JPH02233904A - Combustion control device for liquid heater - Google Patents

Abstract

PURPOSE:To decrease the temp. fluctuation of heated liquid by constituting the title device in such a way that at the changing of an air to fuel ratio when the number of burners is changed, only the supply quantity of combustion air is changed and the supply quantity of fuel is not changed. CONSTITUTION:When hot water supply is started, water flow is detected by a water flow switch 19 and ignition control is performed. When ignition is detected by a flame rod 15, heating quantity Q is determined at a temp. control part 32 and a combustion control part 33 is controlled and combustion based on temp. control is carried out. When the heating quantity Q is changed and a changing command is issued, the elapsed time (t) is counted and in case when the time is over a specified time t1, the heating quantity Q at a blower control part 34 is changed in response to the command from a changing control part 36 and the change of the revolution of a blower 12 is started. Also, a proportional valve control part 35 detects the revolution of the blower 12 and the changing of a flowing current value to a governor proportional valve 23 for fuel supply is started. Then, a timer 36a is started to count time and when a set time ta is passed, an electromagnetic valve 24 is switched over and when the changing of the revolution of the blower 12 is finished, the changing of the opening of the valve 23 is completed.

Description

Detailed Description of the Invention [Field of Application in "Industrial"] The present invention provides a liquid heater that automatically switches the number of burners in use as required, each of which is equipped with a blower for supplying combustion air. relates to a combustion control device that controls the air-fuel ratio to a predetermined state by controlling the amount of fuel supplied to the burner and the operating state of the blower when the flame shifts. [Conventional technology] For example, in a water heater as a liquid heater, in order to be able to supply a large amount of hot water as well as a small amount of hot water, for example, a control valve downstream of a control valve that adjusts the fuel supply amount is required. At the same time, the fuel pipe is branched to provide two burners, and a solenoid valve is installed in the fuel pipe to one burner, and the number of burners used can be changed by opening and closing the solenoid valve depending on the usage condition. There are some that automatically change the amount of heating from small to large. These water heaters are equipped with a blower to supply combustion air to the burner, and while the blower is driven according to the target heating amount determined based on the hot water temperature, etc., the rotation speed of the blower is detected, and the There is one that determines the opening degree of the control valve depending on the rotation speed, and supplies fuel at a constant air-fuel ratio according to the operating status of the blower. Also, when increasing the number of burners (when the flame shifts), some burners change the air-fuel ratio to make the burner rich in gas, taking into consideration the ignitability of the burner being ignited. In this case, conventionally, the blower is driven as is according to the target combustion amount, and only the opening degree of the control valve that regulates the fuel is changed. On the other hand, in order to improve the stability of the outlet temperature after changing the number of burners, for example, as proposed in Japanese Utility Model Publication No. 63-30031, the control valve } after changing the number of burners has been proposed.
Some control valves change the opening degree of the control valve at the same time as switching so that the amount of fuel supplied after switching is the same as before switching, taking into account changes in the load on the upstream side. As a result, the same combustion rate can be obtained before and after switching. [Problems to be Solved by the Invention] However, in a gas water heater that detects the rotational speed of the blower and controls the opening degree of the control valve, the control valve is opened according to the operating state of the blower, which is controlled according to the target combustion amount. Since the opening of the control valve is controlled, when the opening of the control valve is changed to change the air-fuel ratio when the flame shifts, the amount of fuel supplied to the burner will be different from the amount supplied to obtain the target combustion amount. It will be different. For this reason, there is a problem that the temperature of the hot water fluctuates when the fire changes, making it unstable. On the other hand, if the air-fuel ratio is suddenly changed when the flame shifts, the combustion state may become unstable, for example, resonance noise may occur. The present invention provides a liquid heater using multiple burners,
A first object of the present invention is to provide a combustion control device that can reduce the change in combustion amount when switching the number of burners, reduce insufficient heating amount or excessive heating after switching, and obtain a stable heating temperature.

