US2480883A

US2480883A - Control system

Info

Publication number: US2480883A
Authority: US
Publication date: 1949-09-06
Anticipated expiration: 1966-09-06

Description

Sept. 6, 1949. F. B. SCHRAMM CONTROL SYSTEM Filed Nov. 17. 1944 maw ' Patented Sept. 6, 1949 umso STATES PATENT ori-'ics CONTROL SYSTEM Frederic B. Schramm, Cleveland, Ohio Application November 17, 1944 Serial No. 563,910

Y (Cl. 237-8) 18 Claims.

My invention relates to control systems and concerns particularly heat regulating systems and`furnace control systems.

An object of the invention is to provide a heating system which operates with high fuel economy and efficiently employs various types of heaters.

Another object is to provide a heating system in which different types of fuel may be employed alone or in conjunction with each other.

Still another object is t0 provide a system in which a furnace burning one type of fuel may be employed as a stand-by for supplementing. or replacing a normally operating furnace burning the same or a different type of fuel.

A further object of the invention is to provide a system for heating a circulating medium by a plurality of sources of heat and cutting out a source whenever the heat supplied thereby is so low as to impair the efficiency of the system.

A. further object is to providev a system in which either solid or fluid fuel may be employed according to the supply available and the heat demands of the system.

A further object of the invention is to provide a heating system in which both a coalred furnace and a fluid-fuel re'd furnace are employed and the controls tend to put the greater load upon the coal-fired furnace in order to save the fluid fuel for periods of light load and times of sudden demand for extra heat. Likewise it yis an object to provide a heating system in which furnaces are employed burning different types of fuel and the furnace burning cheaper fuel or operating more efficiently at steady load tends to receive greater load.

A further object of the invention is to provide a heating system having a heater of a type which responds quickly to changes in loading and having another heater which utilizes a more abundant fuel or which `supplies heat a minimum of fuel expense for average heater demands.

Other and further objects, features, and advantages of the invention will become apparent as the description proceeds.

In accordance with my invention in its preferred form when used to supply hot water radiators, I employ a fluid-fuel fired boiler such as an oil burner or gas heater, with a second boiler provided with coal grates having combustion control by means of a blower or a damper regulator. Pipe connections are provided for connecting both the fluid-fuel boilerand the solid fuel boiler in a water supply circuit for radiators of the hot water type. Preferably means are provided for cutting a boiler out of the hot water circuit whenever such a boiler does not supply adequate heat to the system. For example, means may be provided cutting out a coal-fired boiler except when needed or for cutting out a slowly-heating boiler during its warm up period.

Temperature regulating means such as a room thermostat is provided in connection with relay means for selectively controlling combustion in the boilers and circulating the heat transfer medium, such as hot water in response to variations in room temperature and temperature of the circulating medium. Interlocks may be provided to prevent the hot water from circulating unless its temperature is above a predetermined minimum, and unless heat is being called for.

A better understanding of the invention will be afforded by the followed detailed description considered in conjunction with the accompanying drawing, and its scope will be set forth in the claims. The single ligure of the drawing is a circuit diagram schematically representing one embodiment of my invention. p

For the sake of illustration a hot water heating system is shown in the drawings comprising boilers Il and I2 adapted to supply hot water to one or more radiators I3 through a circulating system including piping I4. It will be understood, however, that the inventionA is not limited to hot water systems and may be employed in connection with heating systems utilizing any desired type of heat transfer medium or heat circulating medium such as hot water, steam or other vapors, hot air or the like. A closed system has been illustrated with a return portion I5 in the piping, ybut the invention is not limited to such a closed system. In the case of hot air, for example, the heaters il and l2 may be arranged to supply the heat transfer medium to a radiator or duct Without any similar ducts for air-return.

