US2266210A - Off-peak water heating system - Google Patents

Description

Dec.-l6, 1941. JULIAN ETAL 2,266,210

OFF-PEAK WATER HEATING SYSTEM Filed Oct. 12, 1940 2 Sheets-Sheet l INVENTORS Arthur E .C/U/Idfl BY Clark Osterhe/d ATTORNEY A. E. JULIAN ET'AL Filed Oct. 12 1940 2 Sheets-Sheet 2 INVENTOR 2 Arthur Edu/ian flfirk fgOster/w/a AT T ORNEY OFFPEAK WATER HEATING SYSTEM Dec. 16, 1941.

Patented Dec. 16, 1941 OFF-PEAK WATER HEATING SYSTEM Arthur E.-Julian, Chicago, 111., and Clark M.

Osterheld, Stoughton, Wis., assignors to Mc- Graw Electric Company, Elgin, 111., a corporation of Delaware Application October 12, 1940, Serial No. 360,888

11 Claims.

Our invention relates to water heating systems and particularly to.ofl-peak water heating systerms.

An object of our invention is to provide relatively simple means for ensuring heating of water in a hot water tank during the oiT-peak period, means being included in said system to cause prolongation of the heating in case the tank contains less than a predetermined amount of hot water at the end of an ofi-peak period.

Another object of our invention is to provide means whereby continuation of heating of the water in a hot water tank beyond the end of an off-peak period is efiected by thermal control means, in case the tank contains less than a predetermined amount of hot water at the end of an off-peak period.

Another object of our invention is to provide a water heating system wherein the heater will be deenergized during an oiT-peak period after having been energized for some time, in case all of the water in the tank has been heated to a given temperature and which will cause reenergization of the heater during an off-peak period in case of th withdrawal of appreciable quantities of hot water from the tank during that period.

Other objects of our invention will either be apparent from a description of several forms of systems embodying our invention or will be pointed out in the course of such description and set forth in the appended claims.

In the drawings,

Figure 1 illustrates our system as applied to a domestic hot water tank, which latter is shown in vertical section,

Fig. 2 shows a modified form of system embodying our invention, and

Fig. 3 shows a still further modification of electric water heating system embodying our invention. 1

Referring first to Fig. l of the drawings, we have there illustrated generally only a timing device ll. comprising a continuously operative synchronous motor l3 electrically connected to supply circuit leads l5 and I1. A worm gear I9 is mounted on th motor shaft and engages a worm wheel 2| which latter is mounted on a suitable shaft 23. The shaft 23 has also mounted thereon a cam disc 25 which has two different outer peripheral radii. The off-peak period of any day of twenty-four hours corresponds to a part of the periphery of cam disc 25 which has a larger radius and which is designated by 21 in this figure or th drawings. It is to be understood that while we have illustrated a particular peripheral length of portion 21 as being of substantially eight hours in duration, our invention is not limited thereto but it is to be understood that any desired length of off-peak portion 21 of the cam disc 25 may be provided in accordance with the ideas of the engineers operating th utility supplying the electric current for energizing this system. If it is assumed that the duration of an oiT-peak period is substantially eight hours, it may be further assumed that the cam disc 25 is shown in the position which it will occupy at 10 oclock in the evening so that the oiT-peak period may be considered to end substantially at 6 oclock in the morning. It is further to b understood that the cam disc may be adjustably mounted on the shaft 23 so that the ofi-peak period may be started at any time desired by the operating company, may extend for any desired duration and may end at any time desired. While we have shown the motordriven continuously operative timer in outline only, it is to be understood that all such details of constructionas are necessary to cause the device to operate in the desired manner are to be provided but since most of these are mechanical only and constitute no part of our invention, we have not deemed it necessary to illustrate and describe such elements.

We provide a hot water tank 29 having an inlet pipe 3! for cold water connected thereto at the bottom end thereof and an outlet pipe 33 for hot water connected thereto at the top thereof. The tank may be surrounded by a mass 35 of suitable heat insulating material and an outer metal casing 31 may also be provided, all in a manner well known in the art, as these details are shown for illustrative purposes only and constitute no part of our invention.