A second object of the present invention is to provide a combustion control device that can safely and reliably carry out flame transfer and that does not cause the combustion state to become unstable during flame transfer. [Means for Solving the Problems] A first aspect of the present invention is to provide a plurality of burners for heating the liquid passing through a heat exchanger, and to connect a fuel pipe downstream of a control valve for adjusting the amount of fuel supplied. A temperature detection means detects the temperature of the liquid flowing out from the heat exchanger by branching off the fuel pipes, supplying fuel to each of the plurality of burners, and providing a solenoid valve for stopping the fuel supply in the branched fuel pipe. In the combustion control device for a liquid heater, the solenoid valve is opened and closed to switch the number of burners to be used based on the temperature, and the opening degree of the control valve is changed in relation to the opening and closing of the solenoid valve. A blower for supplying combustion air to a plurality of burners is provided, and the control valve is controlled according to the operating state of the blower, and the amount of air and fuel supplied to the burners is controlled when the number of burners to be used is switched. The air-fuel ratio changing means changes the operating state of the blower and adjusts the control valve so as to offset the change in the control valve caused by the change. The technical means is to correct the operating condition. A second aspect of the present invention is to provide a plurality of burners for heating liquid passing through a heat exchanger, and to branch a fuel pipe downstream of a control valve for adjusting the amount of fuel supplied to supply fuel to the plurality of burners. A solenoid valve for stopping the fuel supply is provided in the branched fuel pipe, and the solenoid valve is opened and closed based on the temperature detected by a temperature detection means that detects the temperature of the liquid flowing out from the heat exchanger. In the combustion control device for a liquid heater, the number of the plurality of burners to be used is changed by switching the number of the plurality of burners to be used, and the opening degree of the control valve is changed in relation to the opening/closing of the solenoid valve. and an air-fuel ratio changing means for changing the ratio between the amount of air supplied to the burners and the amount of fuel supplied when switching the number of burners used, and gradually changing the air-fuel ratio. is the technical means. [Function] In the first aspect of the present invention, when switching the number of burners, the operating state of the blower is changed by the air-fuel ratio changing means, so
The amount of combustion air supplied is changed. At this time, the control valve is controlled by canceling out changes depending on the operating state of the blower, so the same amount of fuel is supplied as when the air-fuel ratio is not changed. In the second invention, the air-fuel ratio is gradually changed, and when the air-fuel ratio for switching is reached, the solenoid valve is opened and closed to switch the number of burners. Furthermore, once the switching is completed, the air-fuel ratio is gradually returned to its original value.

[Effects of the Invention] In the first aspect of the present invention, when changing the air-fuel ratio when switching the number of burners, only the amount of combustion air supplied is changed, and the amount of fuel supplied is not changed. Therefore, since the amount of combustion does not change, there is little variation in the temperature of the heated liquid. In the second invention, since the air-fuel ratio is gradually changed when changing the number of burners, there is no sudden change in combustion conditions.

Therefore, the combustion state does not become unstable and stable combustion occurs, so no resonance noise is generated. Also heating J
1, and the temperature of the heated liquid does not become unstable. [Example] Next, the present invention will be described based on an example.

The gas combustion type water heater 1 shown in FIG.
A series of burner groups 11 are provided. Combustor case 10
A blower 12 for supplying combustion air to the burner group 11 is provided below. Soil in the burner group 11 in the combustor case 10. A water tube type heat exchanger 13 is provided on the other side, and water passing through the inside is heated by combustion heat from the burner group 11. Burner group 1 in combustor case 10
1, there are provided a suba force 14 for igniting the burner group 11 and a flame rod 15 for detecting ignition of the burner group 11. Further, one side of the combustor case 10 is provided with an exhaust port 2 for discharging combustion exhaust gas to the outside.

Two nozzle pipes 16 are provided below the burner group 11 to supply fuel gas to each burner group 11, and each nozzle pipe 16 corresponds to each burner of the two burner groups 11. A plurality of fuel injection ports 16a are provided for ejecting fuel gas. The fuel pipes 20 that lead the fuel gas from the fuel supply source to the two nozzle pipes 16 include a source solenoid valve 21 and a main solenoid valve 22 that allow the fuel gas to pass when energized, and a main solenoid valve 22 that controls the supply pressure according to the energized current. Governor proportional valves 23 that adjust the supply amount of fuel gas are provided sequentially from the upstream side, and the fuel pipe 20 branches downstream of the governor proportional valve 23, and one branched gas pipe 20a connects to one nozzle pipe 16. The other branched gas pipe 20b to which the fuel gas is directly guided is provided with a solenoid valve 24 for switching the number of burners in order to stop combustion of one of the two burner groups 11. Gas is guided to the other nozzle pipe 16 depending on whether the solenoid valve 24 is opened or closed.