. The arrangement is specially adapted to cases where it is desired to employ two different types of heaters or energy transfer devices, for example, where one type of fuel is ordinarily more convenient but is not always accessible, or is more expensive than another type of fuel or when different types of fuel are not equally well adapted to the same type of combustion control. In the arrangement illustrated, the heater Il may be a type of heater in which the rate of heat transfer to the heat transfer medium may readily and quickly be changed from high to 3 low rates or completely on and olf such as in an electric heater employing current conducting resistors or in a fluid-fired heater utilizing oil, gas, or finely pulveriaed coal or the like so as to be the equivalent of a fluid fuel-fired heater.

The heater i2 on the other hand may be one which operates well under a static condition of heat transfer but which is not so rapidly transferred from higher to lower rates of heat supply' or fuel consumption, for example, as in the case of a heater utilizing solid fuel such as coal, briquets or the like having either a blower control, stoker, or a damper regulator control for varying the rate of heat transfer in accordance with the demands of the heating system. AS will be explained hereinafter, means are provided whereby one or the other of the heaters II or i 2 may have its combustion control disconnected during periods when the fuel for such a heater is not available or i-is unduly expensive. For the sake of .-illnstration it is assumed that .the fuel for the heater :i 21 is the less Vexpensive and that `the heater Il is to ybe utilized when heat demands are greater than those which can be met by the heater I2, and means to be ldescribed hereinafter are provided for causing the heater I2 to Ibe `brought up to a full load before the heater `II is caused to operate substantially continuously.

The oil burner -boiler II is provided with a .conventional combustion control schematically represented by a blower I6 driven by an elec- .tric motor ITI, and the coal-fired boiler l2 is similarly provided with a suitable combustion y control such as a damper regulator, or a Stoker .system or a simple blower represented schematically by a blower I8 and an electr-ic vdriving motor I9 for the blower I8.

For causing 4the heat transfer medium, in this case vhot water, .to be transferred quickly .from .the boilers IVI and I2 `to the radiator -or `radiators i3, a circulating pump 22I is provided `driven by an -electric motor 22. AIn `Jche arrangement illustrated in whichira .return portion I5 is provided for, the heat transfer medium .pip- .ing .or ducts, the pump 2i .may if desired be `connected in .the return portion I5 of the water circulating piping.

The heaters I I and lI2-are `both connected 4to the outgoing and return circulating pipes I4 and lHi in any desired manner'either in series or parallel as desired, and means are, provided for cutting out one of .the heaters when lit is not to be operat- .ed .for the sake of increasing the eiliciency tof the other heater. For example, in the arrangementillustrated, theboilers I-I and I2 areadapted to Abeconnectedin series. A portion of the circulating piping represented by a pipe 23 leaves the upper `portion -of the boiler I'I, joining a return lf length of piping 2d at the lower portion ofthe boiler I2 which inturnhasan outlet 25 connected to the pipe I4. For disconnecting one of the boilers, .in this case the boiler I2,.a `-by-pass valve 26 is provided -which is adapted to short .out .the boiler I2 and connect :the outlet pipe 23 .of the boiler -II directly to the pipe I4.

Inorder that the .temperature or-energy intensity level in the space occupied by the `radiator Ior energy consumer I3 'may be vmaintained substan- ,f

tially uniform, .a suitable temperature control or regulating system is provided including .a thermostat .Z1-.and a v.thermostat relay 128. Afsuitable source 4of .electric current 29 provided for'venergizing .the motors .I'I, :I9 .and .22, and rthe :theri mostat relay 28 together with any other relays or controls which may be provided.

The thermostat 2l includes a pair of contacts SI and 32 Iwhich are adapted to close when the temperature in the space occupied by the radiator I3 falls below a. predetermined level and to open when the temperature rises above that level. The thermostat relay 28 comprises an operating winding 33 connected in series with the current supply 24 ,and the contacts .3-I and 32 of the thermostat 21, and also includes pairs of contacts 34, 3.5 and 36 adapted to remain open until the relay winding 33 is energized.