An electric heater 39 is shown generally only as being mounted in any suitable or desired manner against the outer surface of the lower part of tank 29 and may be positioned within a tunnel 4| and reference may here be made to copending application Serial No. 284,881, filed July 17, 1939, by Clark" M. Osterheld, in which a clamp-on flexible heater is disclosed and claimed. We desire it, however, to be understood that our invention is not limited to the use of a heater of this particular kind and we have, therefore, shown the heater generally only.

A mechanically actuable main heater control switch 43 may include a relatively rigid contact arm 45 and a resilient contact arm 41 normally yieldingly biased out of engagement with arm 45 but moved into engagement therewith when rotation of the cam disc 25 causes a lug 48, of electrio insulating material, on contact arm 41 to be engaged by the portion 21 which has a larger outer radius than the other part of cam disc 25. Arm 45 is connected by a conductor 49 toone terminal of the electric heater 39 while the other terminal of heater 39 is connected to a substantially rigid contact arm of a thermally actuable switch 53 which includes in addition to the arm 5!, a resilient contact arm 55 normally yieldingly biased into engagement with contact arm 5! but moved out of engagement therewith by an expansion rod 57 mounted in and supported by a tubular member 59 which is secured in a water-tight manner to the tank 29 in a suitable opening in its wall so as to extend through one wall thereof into the tank to be surrounded by the water in the tank at that point. It will be noted that we have illustrated the heater 39 as being located near the bottom of the tank with the thermal switch 53 positioned above the heater but we desire it to be understood that the position of the thermal switch 53 is to be so selected that it will not open the circuit of the heater 39 controlled thereby by moving contact arm 55 out of engagement with contact arm 5|, until substantially all of the water in the tank 29 is hot. conductor 5! to the supply circuit conductor IT. We desire it to be understood also that while we have shown, generally only, a particular form of thermally actuable switch responsive to the temperature of the water in the tank at a predetermined point therein, we do not desire to be limited to such construction and that we may use any other type of thermally actuable switch controlled by tank water temperature without going beyond the limits of our invention.

We provide an electromagnetic switch or contactor 53 including a coil 65, an electromagnetic armature core 57, a pair of fixed contact members 69 and a movable contact bridging memher ll connected with core member 61 to be moved thereby into contact with contact members 59 and to be held in engagement therewith as long as coil 55 is traversed by an electric current. A conductor '13 connects contact arm 5| to one terminal of the coil '55 of contactor 63 while the other terminal of coil 55 is electrically connected to one of the fixed contact members 69 which contact member is connected by a conductor to conductor 35. The other fixed contact member of contactor 53 is connected by a conductor H to supply circuit conductor l5 and therefore also to contact arm 47 of the timercontrolled switch 33.

Let it be assumed that there is an appreciable amount of cold water in the bottom part of tank 25 at the start of an ofi-peak period, this amount being sufficient to surround the tubular member 59 with cold water so that contact arm 55 will be in engagement with contact 5|. It is evident that when portion 21 of cam disc 25 has been moved by synchronous motor l3 into substantially the position shown in Fig. 1 of the drawings, it being understood that discs 2| and 25 are rotated in a clockwise direction, contact arm 41 will be moved into engagement with contact arm :35 and since contact arms 55 and 5| are in engagement with each other, heater 39 will be energized through a circuit starting with supply circuit conductor l5, passing through engaged arms 41, 45, conductor 59, heater 39, engaged Contact arm 55 is connected by a the other supply circuit conductor l7. At the start of an ofi-peak period and coincident with engagement of arms 11 and 45 and the establish-- ment of a flow of current as just above described, contactor 63 will be energized, the circuit being substantially as follows: From supply circuit conductor [5 through the engaged contact arms 61 and 45 of switch 83, apart of conductor 59, conductor 15 to fixed contact member 59, through coil 65, through conductor i3, through the engaged contact arms 5| and 55 and from there through conductors! to the second supply circuit conductor I1. It is to be noted that this is a contactor-energizing circuit to magnetize armature core 61 with resultant upward movement thereof and engagement of contact bridging member H with the two spaced fixed contact members 59. This, it will be noted, establishes a shunt circuit relatively to the timercontrolled switch 53 but this has no other efiect upon the system at this time. A holding circuit for the coil 65 may be traced substantially as follows: From supply circuit conductor I5 through conductor TI, fixed contact members 69 and contact bridging member H, coil 55, conductor 13, contact arms 55 and 5| and from there through conductor 3| to the other supply circuit conductor ll.