A water supply pipe 17 that leads water from a water supply source (not shown) to the heat exchanger 13 includes an electric water flow control device 18 for adjusting the amount of hot water supply, and a water flow switch 19 for detecting water passing through the heat exchanger 13. The hot water supply pipes 17a downstream of the heat exchanger 13 are each provided with a hot water outlet temperature thermistor 25 for detecting the outlet temperature of the hot water flowing out from the heat exchanger 13.

The control device 30 consists of a control circuit centered on a microcomputer, and as shown in FIG. Control your water heater.

The ignition control unit 31 operates the blower 12 and each electromagnetic valve 21 in a predetermined sequence in response to a water flow signal from the water flow switch 19.
, 22 and the sparker 14 to start the ignition operation, and when the water flow signal stops, the blower 12 is stopped, and each electromagnetic valve 21, 22 is closed to stop combustion. In addition, for example, the frame rod 15 may
When the flame is no longer detected, each solenoid valve 21, 22
close to stop the fuel supply and prevent gas leakage. The temperature control section 32 controls the set temperature "l'" of the controller 40.
set and the temperature T detected by the outlet hot water temperature thermistor 25, the water passing through the heat exchanger l3 is adjusted to the set temperature 'x's.
Determine the heating amount (number) Q for heating to et. The combustion control section 33 includes a blower control section 34 and a proportional valve control section 3.
5. The temperature control unit 3 is controlled by each functional unit of the switching control unit 36.
According to the heating amount (number) Q determined in step 2, the blower 12,
Controls governor proportional valve 23 and solenoid valve 24.

Here, prior to controlling the governor proportional valve 23, the voltage applied to the blower 12 is controlled according to the determined heating amount Q, and the current of the governor proportional valve 23 is controlled according to the detected rotational speed of the blower 12. Value controlled.

In addition, in connection with switching the number of burners, the operating states of the blower 12 and the governor proportional valve 23 are changed, and
Solenoid valve 24 is controlled. In this embodiment, for the heating amount (number) Q determined by the temperature control control unit 32, the first combustion is performed in which heating is performed by only one set of the burner group 11 by using the solenoid valve 24, and the solenoid valve Darken 24 2
In the first combustion, as shown by the solid line A in Fig. 3, a combustion area is set in which hot water supply amount of No. 2 to No. 8 can be obtained. ,
In the second combustion, as shown by the solid line B in Fig. 3, a combustion region is set in which the amount of hot water from No. 4 to No. 16 can be obtained. The blower control section 34 blows air tI! according to the heating amount Q of the temperature control section 32 based on the control state of the switching control section 36. 12
Drive. Here, in the case of the first combustion, the blower 12 is driven as it is according to the heating presentation Q determined by the temperature control section 32,
In the second combustion, the control state is switched for the second combustion by the switching control unit 36! The blower 12 is driven according to the heating amount Q of the air temperature control section 32.

Further, when switching the number of burners, switching control is performed by the switching control unit 36, and the operating state of the blower 12 is gradually changed to the operating state for switching, and the supply amount of combustion air is accordingly changed, When the switching is completed, the blower 12 is again driven in accordance with the heating peak Q in each combustion state. In both the first combustion and the second combustion, the proportional valve control unit 35 basically detects the rotation speed of the blower 12 and energizes the governor proportional valve 23 with a current value corresponding to the rotation speed. As a result, even if the governor proportional valve 23 adjusts the downstream pressure in the same way, in the case of the first combustion, the solenoid valve 24 is closed, so the amount of fuel gas supplied is small, and the second During combustion, the solenoid valve 24 is open, so the amount of fuel gas supplied is greater than during the first combustion. Therefore, fuel gas corresponding to the amount of heating Q is supplied. Further, since the fuel gas is supplied in synchronization with the operating state of the blower 12, an appropriate air-fuel ratio can be maintained. Furthermore, when switching the number of burners, the switching control unit 36 performs switching control, and at this time, the change in the operation of the blower 12 that supplies combustion air that is different from the heating amount Q of the temperature control unit 32 is canceled out. The current value to the governor proportional valve 23 is corrected.

The switching control unit 36 determines the number of burners to be used according to the heating amount (number) Q determined by the temperature control unit 32 and controls the solenoid valve 24, and also controls the heating amount Q for each number of burners. Determine the voltage applied to the blower 12 according to the following. Furthermore, when switching the number of burners, air-fuel ratio change control is performed in which the control states of the blower 12 and the governor proportional valve 23 are changed to change the air-fuel ratio to a flammable air-fuel ratio.