The .contacts 34 are interposed in a circuit joining the circulating pump driving motor 22 to the current source 29; the contacts 35 are interposed nfacircuitconnecting the combustion control device, in this case the blower driving motor I'I of the y'oil burner II, to the `current source 29; and the contacts 36 are interposed in a circuit joining the combustion control device for the coal-.fired vboiler i2, in this case the motor I9, to the current source 29. It will be -understood that the combustion control fdevices :for the boilers II and I2, .or the `heat output control devices therefore have two or more settings, in thiscase two settings which may be called a high rate setting or a high fuel consumption setting and a low rate setting or a low fuel consumption setting represented respectively by the conditions when Vthe motors II and I9 are connected -to the current source 29 and the conditions Awhen these motors are disconnected. However, it will be understood that in case a different ltype of combustion control device or 'heat output control device for the heaters is employed, contacts .35 and 36 will be so arranged las to transfer the control devices to the high Yrate setting when the contacts '35 and 36 are respectively closed and totransfer the -control devices to the low rate setting when these Vcontacts are respectively open.

In order to avoid supplying cold -air or water or other heat transfer medium to the radiator I3 when for any reason the heaters ILI and =I 2 are not in operation .or before the heat transfer medium has acquired-a sufficient temperature for effective operation, suitable means are preferably provided for preventing the operation of the circulating motor '22 `until the 'heat transfer medium has reached a Vpredetermined temperature. For example, .a liquid temperature-responsive thermostat 3l such as one of the type sold under the trade :name Aquastat, may 'be provided which is responsive to they temperature of the heat transfer medium, namely the water in the boiler II, and has a pair of contacts 38 which are arranged to open when the temperature of the medium in the -boiler II falls below a setting such as 105 degrees Fahrenheit for example, and which are adapted to Iclose again when the temperature rises above this value, for example when it rises to a temperature such as 120 vdegrees Fahrenheit. It wiil be understood that the temperatures mentioned-are merely illustrative and that the invention is not limited to any ofthe specific temperature settings mentioned in `connection with the thermostat 37 or the other temperature control devices to be mentioned hereinafter. In view of the structural necessities of devices such as the .thermostat 31, it Vwill be understood that the contacts 138 cannot be expected `to open vand close at exactly the same :temperature level, and furthermore, it is desirable to have `a range such as the l5 .degree range mentioned between the vopening and kclosing temperatures in order to vavoid yunduly :frequent starting and stopping of the pump 2I which will obviously lower the temperature of the water in the boiler I I whenever it starts circulating.

For safety purposes and in order to prevent the unnecessary consumption of fuel when the temperature of the heat transfer medium has risen to such a value that sufficient heat is stored therein to supply the radiator I3 without further consumption of fuel, upper limit temperature responsive devices such as liquid temperature-responsive thermostats of the type sold as Aquastats 39 and 4I for example are provided which are responsive respectively to the temperatures of the heat transfer medium, in this case hot water, in the boilers li and I2 respectively. In cases where for any reason one of the heaters I I or I2 is of such a type or nature, or it burns such a fuel, that it is preferred to operate one of the heaters most of the time, the thermostats 39 and 4I are preferably arranged to operate at somewhat different maximum temperatures. Such an arrangement may be desirable, for example where one of the heaters is capable of carrying the entire load until the outside temperature falls below a certain value when the stand-by heater is brought into operation or when one of the heaters is not readily adapted to sudden changes in heat demands and the other heater is brought into operation for short periods of time when additional sudden demands for heat must be met. For the sake of illustration it is assumed that the heater. I2 is the one which operates with the cheaper fuel or which is less well adapted for sudden variations in combustion control so that the load is ordinarily to be carried by the heater I2 so far as possible. Under these circumstances the liquid temperature-responsive thermostat 4I may be set to open its contacts at a temperature slightly higher than the temperature at which the thermostat 39 opens. For example, the thermostats 39 5I may be arranged to cut-off the combustion control devices for the heaters I I and I2 at temperatures of 130 degrees Fahrenheit and 140 degrees Fahrenheit respectively. Y

The thermostats include pairs of relatively

movable contacts

42 and 43 respectively arranged to be opened by mechanism, not shown, responsive to the temperatures of the waters in the boilers II and i2 through connecting pipes 44 and 45 respectively. The contacts 42 are connected in series with the circuit to the combustion control niotor I1, and the contacts 43 are connected in series with the circuit to the combustion control motor I9 of the boiler i2.