Let it be assumed that tank 29 was almost full of hot water at the start of an off-peak period but that energization of heater 39 was efiected as has been hereinbefore described. Let it be assumed further that substantially all of the water in the tank was heated to a desired temperature within a few hours, so that temperature-responsive switch 53 caused disengagement of contact arm 55 from contact arm 5! sometime during an off-peak period, for example, at say 2:00 a. m. with resultant deenergization of the heater. The electromagnetic contactor 83 would also be opened because its holding circuit was interrupted by disengagement of the contact arm 55 from contact arm 5i, but timer-controlled switch 33 would still remain in closed position. If, now, sudden withdrawals of hot water from the tank occur say at 3:00 a. m., that is, before the end of that off-peak period, to such an extent that tubular member 59 is surrounded by cold water, the hereinbefore described circuit through heating element 39 would be reestablished and at the same time contactor 63 would be moved to the position where contact bridging member 1! would be in engagement with fixed contact members 69. Let it be further assumed that a relatively large amount of hot water was withdrawn so that the energized heater 39 would not provide enough heat during the remaining part of the oil-peak period and an appreciable portion of the Water in the tank was still cold at the end of the oil-peak period, disengagement of contact arms 41 and 35 from each other at the end of that off-peak period because of the movement of portion 21 out from under the lug E8 on arm 41 would not cause deenergization of the heater since its heating circuit now is substantially as follows: From supply circuit conductor l5 through conductor H, cooperating contact members 69 and H, conductors l5 and 59, through heater 39, through the engaged contact arms 5i and 55 and through conductor 6| to the other supply circuit conductor ll. It is further evident that energization of the heater beyond the end of the ofi-peak period will continue for as long as the tubular member 59 of the thermal arms 5| and 55 and then through conductor 6| to switch 53 is surrounded by cold water and it may be here pointed out that such energization may continue during a shorter or a longer portion of the following part of the day after the end of an off-peak period, in case additional withdrawals of hot water take place before all of the water in the tank is hot. It is noted here that while reenergization of the heater 39 will occur during an off-peak period in case of first a deenergization of the heater because all of the water in the tank is hot and in case of withdrawal of hot water from the tank, but this is not the case after the end of an oiT-peak period. It may further be pointed out that in this modification of an off-peak water heating system embodying our invention, a prolongation of the energization of the electric heater is effected in case the tank is not full of hot water.

Referring now to Fig. 2 of the drawings, we have there illustrated a modified system embodying our invention which differs somewhat from that shown in Fig. 1 of the drawings, more particularly that prolongation of the energization of the electric heater 39 beyond the end of an off-peak period will be effected in case the amount of hot water in the tank at the end of an off-peak period is approximately less than half the total amount of water in the tank. When a water heating system of the kind embodying our invention is applied to an ordinary domestic hot water tank whose height is several times that of its diameter, the hot water will be located in the upper part of the tank and any cold water in the tank will be found in the lower part thereof with a more or less well defined line of demarcation between the hot and the cold water. The description to be given hereinafter of the system shown in Fig. 2, as well as that shown in Fig. 3, refers more particularly to a tank approximately half full of hot water, but we desire it to be understood that this is a general statement and that, within the scope of our invention, we may so arrange auxiliary thermally controlled elements that prolongation of the heating of the water beyond the end of an off-peak period will occur after the tank is less than one-third full of hot water or is less than two-thirds full of hot water at the end of an off-peak period.

