GB2027851A - Controlling burner fuel supply in fluid heating apparatus - Google Patents

Abstract

The flow rate of the fluid to be heated is detected as a pressure difference by a venturi arrangement 38, which is converted to a force by a pressure receiving unit 58 and a balance mechanism consisting mainly of a spring 83 and a lever 77. The latter applies the force to the loading spring of fuel control valve 70 so that fuel is regulated approximately in proportion to the flow rate of the fluid supply. Accordingly the rise of the temperature of the fluid to be heated is controlled to a set value despite variations in the flow rate. The pressure difference at the venturi is also used to control a valve 34 in the fluid supply line and a valve 56 in the fuel supply line. An adjustable valve 45 is provided in a line bypassing the venturi. The fluid is heated with the heat of combustion of a burner through a heat exchanger 44 such as in an instantaneous gas water heater. <IMAGE>

Description

SPECIFICATION Apparatus for controlling heating of fluid The present invention relates to an apparatus for controlling heating of a fluid, and more particularly to an apparatus for controlling a fuel supply to control the temperature of the fluid to be heated.

It is already known to supply a gas fuel in proportion to a water supply for example for water heaters. Some of such apparatus are adapted to detect the temperature of the water heated to control the gas supply, such as one including a thermistor for electrically detecting the temperature and an electromagnetic gas control valve for controlling the gas supply. Such apparatus have the drawbacks of being complex in construction and expensive. Apparatus in which the temperature is mechanically detected utilizing thermal expansion are not practically useful since the detector has a large heat capacity which leads to marked transient variations in temperature. Apparatus of another type are known which are adapted to control the gas supply in proportion to the water supply, as disclosed in Published Examined Japanese Utility Model Application No. 38623/1974.With the disclosed apparatus, the pressure difference in the water supply is held in balance with the pressure difference in the gas supply utilizing the product of a diaphragm area and level ratio, but since water has 600 to 2,000 times the weight of the gas, the diaphragm area is extremely large or the lever ratio is high, making the control apparatus large-sized. Further because the control force on the diaphragm is usually small, there is the drawback that the frictional force involved in the seal portion or lever mechanism adversely affects the proportionsl relation between the water supply and the gas supply.

Published Unexamined Japanese Utility Model Application No. 34724/1976 further discloses an apparatus including a gas supply control unit incorporating a pilot operated pressure regulator, such that the gas supply is controlled in accordance with a fine displacement of a water supply detecting diaphragm. Accordingly the apparatus requires high machining precision, has a complex construction and is difficult to use for usual small-sized household appliances.

In an apparatus for controlling heating of a fluid by which a fuel is supplied in proportion to the flow rate of the fluid to be heated to heat the fluid to a substantially constant temperature independently of the flow rate of the fluid, the main object of this invention is to control the fuel supply in proportion to the flow rate of the fluid with improved accuracy.

To fulfil this object, the invention provides an apparatus for controlling heating of a fluid comprising a detector for detecting the flow rate of the fluid in terms of a pressure difference, a pressure receiving unit for converting the pressure difference to a force, a control spring acting on a fuel pressure control unit for a burner, a lever assembly for coupling the pressure receiving unit to the control spring, and a detecting spring acting in balance with the output of the pressure receiving unit.

This invention provides, as a preferred embodiment, an apparatus for controlling heating of a fluid which is easily adjustable for use with different kinds of fuels.

The invention further provides, as another preferred embodiment, an apparatus for controlling heating of a fluid incorporating proportional correcting means for correcting the proportional relation between the flow rate of the fluid and the supply of a fuel with heat exchange efficiency considered to heat the fluid to a constant temperature.

According to another preferred embodiment, the invention provides an apparatus for controlling heating of a fluid which assures safety by limiting the amount of combustion of a burner to a preset range.

According to another preferred embodiment, the invention provides an apparatus for controlling heating of a fluid by which the ratio between the fluid flow rate and the fuel supply is easily variable to alter the temperature to which the fluid is to be heated.