It has been assumed for the sake of illustration that the boiler i2 is of the type which is well adapted for steady operation or a continued operation at a substantially uniform rate, and may not be so well adapted for sudden changes in temperature requirements. Accordingly, means may be provided for preventing the boiler I2 from cooling the circulating circuit of the faster operating boiler H when the heating system is turned on by the thermostat 21 unless the temperature of the water in the boiler I2 has attained an adequate value` For this purpose an additional liquid temperature-responsive thermostat 45 may be provided adapted to control an operating winding 41 for the by-pass valve 25. The thermostat 46 may include a pair cf contacts 43 adapted to open below a predetermined temperature and to close above a predetermined temperature for energizing the valve operating winding 41. For example,

the operating temperatures may be and 120 respectively.

The by-pass Valve 26 may be of any suitable type. For'the sake of illustration, it is represented asa gate valve of the two position type having a rotatable member 49 containing a right angle passage-way 5I adapted to make connections between pipes, 25 and 23 or between pipes 23l and 24 according to the angular position of the member 49. For rotating the member 49 suitable mechanism suchl as .a rack and pinion linkage' 53 may be provided controlled by the operating winding 41'throughi the medium of a magnetic plunger 54. For reasons to be explained hereinafter the plunger 54 may also be arranged for operation of normally open contacts 50.V

The passage-way 5I is adapted to communicate between the pipes 23 and an end 52 of the pipe I4 when theoperating winding` 41 is de-energized, and the passage-way 5l is adapted to communicate between the pipes 23 .and 24 .when the valve operating winding 41 is energized. The pipe 25 may make a T connection with the pipe I4.

If desired means may be provided for suspending the operation of either of the heaters II or i2 when'fuel therefor is not available. For example, cut-out switches 55 and 56 may be included in the combustion control circuits, namely in circuits of the motors I9 and I1 respectively.

It will be observed that when the by-pass Valve 25 is in the position shown with the operating winding 41 de-energized, a water circulating circuit is formed from the circulating pump 2| through the return pipe I4, the oil burner boiler il, the pipe 23, the valve passage-way 5I, the pipe-end 52, the supply pipe i4 and the radiator I3 back to the circulatingpump 2 I On the other hand when the valve operating winding 41 is energized which may be considered the normal condition, when the temperature of the water inthe boiler I2 is above a temperature of 120 degrees, for example, the valve passage-way 5I connects thepipes 24 and 23, and the circulating system includes the boiler I2 continuing from the boiler l I through the pipes 23 and 24, through the boiler I2, the pipe 25 to the radiator supply pipe I4. The thermostat 46 may be so designed for example as to open its contacts 48 at temperatures below'l05 degrees and to close the contacts at temperatures above degrees.

It is assumed for the sake of illustration that the temperature inV the space containing Vthe radiator I3 is such that heat is called for, and accordingly the contacts 3| and 32 of the thermostat 21 are closed energizing the thermostat relay winding 33 and closing the contacts 34, 35, and 36. y

An energizing circuit for the relay 28 may be traced from the upper terminal 51 of the current supply source 29 through a conductor 58 to the thermostat blade 32, thermostat contact 3|, a conductor 59, the relay winding 33, and a conductor 6I back to the second terminal 52 of the current supply source 29. The closing of the contacts 34 forms a circuit from the terminal 51 to the conductor 58 and a conductor 63 through the contact 34, a conductor 54, the contacts 38 of the thermostat 31, provided the temperature of the water in the boiler I I is .high enough, through a conductor 65, the motor 22, a conductor 66 and the conductor 6I back to the terminal 52 of the current supply 29.