The air-fuel ratio change control is performed in order to ensure that fire suppression is performed safely and reliably so as not to cause a sudden change in the combustion state when the fire shifts from the first combustion to the second combustion. This is to reduce fluctuations in the amount of heating at that time and stabilize the temperature of the hot water. Here, air correction information Φα for reducing the supply amount of combustion air is set for the air blowing amount information Φ, which is determined according to the heating amount Q, during fire lighting determined by the temperature control unit 32. It is sent to the blower control unit 34 and Φch-Φ, -Φα
・・・ Information on the amount of air blown at the time of fire transfer Φ so that
Determine the ch. At this time, when the blower l2 is controlled accordingly, the fuel supply JIF adjusted by the governor proportional valve 23 decreases by Fα corresponding to the air correction information Φα, corresponding to the heating amount Q at the time of fire transfer. It stops happening. For this reason, fuel correction information Fα for increasing the supply amount of fuel gas is sent to the proportional valve control unit 35 with respect to the fuel supply amount Fch determined according to the air blowing amount information Φch at the time of fire transfer, and F t = FCh10Fα ... By the word ■, corrected fuel supply tF8 is obtained, and air blower 8l1
The fuel supply amount F by the governor proportional valve 23, which is controlled according to the operation of 2, is corrected. As a result, the heating ff at the time of fire transfer
iQ. A corrected fuel supply amount F1 corresponding to the amount is supplied. Therefore, the heating 1tQ of the temperature control section 32. On the other hand, by reducing only the combustion air, the air-fuel ratio is changed and becomes gas-rich, so that reliable fire transfer is obtained, and 31 [t
Since the corrected fuel supply of 1 tFt is supplied, the water passing through the heat exchanger 13 is not heated excessively, and fluctuations in the hot water temperature can be reduced. Furthermore, if the air-fuel ratio changes rapidly due to this air-fuel ratio change control, the combustion state within the combustor case 10 changes and becomes unstable, which may cause abnormal noise such as resonance. Therefore, the switching control section 36 is provided with a timer section 36a, and in the fire transfer control, the air correction information Φα is sent to the blower control section 34 and the fuel correction information Fα is sent to the proportional valve control section 35. Instead of changing the air-fuel ratio all at once, each correction information is gradually increased in accordance with the time measured by the timer section 36a. That is, in the switching control, if ta is the set time from the start of the switching control until the solenoid valve 24 is opened and the number of burners is switched, then the air correction information Φn and the fuel correction at the elapsed unit time n after the start of the switching control. Information FT1 is Φn = Φαxtn/ta ... ■Fn = Fα
Obtained by Xtn/ta −・− ■. Therefore, the elapsed topping position time tn after the start of switching control at the time of fire transfer
The air volume information Φch in is Φch=Φ1−ΦαXt
n/-t. a...■. In addition, the supplementary fuel supply amount F1 at the elapsed time tn after the start of the switching control jump at the time of fire transfer is F1=FCh+FαXt
As shown by n/ta . In addition, in the air-fuel ratio change control, after the transition from the first combustion to the second combustion is completed, the air-fuel ratio is returned to the one before switching.
In this case, the air-fuel ratio is gradually returned to the original value as before switching. In this case, in the switching control, open the solenoid valve 24 to switch the number of burners, and then return to the original air-fuel ratio. Assuming that the setting time until the end of the switch is tb, the air correction information Φn and the fuel correction information Fn in the elapsed unit time tn after switching are as follows: on-Φ(2X (tn -tb), 'tb -
■Fn = FαX (tn -tb)/tb -
The air blowing amount information Φch obtained in step (2) in the elapsed unit time 13n after switching at the time of fire transfer is Φch - Φ1 + Φa × (tn - tb) / tb ”
■It becomes. In addition, the elapsed time of about m after the start of switching control at the time of fire transfer 1,n
M+positive fuel supply amount F1 in is F1 = Fch
Fa×(tn tb),/tb. ．． ．． As shown in ■, the elapsed unit time tn after the start of switching control at the time of fire transfer
Regardless of the heating ffiQ. The corrected fuel supply amount Ft will be according to. In this way, in the switching control, the air-fuel ratio is gradually changed to a floating air-fuel ratio over time, the solenoid valve 24 is opened when the air-fuel ratio is reached, and then the air-fuel ratio is gradually changed, Since the air-fuel ratio is returned to the normal air-fuel ratio, the combustion state will not change suddenly. Therefore, the flame can transfer smoothly and there is no generation of resonance noise. On the other hand, here, the ventilation fil
2 is controlled, and the governor proportional valve 23 is controlled according to the rotation speed of the blower 12. Therefore, when switching between the first combustion and the second combustion, the governor proportional valve 2
Change in fuel supply amount due to step 3 is delayed. As a result, if the solenoid valve 24 were to be switched at the same time as the switching control was started, during the switching transition period, the broken line C in FIG. D
As shown in the figure, since the switching is performed after the heating amount is significantly different from the heating amount before switching, the degree fluctuation in the heating amount becomes large.