Two embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a view in vertical section showing an apparatus for controlling heating of a fluid embodying the invention; Figure 2 is a graph showing the characteristics of a venturi tube and a pressure receiving unit; Figure 3 is a graph showing the characteristics of a lever assembly and a fuel pressure control unit; Figure 4 is a graph showing the relation between the pressure difference in the venturi tube and the gas supply pressure; Figure 5 is a graph showing the relation between the water supply and gas supply; Figure 6 is a view showing another embodiment of the invention; Figure 7is a view taken along the line VII-VII in Fig. 6 and showing a lever assembly; Figure 8 is a graph showing temperature control performance;; Figure 9 is a graph showing thermal efficiency characteristics; Figure 10 is a graph showing the relation between the water supply and the gas supply when the thermal efficiency is corrected; and Figure ii is a graph showing the characteristics of the lever assembly and the fuel pressure control unit when the thermal efficiency is corrected.

Fig. 1 shows an instantaneous gas water heater embodying this invention. Water is supplied from a water pipe 1 to a venturi tube 4 having a throat 2 and a diffuser 3 continuous therewith, then heated by a heat exchanger 7 and run off from a faucet 8. The pressure of the water upstream from the throat 2 of the venturi 4 is detected at a high pressure outlet 5, and the pressure of the water at the throat 2 is detected at a low pressure outlet 6, and the pressures are fed to the pressure receiving unit 1 3 and gas valve operating means 29 to be described later.

Gas is supplied from a gas pipe 9, passed through a gas valve 10, then controlled by a fuel pressure control unit 11 to a supply pressure in accordance with the water supply and thereafter burned by a burner 12. The pressure receiving unit 13 comprises a main diaphragm 14, two balance diaphragms 1 5, 1 6 having a smaller diameter than the main diaphragm 14 and arranged concentrically therewith, and an operating rod 1 9 for fixedly holding these three diaphragms 14, 1 5, 1 6 together. One of the pressure chambers defined by two of the three diaphragms and provided therebetween is a high pressure chamber 1 7 coupled to the high pressure outlet 5. The other pressure chamber is a low pressure chamber 1 8 and is connected to the low pressure outlet 6.The balance diaphragms 1 5, 16 are exposed to the atmospheric pressure on their outer sides. A detecting spring 20 acts on the upper end 1 9a of the operating rod 1 9 in the direction of from the low pressure chamber 1 8 toward the high pressure chamber 17. The operating rod 19 is in engagement, at a bearing portion 19b, with an operating lever 21. By virtue of the arrangement of these elements relative to a lever pin 22, the displacement of the operating rod 19 is enlarged at the free end of the operating lever 21.

The fuel pressure control unit 11 comprises a valve disk 24 for varying a valve opening 24 in the gas flow channel. The valve disk 23 is fixed to a fuel diaphragm 25. A control spring 26 has one end acting on the diaphragm 25 in the direction to open the valve disk 23 and the other end bearing against a pressure adjusting screw 27 screwed into the inner side of a slider 28 which is slidable in the opening and closing directions of the valve disk 23. The pressure adjusting screw 27, when turned, optionally varies the load of the control spring 26 acting on the fuel diaphragm 25, independently of the displacement of the operating lever 21.The pressure of the gas flowing into the fuel pressure control unit 11 acts both on the diaphragm 25 and on the valve disk 23, but since the effective area of the fuel diaphragm 25 to be subjected to the gas pressure is designed to be approximately equal to the area of the valve opening 24, the force acting on the diaphragm 25 to close the valve 23 and the force acting on the valve 23 itself to open the valve are substantially equal and in ipposite directions, with the result that the gas pressure will not produce a force that would move the disk 23. On the other hand, the pressure of gas, controlled when the gas has passed through the valve opening 24 as a supply to the burner 12, acts on the valve 23 to close the valve. The valve 23 remains stationary at the position where this pressure comes into balance with the force of the control spring 26 acting to open the valve disk 23.Thus the supply pressure to the burner 1 2 is automatically controlled in accordance with the load of the control spring 26. When the slider 28 remains stationary, the load of the control spring 26 will not alter, so that if the pressure of inflowing gas alters, the valve disk 23 moves in accordance with the variation as is the case with usual gas pressure regulators, thus maintaining the gas supply pressure to the burner 1 2 substantially constant.