Those parts of the system illustrated in Fig. 2

of the drawings which are the same as those in the system shown in Fig. 1 of the drawings are provided with the same reference numerals and we may point out that we provide a continuously operative timer H, a tank 29, a tank heater 39, a main thermally actuable heater control switch 53 positioned near the bottom of the tank, an electromagnetic heater control switch 63, but in addition thereto we provide an auxiliary thermally actuable switch 19 which includes a tubular member 8|, an expansion rod 83 therein engaging, when expanded, a resilient contact arm 85 which is normally yieldingly biased into engagement with a relatively rigid contact arm 81. In general, this auxiliary switch is substantially the same as the main thermal switch 53 and the same comments as to the use by us of a different form of thermally controlled switch apply here also. We have shown the thermally actuable switch 19 as located approximately at half the height of the tank but do not desire to be limited thereto as it may be located either higher up or lower down in the tank and where we have used the words half full of water or similar words, it is to be understood that this can be changed to correspond to the ratio of hot or cold water in the tank corresponding to the fixed position of the auxiliary thermally actuable switch 19. Contact arm 5| is connected by a conductor 89 to contact arm 31 and contact arm is connected through a conductor 9| to one terminal of coil 65 of contactor 63, the other connections of the contactor being as hereinbefore described.

Let it be assumed that timer-controlled switch 43 is in the position shown in Fig. 2 of the drawings, having been moved into this position by engagement with portion 21 of cam disc 25 at, say, 10:00 p. m. In case of the presence of an appreciable amount of cold water in the tank sufllcient to surround tubular member 59 at the start of an off-peak period, contact arms 55 and 5| will be in engagement with each other and an energizing circuit through heater 39 will be established, it being noted that timer-controlled switch 43 and tank water temperature-controlled switch 53 are connected in series circuit relation relatively to each other. Let it be assumed that there was only a relatively small amount of cold water in the tank at the start of an off-peak period and that all of the water in the tank became hot on or before 2:00 a. m. when switch '53 would interrupt the energizing circuit through heater 39 to cause deenergization thereof. If withdrawals of hot water were effected at say 3 or 4 oclock in the morning, that is, during the off-peak period, switch 53 would again close, provided that an appreciable amount of hot water was withdrawn, with resultant reenergization of heater 39.

Let it be assumed that withdrawal of hot wa ter from the tank was effected to such an extent that the tank was less than two-thirds full of hot water, less than one-half full of hot water, or less than one-third full of hot water in accordance with the position of the auxiliary thermal switch so that the auxiliary thermally actuable switch 19 was surrounded by cold water just before the end of an off-peak period. Contact arms 85 and 81 would then be in engagement with each other and would close an energizing circuit through coil 65 of contactor 63 substantially as follows: From supply circuit conductor |5 through engaged contact arms 41 and 45,

through conductors 49 and 15 to contact member 69, through coil 65, conductor 9|, through engaged contact arms 85 and 81 of thermal switch 19, through conductor 89 and engaged contact arms 5| and 55 and through conductor 6| to the other supply circuit conductor IT. A holding circuit for coil 65 is also established, extending substantially as follows: From supply circuit conductor I5 through conductor 11, through contact members 69 and 1|, coil 65, conductor 9|, engaged contact members 85 and 81, conductor 89, engaged contact arms 5| and 55 and through conductor 6| to the other supply circuit l1.