The closing of the contacts 35 forms a circuit from the terminaly 51 through the conductor 58, a Vconductor 61, the normally closed contacts 42 aai-emacs o'f :the @thermostat 139', .a :conductor 8, `the relay contacts 35, a conductor 69, the combustion control motor :I1 for-theiboiler .I I, a conductor 1:I,ithe cut-out 1switch 56, a conductor 212 Aand :the conductor '.61 back to the terminal 621of.the.current source .29. isimilarily, .rthetclosing .of 4the 'contacts 3S forms 1a :circuit .from :the terminal :51 through a conductor @13, the normally `closedcontacts:113 of the thermostat 4I, a conductor 14, the control cut-out `switch :55, 'the :combustion control .motor :t9 offthe boiler i2, a-conductor 15, the contacts 56, a .conductor .16 back to :the conductor t(i1 and the terminal 52 of the current source 29.

As soon .as the motor 'il has run long enough .to bring the `tennoerature :of lthe heat .transfer medium in 'the boiler iiI to 'the proper value, the contacts '38 of 'the thermostat '3l' close permitting the operation to .take-,place as described. If the oil burner :H is not in use, assuming the switch 5.8 is open Ifor example, the thermostat contacts 38 .remain open until :the temperature of the Water rin tlre circulating system has been raised :to a suiiicientzvalue:to'aranser heat to the :radiator i3. 'Whetheror not-,the oil burner II is'in loperation, as soon as the-.temperature of the water in the coal-fired fboiler :I2 has reached the proper value fthe 'thermostat contacts 48 close, energizlngithe valve :operating winding el' to permit the circulating pump f2.2 to .drive :water .through v.the coal-lired.boiler lIZ andthe oil `burner II as well as the 'remainer of :the system. In the event-that the temperature of the heat transfer medium in the Vboiler H should beftoo low cto close the thermostat contacts 38,1the circuit -.to the circulating pump lmotor 2v2 is 'inany event closed by the `closngfo'f :the contacts 5B carried lby the plunger r54 of the valve operating winding '41. In this case `a circuit ifor the `circulating pump motor 2-2 yis formed from zthercontact :3d :through a conductor TI throughthe contacts :59, .aconductor 18 and .the conductor 55 :to the imotor22, the remainder of the-circuit-beingthe same as before. Thus it is apparent that means rare `,provided for vcausing the operation of the :circulating pump 2I -whenever thetemperature of the fheatftransfer lmedium in eithervof the boilers -IfI or 12 reaches -Tthe proper minimum value. In ,thisiway overheating of the boilers is prevented as .well as utilization of the .heat storage capacityof the-heat transfer medium whenever available. soon -asthe temperature ,of the heattransferfmedium inthe boiler I-I rises ,to the upperlimitior which theithermostat 39 is set, for example.130.degrees, the combustioncontrol .for the boiler :.L-I ris cut-,out by the opening of the-oontacts?, fand the -other 4boiler YI continues tosupplyall-oftherequired heat for-the circulating system. The boiler I2 continues in operation ,until its Athermostat contacts 43 ropenat a -temperature which `is set `above vthatof the thermostat.

In theevent-that the'boiler I2 patin-operation the valvefoper-ating winding-47 remains deenergized andthe fvalve -4Z5 remainsin-thefby-pass Ycondition sothat no heat from the oil, vburner JI is wasted inraisingthe temperature of thecoaliired .boiler AI-2, and `the :entire yheating ,requirementsofthesystem are supplied byfthe oilfburner H It will be understood `that the kapparatussuch as theo-il burner yII .maylbe 'provided .with relativelysmall water storage facilities in'viewof the fact that the combustion `fcontrols Vare easily -arranged for quick transfer fromtonitofoff operation or .between high andf1owratefoperation. On'the other hand aboiler suchas a coal-'fired boiler I2 ordinarily -is built nwith a 'bulkier and heavier :construction so that Litihasfa :larger water storage :compartment and ygreater :loss (of 'heat Lis represented by Ethe use of the oil iburner 44,I'I :to heat the Water in the boiler I2, than is `repre sented by :the use fof the `coal-iired boiler i12 to raise the temperature of .the .water in the boiler il. -In cases xwhere the heaters IIl'and :Ilareion a :par `as :to :the ,desirability .of avoiding energy loss :by .heating 4tl'lejheat transfer `medium :therein when'theneateris not-.in use, parallel connections of vthe :heaters to zthe .supply iin the .return pipes Miami iIfmay be provided in which casethe arrangement ;for cutting out one r,or the other of theboilersimay simplyitake ,the form of .afsluit-nil' valve 'interposed in the ,connection of :the :heater to the circulating Water system.