The slider 28, when shifted, gives an altered load to the control spring 26, varying the gas supply pressure.

The gas operating means 29 is connected to the high pressure outlet 5 and the low pressure outlet 6 of the venturi tube 4, such that the pressure difference therebetween produces a force to open or close the gas valve 10 on the gas circuit.

A description will now be given of the proportional relation between the water supply and the gas supply according to this invention. The relation between the water supply Ow at the venturi in Fig. 1 and the pressure difference Pw involved is as follows: Pw = k,Qw2 (1) in which kw is a constant dependent on the shape of the venturi tube 4 and the kind of the fluid.

Since the pressure difference APw is fed to the high pressure chamber 1 7 and the low pressure chamber 1 8 of the pressure receiving unit 13, the force Fw produced by the unit 1 3 is as follows: Fw = (S - s)APw (2) wherein S is the effective pressure receiving area of the main diaphragm 14, and s is the effective pressure receiving area of the balance diaphragms 1 5 and 1 6.If AS = S - s, the operating lever 21 will be shifted by a variation of force ASAPw. Assuming that the distance from the lever pin 22 to the position 1 9 b where the operating rod 19 is in engagement with the operating lever 21 is X, the distance from the lever pin 22 to the position where the lever 21 is in contact with the slider 28 is Y, the displacement of the operating lever 21 due to a variation #Qw of the water supply is Ax at the position 1 9 b and dy at the position of contact between the lever 21 and the slider 28, the spring constant of the detecting spring 20 is K, and the spring constant of the control spring 26 is k, there is the following relation:: ASAPwX = AxKX + AykY (3) if the operating lever 21 is a rigid body, there is the relation of: Y Ay - = -&alpha; (4) X Ax wherein a is referred to as "lever ratio." From Equation (3) and Equation (4),

#S#Pw = + &alpha;k) #y (5) Ay is the variation of the force of the control spring 26 and is further the variation APg of the gas supply pressure to the burner 12. Suppose the effective pressure receiving area of the fuel diaphragm 25, namely the area of the valve opening 24 is A, APgA=Ayk (6) Accordingly AS k APg=- .APw (7) A K - + ak a Assuming that integration constant is C, AS k Pg=- . Pw+C Pw + C (8) A K -- + &alpha;k &alpha; If Pg is controlled to zero when Pw = 0, c=0 AS k Pg = - . Pw (9) A K -+ &alpha;k a Generally there is the following relation between the gas pressure Pg and the gas supply Qg.

Pg=kgQg2 (10) wherein kg is a constant dependent on the shape and dimensions of the gas nozzle of the burner 12, and on the kind of the gas.

From Equations (1), (2) and (10),

Thus the gas is supplied at the rate of Qg in proportion to the water supply Qw.

Fig. 2 shows the relation between the water supply Qw and the pressure difference Pw in the venturi 4, namely Equation (1), and the relation between the pressure difference Pw in the pressure receiving unit 1 3 and the force Fw, namely Equation (2). Fig. 3 shows the relation of the force Fw produced by the pressure receiving unit 1 3 with the displacement Ax of the operating rod 19 and with the displacement Ay of the lever 21, Fig. 3 further showing the variation APg due to Ay. Fig. 4 shows the relation between the gas supply pressure Pg and gas supply Og, namely Equation (10). Fig. 5 shows the relation between the water supply Qw and the gas supply Qg.