Therefore, in order to perform the actual switching of the blower 12 and the governor proportional valve 23 and the switching of the solenoid valve 24 at the same timing, the timer section 36a waits until the switching of the blower 12 and the governor proportional valve 23 is started. When the above-mentioned set time ta has elapsed, the solenoid valve 24 is switched from "C". As a result, the burners are switched as shown by the broken lines E and F, so the feed fflf
The influence of response delay of il2 can be reduced.

The switching timing of the solenoid valve 24 is not only performed gradually when the number of burners increases, but also when the number of burners decreases. In this way, in this embodiment, it is possible to reduce fluctuations in the outlet temperature due to switching of the number of burners, but even so, when the number of burners is switched, Since the control system changes due to
fluctuations cannot be avoided. Therefore, when switching the number of burners is not absolutely necessary, in order to avoid switching as much as possible, the heating amount Q determined by the temperature control section 32 is changed, and in the first combustion If the heating amount Q increases and exceeds the switching number to the second combustion, in the second combustion, the switching command is set when the heating IiQ decreases and falls below the switching number to the first combustion. At this time, the duration t of this switching command is measured by the timer section 36a, and if the duration t exceeds a certain time t1, switching control is performed, and if the duration t is less than the certain time t, In this case, switching is not performed.

The Mercury control unit 37 adjusts the opening degree of the electric water flow control device 18 based on the temperature of the incoming water, and limits the flow of water into the heat exchanger 13 in excess of the heating capacity.

The operation of the gas water heater of this embodiment having the above configuration is explained in the fourth section.
This will be explained based on the diagram.

When the user operates a hot water tap (not shown) to start hot water supply, water flow is detected by the water flow switch 19, and ignition control is performed in a predetermined sequence (Step 1). When ignition is detected by the flame rod 15, the heating amount Q is determined in the temperature control section 32, and the combustion control section 33 is controlled based on it, so that combustion is performed under temperature control (step 2). During combustion by temperature control, the amount of heating Q
increases or decreases, and if a switching command is issued (YES in step 3), the elapsed time t of the praise command is measured (step 4). If the switching command is continuously issued for a certain period of time t or more (YES in step 5), switching control (step 6) is performed and a predetermined switching operation is performed. Next, the switching control in step 6 will be explained based on FIG. 5.

In response to the switching command from the switching control section 36, the heating 1tQ of the blower control section 34 that drives the blower 12 is changed according to the heating amount Q of the temperature control section 32, and the rotation speed of the blower fil2 starts to be changed. (Step 11). Further, the proportional valve control unit 35 detects the rotation speed of the blower 12 whose rotation speed has been changed, and changes the value of the current flowing to the governor proportional valve 23 according to the switching command from the switching control unit 36. 《Step 12). When the change of the governor proportional valve 23 is started, the timer 36a
starts timing, and when the set time ta has elapsed (step 13), the solenoid valve 24 is switched (step 14). After that, the rotation speed of the blower 12 is changed, and the blower 12
When the change in the rotation speed of the governor proportional valve 23 is completed (step 15), the change in the opening degree of the governor proportional valve 23 is subsequently completed (step 16). In this switching control, in the case of fire transfer, the air flow rate information Φch is gradually decreased with the elapsed unit time tn according to the relationship Φch - Φ1 - ΦαXtn/ta from the above formula (■), so that the combustion air The amount gradually decreases.