In accordance with the provisions ,of ithematent statutes, .I jhave described the principle of operation :of my invention together -ivith the apparatus which I mow believe tto represent the bestembodiment thereof, buttdesire to have it .understood that the apparatus shown and Ydescribed is only illustrative and that my invention may bancarried lout by other arrangements.

'What i claim is:

1. Aheating systemcomprising in combination a -boiler `with-an oil burner, -andavcoaleflred boiler for heating :uid circulating medium, ;a radiator for such a medium, -a lcirculator for causing the medium vto travel through ithe `said Iboilers and through the radiator, electrically operated .combustion -control devices for :each l,of said boilers, each control device having Ya flow Efue] consumption. setting and a high vfuel consumption 'setting, room temperature responsive means :for :simultaneously :tending to translate the combustion control -devices -to the high .fuel consumption settings and -for -,energizing the simulator, .means vre ysponsive to temperature of the circulating :medium for preventingoperation of :the zcirculator when the temperature of :the circulating Amediiutri is below .fa predetermined minimum temperature, means 'for transferring the combustion VVcontrol device .rior the oil burnerzto ithealow 'fuel lconsumption setting when -the temperature of thecirculating medium rises above a predetermined `maximum, means associated with thecoal-lfirediboiler for transferring :its combustion control ,device Yto the low fuel consumption setting when the ternperature of Athe medium rises above a predetermined maximum temperature higher than 4that for which the upper limit `control of the oil burner lis set and means'for cutting outthecoal-'red boiler from the circulatingsystem of the oil burner boiler and the radiator When'the temperature of the :medium -infthecoal-ired boiler :falls below a predetermined-minimum temperature.

2. A. heating system comprising-in combination aluid-fuel heateiuadapted to `heat a uid Vheat circulating medium,a solid fuel-fired heater also adapted to heat-a fluid circulating mediunna radiator 'for such a-medium, a circulator for circulating the1medium through said heaters and said radiator, a `combustion control device for each of said heaters having low fuel consumption 'and high fuel consumption settings, means responsive to ,room temperature for simultaneously tending to transfer the combustion control devices Ato .the high fuel consumption setting andto energize ,the said circulator, means for cle-energizing said circulator When'the temperature of the heattransfer medium inthe iiuid fuel-fired heater falls below a pre-determined temperature, means for transferring the combustion control device ofthe uidfuel 4heater to the low fuel consumption settingV when the temperature'of the heat transfer medium in the uid-fuel heater rises above a predetermined temperature, means for transferring the combustion control device of the solid fuelred heater to the low consumption setting when the temperature of the heat transfer medium therein rises to a still higher temperature and means for disconnecting the solid fuel-fired heater from the circulating system of the other heater and the radiator when the heat transfer medium in the solid fuel-fired heater falls below a predetermined temperature.

3. A heating system comprising in combination with a radiator and a heat transfer medium, a first heater for heatingr the heat transfer medium, a second heater for heating the heat transfer medium, and a circulator, said radiator, circulator and heaters normally being in operative relation to each other, heating-rate control devices for each of said heaters having low and high rate settings, means responsive to room teniperature below a predetermined value tending to transfer said control devices to the high rate settings and to energize the circulator for the heat transfer medium, and means responsive to temperature of the heat transfer medium below a predetermined value for diverting load to one of said heaters.