If, now, this condition was brought into existence just before the close of an off-peak period, that is, withdrawals of hot water from the tank occur to such a degree that the upper auxiliary thermal switch 79 was subjected to cold water, and if this continued to the end of the off-peak period when contact arms 41 and 45 moved out of engagement with each other, the electromagnetic contactor 63 would remain in heater circuit closing position, this circuit exfollows: From supply l5 through conductor 11, and H, 49, heater 39, enand 55 and from there tending substantially as circuit conductor through engaged contact members 69 through conductors l5 and gaged contact members 5| of the main thermal switch 53 through conductor to the supply circuit conductor II. It is obvious that when the auxscribed holding, circuit of contactor 93 and deenergizationof the heater 39. Whereas in the system shown in Fig. l of the drawings prolongation of the heating efiect of the energized heater 39 on the water in the tank continues until the tank is full of hot water, our system shown in Fig. 2 of the drawings is effective to continue energization of the electric heater in case the tank is less than one-half full of hot "water at the end of an ofi-peak period or less than some other predetermined portion as described hereinbefore.

Referring now to Fig. 3 of the drawings, we have there illustrated a still further modification of an oiT-peak water heating system embodying our invention in which a series type of electromagnetic contactor coil may be used. Such parts of the system as are the same are provided with the same reference numerals as in Figs. 1 and 2 of the drawings. In this systern the relatively rigid contact arm 45 is connected through a conductor 93 to one of the fixed contact members 95 of an electromagnetic contactor 91 which includes in addition to a coil 99, a magnetizable core member IOI which has connected thereto a contact bridging member I03 adapted to be moved into engagement with contact members 95 upon energization of coil 99. One terminal of coil 99 is connected to that fixed contact member 96 which has conductor 93 connected thereto while the other terminal of coil 99 is connected by a conductor I to contact arm 55 of the main thermally actuable switch 53. An auxiliary thermally actuable switch I0I includes a tubular member I09, an expansion rod III therein adapted to engage-and move a resilient contact arm II3 normally yieldingly biased into engagement with a relatively rigid contact arm II5. Contact arm H3 is connected by a conductor II! with the other fixed contact member 95 of contactor 91 while contact arm H5 is connected by a conductor II9 to contact arm 4? of the timer controlled switch 33.

Let it be assumed that the main thermally actuable switch 53 is subjected to the temperature of cold water in the lower part of the tank so that contact arms 55 and 5! are in engagement with each other at the start of an oifpeak period when contact arm M is moved into engagement with contact arm 95, as shown in Fig. 3 of the drawings. An energizing circuit through the heater willbe established as follows: From supply circuit conductor I5 through engaged contact arms 4'! and 45, through conductor 93 and through coil 99, conductor I05, through engaged contact arms 55 and 5I to the heater 39 and from there through conductor 6I to the other supply circuit conductor IT. This will cause energization of heater 39 and attendant heating of the cold water in the lower part of the tank The energized coil 99, traversed by the current flowing through heater 39 as above described, causes energization of armature core. ml with resultant upward movement thereof and engagement of contact bridging member I03 with contact members 95. This energized condition of the coil 99 will continue for as long as current flows through heater 39. In case all of the water in the tank is heated to a temperature suflicient to cause disengagement of contact arms 55 and SI, deenergization of the heater 39 is effected, should all of the water in the tank be heated before the end of an ofl-peak period. In case hot water is thereafter withdrawn from the tank, to an appreciable extent, the main thermal switch 53 may be moved to circuit closing position and the heater 39 will be reenergized.

If it be assumed that the amount of hot water withdrawn during the latter part of an off-peak period was so much that the auxiliary thermally actuable switch I0'l was subjected to cold water, prolongation of the energization of heater 39 would result through a circuit substantially as follows: From supply circuit conductor I5 through conductor II9, through engaged contact arms H5 and H3, through conductor II'I, through engaged contact members and I03, through coil 99, through conductor I05, through engaged contact arms 55 and SI, through heater 39 and through conductor SI to the other supply circuit conductor IT. This energization of heater 39 would continue until the amount of hot water in the tank would be sufiicient to immerse tubular member I09 of switch I0'I in hot water, when contact arm I I3 would be moved out of engagement with contact arm II5 with resultant interruption of the energizing circuit through heater 39. The same comments as regards the position which auxiliary thermally ac tuable switch I01 may occupy made hereinbefore with regard to auxiliary switch I9 apply also to switch I 07.

It is therefore evident that the water heating system embodying our invention as herein described in its several modifications provides for start of the energization of the electric heater of a hot water tank at the start of an off-peak period. means responsive to the temperature of a predetermined part of water in the tank or to the temperature of water in a desired point in the tank to eiTect prolongation of energization of the tank heater beyond the end of an off-peak period, this energization continuing until a predetermined part of the water in the tank, which may be either the whole contents or a desired part thereof, has been heated to a predetermined tempielrature which we have designated broadly as ot.

While We have illustrated and described certain embodiments of our invention now preferred by us, we, do not desire to be limited thereto and any further modifications clearly coming within the scope of the appended claims are to be considered as being covered thereby.

We claim as our invention:

1. An oiT-peak water heating system for a hot water tank having a single electric heater adjacent to the bottom of the tank, comprising a continuously operative timing means, a heater control switch adapted to be held closed by said timing means during an oiT-peak period, a thermal It provides further thermally controlled heater control switch in series circuit with said timer-controlled switch subject to water temperature and maintained in closed position until the tank is full of hot water and auxiliary heater control switching means controlled jointly by the timing means and by thermal means subject to the temperature of the water in the tank to cause continuation of the energization of the heater beyond the end of the off-peak period in case the thermal means subject to the temperature of th water in the tank is subject to relatively cold water at the end of an oil-peak period.

2. An off-peak water heating system for a hot Water tank having a single electric heater adjacent to the bottom of the tank and comprising a continuously operative timing means and a plurality of electrically connected switching means controlled separately and jointly by the timing means and by the tank water temperature for causing energization of the heater at the start of an off-peakperiod in case the tank contains an appreciable amount of cold water at the start of an off-peak period, for causing continuation of the energization of the heater beyond the end of the off-peak period in case the tank contains less than a predetermined amount of hot water at the end of an oiT-peak period and for causing deenergization of the heater when the tank contains more than said predetermined amount of hot water.

3. An olT-peak water heating system for a hot water tank having a single electric heater adjacent the bottom of the tank and comprising a continuously operative timing means and a plurality of electrically connected switching means controlled separately and jointly by the timing means and by the tank water temperature for causing energization of the heater at the start of an oiI-peak period in case the tank contains an appreciable amount of cold water at the start of an off-peak period, for causing continuation of the energization of the heater beyond the end of the off-peak period in case the tank contains less than a predetermined amount of hot water at the end of an oiI-peak period, for causing deenergization of the heater during an off-peak period when the tank contains more than said predetermined amount of hot water and for causing reenergization of the heater during the oiI-peak period in case withdrawals of hot water from the tank reduce the amount of hot water in the tank to less than said predetermined amount.

4. An off-peak water heating system for a hot water tank comprising an electric heater, a control switch therefor, a continuously operative timer for causing closing of said control switch at the b inning of an off-peak period and holding it closed for a predetermined length of offpeak time, and thermally and electromagnetically controlled switching means to cause continuance of the energization of said heater beyond the opening of the timer-controlled switch until the temperature of substantially all of the water in the tank is at a predetermined value.

5. An off-peak water heating system for a hot water tank comprising an electric heater, a control switch therefor, a continuously operative timing means for causing closure of said switch at the beginning of an oiT-peak period and for holding it closed for a predetermined length of off-peak time, a normally closed thermally-eontrolled switch connected in series circuit with said first named switch for causing deenergization of the heater when substantially all of the water in the tank has been heated to a given temperature and an electromagnetic switch connected in parallel-circuit with said timer-controlled switch to cause continuation of heating of the water in the tank by said heater after opening of the timer-controlled switch at the end of the oil-peak period until substantially all of the water in the tank has been heated to said given temperature.

6. An off-peak water heating system for a hot water tank comprising an electric heater, 9. control switch for said heater, a continuously operative timing means for causing closing of said switch at the beginning of an off-peak period and for holding it closed for a predetermined length of off-peak time, a thermally-controlled switch for said heater subject to the temperature of water in the tank, an electromagneticallycontrolled switch connected in parallel-circuit relation with said timer-controlled switch, circuit connections between all of said switches to cause closure of said electromagnetically-controlled switch on closure of said timer-controlled switch and to cause opening of the thermally-controlled switch and of the electromagnetically-controlled switch when the temperature of all of the water in the tank is above a predetermined value irrespective of the operative position of the timercontrolled switch.

7. An off-peak water heating system for a hot water tank having a single electric heater located adjacent to the lower end of the tank and comprising a continuously operative timing means, a heater control switch adapted to be held in heater-energizing position by said timing means during an off-peak period, a thermally-actuable heater control switch cooperating with said timer-controlled switch to efiect energization of the heater at the start of an off-peak period in case of the presence of an appreciable quantity of cold water in the tank at the start of an 01!- peak period and thermally-controlled switching means for ensuring continuation of the energization of the heater beyond the end of an off-peak period in case of the presence of a predetermind quantity of cold water in the tank at the time of movement of the timer-controlled switch to heater-deenergizing position.

8. Anon-peak water heating system for a hot water tank having a single electric heater adjacent to the bottom of the tank and comprising a continuously operative timing means, a heater control switch adapted to be held in heaterenergizing position by said timing means during an off-peak period, a thermally actuable heater control switch cooperating with said timer-controlled switch to effect energization of the heater at the start of an ofl-peak period in case or the presence of an appreciable quantity of cold water in the tank at the start of an off-peak period and a thermally-controlled electromagnetic switch electrically connected in shunt circuit relation with said timer-controlled switch for continuing the energization of the heater beyond the end of the off-peak period in case of the presence of a predetermined quantity of cold water in the tank atthe end of an off-peak period, said energization continuing until the tank contains appreciably less than said predetermined quantity of cold water.

9. An off-peak water heating system for a hot water tank having a singl electric heater located near the lower end of the tank, comprising a supply circuit for said heater, a continuously operative timer, means including an electromagnetic switch in said circuit for controlling the energization of said heater, a plurality of switches controlled respectively by tank water temperature and by said timer and electric connections between said plurality of switches, the coil of the electromagnetic switch and the supply circuit ahead of said timer controlledswitch for causing energization of said heater at thestart of an ofi-peak period in case said switch controlled by tank water temperature is subject to relatively cold water and to continue energization of the heater beyond the end of an off-peak period in case said switch controlled by tank water temperature is subject to relatively cold water at the end of an off-peak period.

10. An off-peak water heating system for a hot water tank having a single electric heater located adjacent the bottom end of the tank, comprising a supply circuit for said heater, a continuously operative timing means, a heater control switch in said circuit adapted to be held 11. AnofiE-peak water heating system for a hot water tank having a single electric heater located near the lower end of the tank, comprising a supply circuit for said heater, a continuously operative timer, means including an electromagnetic switch in said circuit for controlling the energization of said heater, a plurality of switches controlled respectively by tank water temperature and by said timer and electric connections between said plurality of switches, the coil of the electromagnetic switch and the supply circuit ahead of said timer controlled switch for causing energization of said heater at the start of an off-peak period in case said switch controlled by tank water temperature is subject to relatively cold water, for causing deenergization of the heater during an ofi-peak period when the switch controlled by tank water temperature is subject to relatively hot water, for causing reenergization of the heater during an oiT-peak period in case withdrawals of hot water from the tank cause the switch controlled bytank water temperature to be subject to relatively cold water and for causing continuation of the energization of the heater beyond the end of the ofi-peak period in case the switch controlled by tank water temperature is subject to relatively cold water at the end of an oiT-peak period.

ARTHUR E. JULIAN.

CLARK M. OSTERHELD.

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