Conversely, the corrected fuel supply amount F1 is calculated from the above formula (■) by Fs
Due to the relationship =FCh+FαXtn/ta, it is gradually corrected according to the elapsed unit time 1, n after the start of switching control,
A fixed amount is supplied according to the heating amount Q. Therefore, the air-fuel ratio gradually becomes gas-rich, ensuring smooth flame transfer at the time of switching. Additionally, since the air-fuel ratio changes gradually, combustion conditions do not become unstable. Furthermore, since the amount of fuel supplied does not change when the fire changes, the temperature of the hot water does not become unstable. At this time, the heating amount Q changes as shown by the broken line E in FIG. 3 in accordance with the change in the opening degree of the governor proportional valve 23, so the rotation speed of the air blower 1s12 is changed as the start of the burner number switching operation. The difference between the amount of combustion immediately before starting and the amount of combustion immediately after the solenoid valve 24 is switched is smaller than, for example, in the conventional case shown by the broken line C, so that an excessive amount of heating is not applied due to the switching. I never get caught. Conversely, when the number of burners decreases, the heating amount changes as shown by the broken line F in FIG. 3, and therefore becomes smaller compared to the conventional case shown by the broken line D. There is no significant shortage of heating amount. If the switching command is completed in a short time and is less than the fixed time t1 (No in step 5), switching control is not performed and combustion is performed according to temperature control with the same number of burners. As described above, according to the present invention, a plurality of burners are provided,
In water heaters where the number of burners used can be changed, the amount of heating does not change excessively when the number of burners is changed, and it shows a continuous change compared to conventional systems, so there is less fluctuation in the hot water temperature when switching. Become. In addition, since the air-fuel ratio is gradually changed during flame transfer, the combustion state does not become unstable and resonance noise does not occur.

In this embodiment, only the temperature of the outlet water is detected, but the water heater may be equipped with an inlet temperature thermistor that detects the temperature of water flowing into the heat exchanger or a flow rate sensor that detects the amount of water flowing into the heat exchanger. In this example, a gas combustion type water heater was described, but
Stable hot water supply can be similarly achieved using other fuels such as oil. Furthermore, the liquid to be heated is not limited to water.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the functional structure of a control device for a gas water heater according to an embodiment of the present invention, FIG. 2 is a block diagram showing an outline of the gas water heater according to the embodiment, and FIG. A characteristic diagram showing the opening characteristics of the governor proportional valve with respect to the required heating amount in the first combustion and the second combustion of the embodiment, FIG. 4 is a flowchart for explaining the operation of this embodiment, and FIG. This is a flowchart for explaining the operation of switching control. In the figure, 13... Heat exchanger, 11... Burner group (multiple burners), 12... Blower, 23... Governor proportional valve (control valve), 20... Fuel pipe, 24... ··solenoid valve,
25... Hot water temperature thermistor (temperature detection means), 30...
...Control unit W (combustion control device for water heater), 36...Switching control section (air-fuel ratio changing means).

Claims (1)

[Claims] 1) A plurality of burners are provided for heating the liquid passing through the heat exchanger, and a fuel pipe is branched downstream of a control valve for adjusting the amount of fuel supplied to the plurality of burners. A solenoid valve for respectively supplying fuel and stopping the fuel supply is provided in the branched fuel pipe, and the solenoid valve is operated based on the temperature detected by the temperature detection means for detecting the temperature of the liquid flowing out from the heat exchanger. A combustion control device for a liquid heater that switches the number of the plurality of burners in use by opening and closing them, and also changes the opening degree of the control valve in relation to the opening and closing of the solenoid valve, wherein combustion air is supplied to the plurality of burners. an air blower for controlling the control valve according to the operating state of the blower, and for changing the ratio between the amount of air supplied to the burners and the amount of fuel supplied when switching the number of burners to be used; The air-fuel ratio changing means includes a fuel ratio changing means, and the air-fuel ratio changing means changes the operating state of the blower and corrects the operating state of the control valve so as to offset a change in the control valve due to the change. Combustion control device for liquid heater. 2) A plurality of burners are provided for heating the liquid passing through the heat exchanger, and a fuel pipe is branched downstream of a control valve for adjusting the amount of fuel supplied to supply fuel to each of the plurality of burners. , a solenoid valve for stopping the fuel supply is provided in the branched fuel pipe, and the solenoid valve is opened and closed based on the temperature detected by the temperature detection means for detecting the temperature of the liquid flowing out from the heat exchanger, and the plurality of A combustion control device for a liquid heater that switches the number of burners in use and changes the opening degree of the control valve in relation to opening and closing of the solenoid valve, comprising: a blower that supplies combustion air to the plurality of burners; A liquid heater comprising an air-fuel ratio changing means for changing the ratio between the amount of air supplied to the burners and the amount of fuel supplied when switching the number of burners used, and gradually changing the air-fuel ratio. combustion control device.