4. In a heating system utilizing a heat transfer medium and a distribution system for said medium, a heater and a combustion control device for the heater, both adapted for relatively rapid transfer from low rate to high rate combustion and vice versa, a second heater adapted for steady combustion, and a combustion control device therefor adapted for gradual transfer from low rate to high rate combustion and vice versa, means responsive to room temperature for transferring both of the combustion control devices to high rate condition when room temperature falls below a predetermined level and for transferring both heaters to low rate combustion condition when the room temperature rises above a predetermined level, means responsive to temperature ,of the heat transfer medium for transferring the combustion control device of the first heater to low rate condition when the temperature of the heat transfer medium in said heater rises above a predetermined temperature while permitting the second heater to operate at high rate conditions and means for cutting out the second heater when the temperature of the heat transfer medium therein falls below a predetermined minimum temperature the connections being such that the first heater remains atY such minimum temperature.

5. Apparatus as in the preceding claim wherein the second heater is provided with means for transferring the combustion control device to low rate combustion condition when the temperature of the heat transfer medium in the second heater rises to a predetermined temperature higher than that for which the upper limit control of the first heater is set.

6. A heater for a heat transfer medium and a combustion control device therefor both adapted for relatively rapid transfer from high rate to loi-v rate combustion condition and vice versa and a second heater adapted to be interconnected to the first heater having a relatively steady combustion condition and a combustion control device therefor adapted for gradual transfer from high rate to low rate heat combustion condition and vice versa, an upper limit control for the first heater adapted to transfer the cornbusl tion control device to low rate condition when the temperature of the heat transfer medium in the first heater rises above a given predetermined temperature and an upper limit control for the second heater adapted to transfer the combustion control device of the second heater to a low rate condition after the temperature of the heat transfer medium in the second heater has risen above that for which the upper limit control of the first heater is set.

7. A thermostat responsive to temperature in the space to be heated, a relay controlled by said thermostat, a circulator for heat transfer medium controlled by said relay, irst and second heaters connected to said circulator, a relatively fast combustion control device also controlled by said relay, a less fast acting combustion control device controlled by said relay, the first and second heaters being controlled by the first and second combustion control devices, respectively, and means for disconnecting the second heater when the temperature thereof falls below a predetermined value.

8. A heating system comprising in combination first and second heaters with electrically operated combustion control devices with electrical control circuits, a common circulating pump for said heaters with an electric motor for driving said pump and a thermostat relay having three sets` of contacts arranged to open and close together, one of said sets of contacts being connected to the combustion control device of the rst heater for opening and closing the electrical circuit thereof, another .set of said contacts being connected to the combustion control device of the second heater for opening and closing the electrical circuit thereof and the third set of contacts being connected in circuit with the circulating pump motor for opening and closing the electrical circuit thereof.

9. Apparatus as in claim 8 wherein one of the first two sets of contacts of the thermostat relay has connected in series therewith a pair of ternperature responsive contacts which open at a temperature of the circulating medium exceeding-a predetermined value whereby one of said heaters operates only when the other is incapable of carrying the load.

l0. Apparatus as in claim 8, wherein the first two sets of thermostat relay contacts each have included in series therewith temperature responsive contacts which open at two different predetermined maximum temperature values of heat circulating medium in the two different heaters ,A at which they are associated.

11. A heating system comprising in combination' with a circulating system for heat transfer medium `rst vand second heaters connected in series with'saidfcirculating system and a by-pass valve responsive to temperature of the heat circulating medium in the second heater for cutting .out the second heater from the circulating system when the temperature of the medium in said heater falls below a predetermined value.

1.2. In a heating system, the combination coinprising first and second heaters for a heat transfer medium, a common circulator for said heaters, means responsive to the temperature in either of said heaters for starting the circulator when the temperature in either of the heaters rises above a predetermined value and for shutting off the circulator when the temperature in both heaters falls below a predetermined value and means responsive to the temperature in one of the heaters for connecting it in the circuit 11 only when the temperature`- thereof is above apredetermined value;

13. In a heating system, the combination com-- prising rst and second heaters for a heat transfer medium, a common circulator for` said heaters and means responsiveA to the temperature in either of said heaters for starting the circulator when the temperature in` either of the heaters rises above a predetermined valueand for shutting off the circulator When the temperature in both heaters falls below a predetermined value.