The displacement of the operating lever 21 actually varies the effective pressure receiving area of the diaphragms, and mechanical frictional forces adversely affect the proportional relation of Equation (1) especially at low rates of water supply. Additionally at low rates of gas supply, the burner 1 2 undergoes incomplete combustion, or the flame will not be transferred from burner to burner in the case where a large number of burners are used. Accordingly it is desirable to stop the supply of gas in the range of small water supplies. Thus the differential pressure in the venturi tube is fed to the gas valve operating means 29 to cause the gas valve 10 to stop the gas in the range of low water flow rates.

Since the weight of water differs greatly from that of gas as already mentioned, water inherently differs from gas in pressure differences involved. The pressure difference given by the venturi tube 4 should be at least 1,000 mm Aq. even at low water supply rates to assure accurate detection, whereas the gas supply pressure to the burner 1 2 is usually up to 50 to 250 mm Aq. if highest and must be controlled to 5 to 25 mm Aq. in the range of low gas supply rates. Thus the Pw to Pg ratio expected in Equation (9) should be 40 to 200. In Equation (9), the decrease of AS means a reduction in the effective pressure receiving area of the diaphragm of the pressure receiving unit 1 3 and results in reduced detection precision, hence undesirable.

There is a limitation on the increase of A since the fuel pressure control unit 11 will then become larger. It is therefore impossible to reduce AS/A greatly. However, the spring constants of the two springs can be altered easily. The spring constant is variable by varying the coil diameter or wire diameter without enlarging the spring while ensuring accuracy. A lever can be made with high precision if the lever ratio a is not large. Consequently k K + ark a can be reduced by suitably selecting the spring constants K and k of the two springs, and the lever ratio a. Especially, since the ratio of the spring constants can be increased to 1 ,000 or more with ease, the Pw to Pg ratio of 40 to 200 is available with high precision by a simple construction.

Fig. 6 shows another embodiment of the invention. A water supply pipe 31 is in communication with a water supply control unit 32. Water flows into a pressure chamber 34 at a rate controlled by a valve 33. A diaphragm 35 and a cap 36 define a pressure chamber 37, to which the pressure of a low pressure portion of a venturi tube 38 is applied through a bore 39.

Accordingly the pressures of the chambers 34 and 37 act on the opposite sides of the diaphragm 35, which is therefore subjected to the pressure difference of the venturi tube 38.

The force resulting from the pressure difference is in balance with the load of a water pressure adjusting spring 40 provided within the pressure chamber 37, operating the water supply control valve 33 which is held against the diaphragm 35 by a spring 41. The water regulated by the control valve 33 flows into a pressure chamber 42 and then dividedly into the venturi tube 38 and a bypass 43. The throat of the venturi tube 38 produces a pressure difference in the water flowing into the tube 38. The pressure of the low pressure portion is fed to the pressure chamber 37 as already mentioned. The water through the venturi tube 38 passes through a heat exchanger 44 connected to the tube 38, whereby the water is heated. The water flowing from the pressure chamber 42 into the bypass 43 passes through a regulating valve 45 and then through a bypass pipe 46 and flows into a mixing portion 47 at the outlet of the heat exchanger 44. The pressure chamber 42 is provided with a diaphragm 48 which, along with a cap 49, defines a pressure chamber 50. The pressure of the low pressure portion of the venturi tube 38 is applied to the chamber 50 via a pressure duct 51. The diaphragm 48 provides gas valve operating means 52 for opening or closing a gas valve 56 in accordance with the water supply through the venturi tube 38. A valve stem 54 cooperative with a pressure receiving plate 53 opens the gas valve 56 against the force of a valve closing spring 55. A pressure receiving unit 58 includes a diaphragm 59 and a casing 60 which define a pressure chamber 61.The pressure of the pressure chamber 42 is applied to the pressure chamber 61 through a pressure duct 57. The pressure of the low pressure portion of the venturi tube 38 is fed, through a pressure duct 64, to a pressure chamber 63 formed by the diaphragm 59 and a casing 62. Gas flows into a gas inlet 65, then passes through the gas valve 56 into a pressure chamber 67 of a fuel pressure control unit 66, is subjected to pressure control by a diaphragm 68, a control spring 69 and a valve disk 70, flows through a gas pipe 71 and jets out from a burner nozzle 72. The gas is mixed with air and burns in a burner 73. The pressure to which the gas is controlled by the valve disk 70 is determined by the intensity of the force of the control spring 69. The spring 69 has one end engaged by a pressure adjusting screw 74 which is screwed in a slider 75.The screw 74 is set for the desired pressure when the unit is assembled. After the pressure setting, the screw 74 is attached to the slider 75 with an adhesive. The slider 75 is slidable in a cylinder portion of a cap 76. As seen in Fig. 7, a control lever 77 is supported by a lever pin 78 on a portion 62a of the casing 62 and has one end bearing on an operating rod 80 movable with a pressure receiving plate 79 which is cooperative with the diaphragm 59 of the pressure receiving unit 58. The other end of the lever 77 bears on a pin 81 projecting from the slider 75 of the fuel pressure control unit 66. A spring 82 acting on the pin 81 presses the slider 75 against the opering lever 77 at all times. Accordingly when water is passed through the venturi tube 38, the diaphragm 59 is displaced against the action of a detecting spring 83.

The displacement of the diaphragm 59 is delivered through the operating rod 80 and the operating lever 77 to the slider 75, which in turn compresses or stretches the control spring 69, consequently altering the pressure of gas supply to the burner 73. The operating lever 77 and the portion 62a of the casing 62 are formed with a bore 78a other than the bore for passing the lever pin 78 therethrough. When the lever pin 78 is inserted into the bore 78a, an altered lever ratio can be obtained. Additionally the operating lever 77 is provided with a screw 84, which when coming into contact with the casing 62, prevents the operating lever 77 from turning further counterclockwise in Fig. 6. Thus even when the water supply is very small, the gas supply will not become lower than the level set for the lowest amount of combustion.

Furthermore, the pressure receiving plate 79 on the diaphragm 59 has a projection 79a. When the projection 79a is in contact with a portion 62b of the casing 60, the operating lever 77 will not turn further clockwise in Fig. 6 even if the pressure difference on the diaphragm 59 increases. As a result, even when the water supply increases greatly, the gas supply will not increase beyond the level set for the largest amount of combustion.

Basically the embodiment of Fig. 6 operates in the same manner as the embodiment of Fig.

1. The differential pressure produced by the venturi tube 38 displaces the diaphragm 59 of the pressure receiving unit 58 by an amount in balance with the force of the detecting spring 83 to turn the operating lever 77 about the lever pin 78. This movement causes the slider 77 to alter the force of the control spring 69 to control the gas supply. The regulating valve 45, when shifted axially thereof, varies the resistance to the fluid flowing into the bypass 43. While the water flows from the pressure chamber 42 dividedly into the venturi tube 38 and into the bypass 43, the regulating valve 45, when operated, also varies the ratio of the divided flows.

Since the pressure receiving unit 58 is driven by the differential pressure given by the venturi tube 38, the variation of the flow ratio between the venturi tube 38 and the bypass 43 alters the pressure difference of the venturi tube 38 relative to the total water supply and therefore alters the ratio of the gas supply relative to the total water supply, consequently varying the temperature of the heated water available at the mixing portion 47. The regulating valve 45 thus serves as a temperature setting valve. When the opening degree of the regulating valve is fixed, the variation of the water supply, of course, will not alter the flow ratio between the venturi tube 38 and the bypass 43 but affords a gas supply in proportion to the water supply.If the regulating valve is opened to a larger degree, the water flow through the bypass 43 increases, affording hot water of relatively low temperature at the mixing portion. Conversely when the regulating valve 45 is closed, hot water is available at a relatively high temperature.

Owing to the relation between the amount of hot water available and the rise of the temperature of the water, a larger amount of water is available at a relatively low temperature than at a higher temperature. Fig. 8 is a graph showing the relation between the water supply and the rise of the temperature of water. Curve A represents maximum output, Curve B represents controllable minimum output, and the space between Curves A and B indicates the range of output available. Line H indicates a high temperature setting with a temperature control range of bc. Line M indicates a medium temperature setting with a temperature control range of D-g.

At a low temperature setting of Line L, large amounts of water are available, but there is the necessity of reducing pressure losses involved in the water channel for an increase in the output.

Since the pressure losses in the water channel of instantaneous water heaters occur predominantly in the venturi tube 38 and the heat exchanger 44, bypassing these elements is effective in mitigating pressure losses. A poinf kin Fig. 8 represents a maximum quantity of water when the venturi alone is bypassed, and a point K a maximum when both the venturi tube 38 and the heat exchanger 44 are bypassed. After the water supply has been stopped, the water within the heat exchanger 44 will be heated owing to the heat capacity of the heat exchanger 44 and to the presence of a pilot burner (not shown), with the likelihood that hot water will jet out from the heat exchanger 44 when water is supplied again, but this is avoidable since cold water through the bypass 43 and the bypass pipe 46 is admixed with the hot water at the mixing portion 47.

With a water supply exceeding the point g on Line M of the medium temperature setting, the burner is at the upper limit of its capacity, delivering water at a reduced temperature of point h.

From the viewpoint of temperature control, this is not desirable; the water supply should be limited to the point g. For this purpose, the differential pressure of the venturi tube 38 is applied to the diaphragm 35; the water supply control valve 33 of the water supply control unit 32 controls the water supply to a level not exceeding the point g. When the temperature setting is changed, the water supply control point automatically alters. Thus the venturi tube 38 involves the same pressure difference at the point con Line H of high temperature setting and at the point g although the total water supply differs as will be apparent from Equation (9).

Since this differential pressure is given to the diaphragm 35, the water supply is controlled to the points C and g. Briefly the venturi tube 38 maintains the same pressure difference on Curve A. The amount of water controlled by the valve 33 automatically varies with the variation of the temperature setting by the regulating valve 45. Thus a set temperature is given with stability without an increase in the amount of water.

The lever ratio of the operating lever 77 is variable by inserting the lever pin 78 into the bore 78a. As well known, town gas and natural gas are supplied to the burner at different pressures because of the difference in properties. The present apparatus is usable for such different fuels without necessitating complex adjustment but easily by varying the lever ratio a in Equation (11), namely by shifting the lever pin 78.

If the gas supply is proportional to the water supply and the heat exchange efficiency is constant, the water will be heated to a constant temperature since the amount of heat absorbed by water is in proportion to the water supply. The heat exchange efficiency, however, varies with the fuel-to-air ratio (excess air factor). With reference to Fig. 9 showing the relation between the amount of combustion and the heat exchange efficiency, Line A represents the relation as established by a usual Bunsen burner of the atmospheric type without controlling the excess air factor, and Line B is the relation as determined with the excess air factor controlled to a constant value.With the combustion device having the heat exchange efficiency characteristics of Line A, heat is supplied to water at a rate higher than the rate of increase of the water supply, whereas with Line B, the supply of heat is at a rate lower than the rate of increase of the water supply. For this reason, the amount of heat absorbed by water is not proportional to the variation of the water supply. This makes it impossible to heat the water to a constant temperature. The temperature can be raised to a constant level independently of the water supply by varying the proportional relation of the fuel supply to the water supply for correction in view of the heat exchange efficiency.Thus in the case of the heat exchange efficiency of Line A, the gas supply is made slightly larger than the proportional relation when the water supply is small as represented by Line A in Fig. 10 to compensate for the reduction in the heat exchange efficiency, while in the case of the heat exchange efficiency of Line B in Fig. 9, the gas supply is made slightly smaller than the proportional relation when the water supply is small as represented by Line B in Fig. 10 to correct the increase in the heat exchange efficiency. The characteristics of Fig. 10, can be realized by establishing the relation of Pg = PgO when Pw = 0 in Equation (8). Stated more specifically, the detecting spring 83 is made shorter to give a displacement of x0 even in the absence of the pressure difference as seen in Fig. 11. The displacement x0 is enlarged by the operating lever 77, giving a displacement of Yo to the slider 75 and affording a gas supply pressure Pug,. Since the gas valve operating means 52 is out of operation at this time, there is no gas supply. From Equation (11), the gas supply relative to the water supply is:

Because the gas supply is made larger than the proportional relation when the water supply is small, while gas is supplied nearly in proportion to the water supply when the water supply is large, variations in the temperature of the hot water available due to variations in the heat exchange efficiency can be corrected. In the case of the heat exchange efficiency characteristics of Fig. 9, Line B, use of a longer detecting spring 83 produces an effect reverse to the foregoing, giving the characteristics of Fig. 10, Line B. Alternatively the same correction can be realized by the use of an adjusted control spring 69, namely by providing the spring 69 with a greater force, whereby the gas supply pressure PgO shown in Fig. 11 is available.

Although the embodiments described are instantaneous gas water heaters adapted for use with water as the fluid to be heated and gas as a fuel, any fluids and fluid fuels are usable. The invention is not limited to the illustrated embodiments, therefore.

Claims (11)

1. An apparatus for controlling heating of a fluid comprising a burner, a heat exchanger, a detector for detecting the flow rate of the fluid in terms of a pressure difference, a pressure receiving unit for converting the pressure difference to a force to give output, a detecting spring acting in balance with the output of the pressure receiving unit, a fuel pressure control unit for controlling fuel supply pressure to the burner, a control spring acting on the fuel pressure control unit and a lever assembly for coupling the output portion of the pressure receiving unit to the control spring, whereby the fuel supply pressure is controlled in relation with the flow rate of the fluid to be heated.

2. An apparatus as defined in claim 1 wherein the detecting spring has a greater spring constant than the control spring.

3. An apparatus as defined in claim 1 which is provided with a fuel stop valve which is openable and closable by the pressure difference detected by the flow rate detector.

4. An apparatus as defined in claim 1 which is provided with means for adjusting the load of the control spring.

5. An apparatus as defined in claim 1 wherein the lever assembly transmits a force in a variable ratio.

6. An apparatus as defined in claim 1 wherein the flow rate of the fluid to be heated and the fuel supply are controlled in a substantially proportional relation with each other, and one of the pressure receiving unit, the lever assembly and the fuel pressure control unit is provided with means for correcting the proportional relation in accordance with a variation of heat exchange efficiency relative to the amount of combustion of the burner and the flow rate of the fluid.

7. An apparatus as defined in claim 1 wherein one of the pressure receiving unit, the lever assembly and the fuel pressure control unit is provided with engaging means for regulating the amount of combustion in the burner to a preset maximum or minimum value.

8. An apparatus as defined in claim 7 wherein the engaging means for regulating the maximum amount of combustion is provided so as to limit the displacement of a diaphragm included in the pressure receiving unit.

9. An apparatus as defined in claim 7 wherein the engaging means for regulating the minimum amount of combustion is provided so as to limit the displacement of the lever assembly.

10. An apparatus as defined in claim 1 which is provided with a bypass for bypassing both the flow rate detector and the heat exchanger, and the bypass includes a flow regulating valve, whereby the pressure difference to be delivered from the flow rate detector is altered relative to the overall flow rate of the fluid to be heated.

11. An apparatus as defined in claim 1 which is provided with a flow rate control unit for the fluid to be heated, the flow rate control unit being controllable by the pressure difference given by the flow rate detector.

1 2. Apparatus for controlling heating of a fluid by controlling the supply of fuel to a burner substantially as described herein with reference to and as illustrated in Figs. 1 to 5 of the accompanying drawings.

1 3. Apparatus for controlling heating of a fluid by controlling the supply of fuel to a burner substantially as described herein with reference to and as illustrated in Figs. 6 to 11 of the accompanying drawings.