherein the room thermostat includes a heat section and a cool section.
- 9. The circuit of claim 8 wherein the heat section is adapted to allow a user to activate the resistance heater and the cool section is adapted to allow the user to activate an air conditioner unit.
- 10. The circuit of claim 1 wherein the power circuit includes three resistance heaters coupled in parallel and each being controlled by a respective dual thermostat.
- 11. The circuit of claim 1 wherein the first temperature is about 140° C. to 160° C., the second temperature is about 170° C. to 190° C. and the third temperature T3 is greater than T2.
- 12. A circuit for controlling the temperature of a heater comprising:a power circuit including a first thermostat means in series with a second thermostat means for providing thermal overload protection, a first switching means coupled to a first contactor means for controlling the operation of the first switching means, and heater means coupled in series between the first switching means and at least one of the first and second thermostat means, wherein the first thermostat means cycles open and closed automatically and is set to trip at a first temperature T1 and the second thermostat means is a one-shot thermostat and is set to trip at a second temperature T2, where T2 is higher than T1; and a control circuit including a third thermostat means coupled in series with the first contactor means wherein energizing and de-energizing the first contactor means causes the first switch to close and open, respectively, and wherein the third thermostat means is a one-shot thermostat and is set to trip at a third temperature T3, where T3 is higher than T2, and wherein when the temperature exceeds T3, the third thermostat trips, thereby de-energizing the first contactor means, causing the first switch means to open in the power circuit and preventing current from being supplied to one end of the heater means, whereby continual oscillation in the opening and closing of the first thermostat means as a result of oscillation in the temperature above and below T1, but less than T3, is prevented by said second thermostat means having a trip temperature above T1 but less than T3.
- 13. The circuit of claim 12 wherein the heater means is used in a heating, ventilation and air conditioning system.
- 14. The circuit of claim 12 wherein a blower motor is coupled in parallel across the first and second thermostat means and resistance heater.
- 15. The circuit of claim 14 wherein the blower motor is not powered off until the temperature exceeds T3.
- 16. The circuit of claim 12 wherein the first, second and third thermostat means comprise bi-metal disc thermostats.
- 17. The circuit of claim 12 wherein the control circuit includes a transformer coupled in parallel across the power circuit.
- 18. The circuit of claim 17 further including a room thermostat coupled in series with the third thermostat means and first contactor means.
- 19. The circuit of claim 18 wherein the room thermostat includes a heat section and a cool section.
- 20. The circuit of claim 19 wherein the heat section is adapted to allow a user to activate the heater means and the cool section is adapted to allow the user to activate an air conditioner unit.
- 21. The circuit of claim 12 wherein the power circuit includes three heater means coupled in parallel and each being controlled by respective thermostat means.
- 22. The circuit of claim 12 wherein the first temperature is about 140° C. to 160° C., the second temperature is about 170° C. to 190° C. and the third temperature T3 is greater than T2.
- 23. A method for controlling the temperature of a resistance heater comprising:providing in a power circuit a dual set of thermostats having a first and second thermostat in series with each other, a first switch coupled to a first contactor for controlling the operation of the switch, and a resistance heater coupled in series between the dual set of thermostats and the switch, wherein the first thermostat cycles open and closed automatically and is set to trip at a first temperature T1 and the second thermostat is a one-shot thermostat and is set to trip at a second temperature T2, where T2 is higher than T1; and providing in a control circuit a third thermostat coupled in series with the first contactor wherein energizing and de-energizing the first contactor causes the first switch to close and open, respectively, and wherein the third thermostat is a one-shot thermostat and is set to trip at a third temperature T3, where T3 is higher than T2, and wherein when the temperature exceeds T3, the third thermostat trips, thereby de-energizing the first contactor, causing the first switch to open in the power circuit and preventing current from being supplied to one end of the resistance heater, whereby continual oscillation in the opening and closing of the first thermostat as a result of oscillation in the temperature above and below T1, but less than T3, is prevented by said second thermostat having a trip temperature above T1 but less than T3.
- 24. The method of claim 23 wherein the resistance heater is used in a heating, ventilation and air conditioning system.
- 25. The method of claim 23 wherein a blower motor is coupled in parallel across the dual thermostat and resistance heater.
- 26. The method of claim 25 wherein the blower motor is not powered off until the temperature exceeds T3.
- 27. The method of claim 23 wherein the first, second and third thermostats comprise bi-metal disc thermostat.
- 28. The method of claim 23 wherein the control circuit includes a transformer coupled in parallel across the power circuit.
- 29. The method of claim 28 further including a room thermostat coupled in series with the third thermostat and first contactor.
- 30. The method of claim 29 wherein the room thermostat includes a heat section and a cool section.
- 31. The method of claim 30 wherein the heat section is adapted to allow a user to activate the resistance heater and the cool section is adapted to allow the user to activate an air conditioner unit.
- 32. The method of claim 23 wherein the power circuit includes three resistance heaters coupled in parallel and each being controlled by a respective dual thermostat.
- 33. The method of claim 23 wherein the first temperature is about 140° C. to 160° C., the second temperature is about 170° C. to 190° C. and the third temperature T3 is greater than T2.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
3046380 |
Carlson |
Jul 1962 |
|
3606970 |
Kompelien |
Sep 1971 |
|