14. In a heating system, irst and second heaters for a heat transfer medium, a common` circu lator for the heat transfer` medium` in said-heat-v ers, means responsive to the temperature in the` first heater for causing operation of the circulator when the temperature of the heat transfer medium is above a predetermined value and means responsive to the temperature in the second heater for connecting it in circuit With therst heater when the temperature: of the heat transfer medium in the second heater-rises above a predetermined value.

15. Apparat-us as in the preceding claim in which the cut-in means for the second heater includes mechanism for causing operation of the circulator simultaneouslyv with the inter-connection of the second heater.

16. A heating system comprising in. combination With a radiator and a heat transfer medium, a first heater for heating the heat transfer medium, a second heater for heating the heat transfer medium, a circulator, said radiator, circulator and heaters normally beng'in opera tive` relation to each other;l heating rate control: devices for each of said heaters having low and high rate settings, means: responsive to room temperature below a predetermined value tend.- ing to transfer said control devices to the high rate settings and to energize the circulator for the heat transfer mediums, means for normally diverting load to the secondA heater, and means responsive to temperature of the heat transfer medium in the second heater for disconnecting said heater from the-circulating system when the temperature of the heat transfer medium inthe second heater falls below a predetermined temperature.

17. A heating system comprising in combination a rst heater employing' uid fuel adapted to heat a iiuid heat transfer medium', a solid fuel-fired heater also adapted toheat a uid heat' transfer medium, a radiator for such av medium, a circulator for-circulatingl the medium through said heatersandsaidradiatc-r, a combustion control device for each of said heaters having low fuel consumption and high fuell consumption settings, means responsive to room temperature below apredetermined value for simul-A taneously tending to transfer the combustion: control' devices to the high fuel consumption'set.- ting, andv means responsive to theV temperatura of the heat transfei-medium of therst. heaterl for-transferring the control device thereo to the: 10W rate setting Wheny the temperature of the heat, transfer medium rises above a. predeterminedvalue;

18;. A heatingv system comprising in combina tion with a radiator and. a heatinansfer'medilmn. 'a first heater for heating the heat. transfer: medium, a second heater for heating the heat transfer medium, and a circulator, said radiator` circulator and heaters normally being' in oper-f ative` relation to each other, heating-rate con;- trol devices for each of said heaters having low' and high rate settings, means responsive to room; temperature below a predetermined ValueY tend:- ing to transfer said. control devices tothe high rate settings and to energize the circulator for the heat transfer medium, upper-limitV means responsive to the temperature of the heat transfer mediumof the iirst heater for transferring the control device thereof tothe low rate set ting when the temperature of the. heat, transfer medium rises above a predetermined-temperaturel and means responsive to the temperature of they heat transfer mediumy in the second` heater for transferring the control device of said heater to.l the low rate position when the temperature of the heat transfer medium in the second heater rises above a maximum temperature greatery than that for which the upper limit means of the rst heater is set.

FREDERIC'B.. SGIRAMM.

REFERENCES CITED- The following references are of record in the l'e of this patent:

UNITED STATES PATENTS Number Name Date 1,683,371' Peabody Sept. 4, 1928 1,811,378 Bannister June 23, 1931 1,884,256 Rogers et al. Oct. 25, 1932 2,009,823v Van Vulpen July 30, 1935 2,073,676 Broderick Mar. 16, 1937 2,083,612 Midyette June 15, 1937 2,129,526 Crago Sept. 6, 1938l 2,159,284 Miller May 23, 1939 2,300,011 Rose Oct. 27', 1942 FOREIGN PATENTS f Number' Country Date 243,211 Germany Feb. 6, 1912 642,180 France May 2, 1928 850,923: France Sept. 25, 1939: