JPH0532652B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a liquid heater that automatically switches the number of burners equipped with a blower for supplying combustion air as necessary. The present invention 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 during the time of flame transfer.

[Prior Art] For example, in a water heater as a liquid heater, in order to be able to handle a large amount of hot water supply as well as a small amount of hot water supply, 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 a small amount to a large amount of heating.

Such 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., for example, the rotation speed of the blower is detected, There is a system in which the opening degree of the control valve is determined according to the rotation speed, and fuel is supplied at a constant air-fuel ratio according to the operating state of the blower.

Also, when increasing the number of burners (when the flame shifts), consider the ignitability of the burner on the side that is lit.
There are some that change the air-fuel ratio to become gas rich. In this case, conventionally, the blower is driven as it is according to the target combustion amount, and only the opening degree of the control valve for regulating the fuel is changed.

On the other hand, in order to improve the stability of the hot water temperature after changing the number of burners, for example,
As proposed in Publication No. 30031, the opening degree of the control valve is adjusted so that the amount of fuel supplied after switching is the same as before switching, taking into account the change in the load on the downstream side of the control valve after switching the number of burners. There are some things that change at the same time as switching. As a result, the same amount of combustion 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 becomes different.

For this reason, when the fire changes, the temperature of the hot water fluctuates,
There is a problem with instability.

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.

In a liquid heater using multiple burners, the present invention provides combustion control that reduces the change in combustion amount when switching the number of burners, reduces excess heating due to insufficient heating amount after switching, and provides stable heating temperature. The first objective is to provide an apparatus.

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. The fuel pipe is branched to supply fuel to each of the plurality of burners, and a solenoid valve for stopping the fuel supply is provided in the branched fuel pipe, and the temperature detection means detects the temperature of the liquid flowing out from the heat exchanger. A combustion control device for a liquid heater that opens and closes the electromagnetic valve to switch the number of burners in use based on the detected temperature, and changes the opening degree of the control valve in relation to the opening and closing of the electromagnetic valve, A blower for supplying combustion air to the plurality of burners is provided, and the control valve is controlled according to the operating state of the blower, and when the number of the plurality of burners to be used is switched, the amount of air supplied to the burners and the amount of fuel are controlled. The air-fuel ratio changing means changes the operating state of the blower, and the air-fuel ratio changing means changes the operating state of the blower.
The technical means is to correct the operating state of the control valve so as to offset the change in the control valve caused by the change.

A second aspect of the present invention provides a plurality of burners for heating the liquid passing through the heat exchanger, and branches a fuel pipe 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. In the combustion control device for a liquid heater, the number of burners to be used is changed by opening and closing the plurality of burners, and the opening degree of the control valve is changed in relation to the opening and closing of the solenoid valve, wherein combustion air is supplied to the plurality of burners. A blower for supplying air 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 to be used, and gradually changing the air-fuel ratio. Use technical means.

[Function] According to the first aspect of the present invention, when the number of burners is changed, the operating state of the blower is changed by the air-fuel ratio changing means, so that 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 reaches the switching air-fuel ratio, the electromagnetic 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 is performed, so that resonance noise is not generated. In addition, it has little effect on the amount of heating,
The temperature of the heated liquid does not become unstable.

[Example] Next, the present invention will be explained based on examples.

The gas combustion type water heater 1 shown in FIG. 2 includes two burner groups 11 in which a plurality of burners are arranged in two rows in a combustor case 10 that forms a combustion chamber. A burner group 11 is located below the combustor case 10.
A blower 12 is provided for supplying combustion air to the combustion chamber. Burner group 11 in combustor case 10
A water tube type heat exchanger 13 is provided above the
Water passing through the interior is heated by combustion heat from the burner group 11. A sparker 14 for igniting the burner group 11 and a flame rod 15 for detecting ignition of the burner group 11 are provided near the burner group 11 in the combustor case 10. Furthermore, an exhaust port 2 is provided above the combustor case 10 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. Therefore, governor proportional valves 23 that adjust the supply amount of fuel gas are provided in order from the upstream side, and downstream of the governor proportional valves 23, the fuel pipe 20 branches, and one of the branched gas pipes 2
From 0a, fuel gas is led directly to one nozzle pipe 16, and to the other branched gas pipe 20b,
In order to stop combustion in one of the two burner groups 11, a solenoid valve 24 for switching the number of burners is provided, and fuel gas is guided to the other nozzle pipe 16 according to the open/closed state of the solenoid valve 24. It will be destroyed.

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 temperature thermistor 25 for detecting the 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 1 in a predetermined sequence in response to a water flow signal from the water flow switch 19.
2. Control each solenoid valve 21, 22 and sparker 14 to start the ignition operation, and when the water flow signal stops, stop the blower 12, and each solenoid valve 2
1 and 22 to stop combustion.

For example, when the flame is no longer detected by the flame rod 15 due to extinguishing, etc., each electromagnetic valve 21, 22 is closed to stop the fuel supply.
Prevent gas leaks.

The temperature control unit 32 controls the set temperature Tset of the controller 40 and the detected temperature T of the hot water temperature thermistor 25.
Based on, the heating amount (number) Q for heating the water passing through the heat exchanger 13 to the set temperature Tset
Determine.

The combustion control unit 33 controls the heating amount (number) determined by the temperature control unit 32 by each functional unit of the blower control unit 34, the proportional valve control unit 35, and the switching control unit 36.
The blower 12, governor proportional valve 23, and solenoid valve 24 are controlled according to Q.

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. Further, in connection with switching the number of burners, the operating states of the blower 12 and the governor proportional valve 23 are changed, and the solenoid valve 24 is controlled.

In this embodiment, the first combustion is performed in which the solenoid valve 24 is closed and the solenoid valve 24 is heated by only one set of the burner group 11, and the solenoid valve 24 is A second combustion is set in which the burner is opened and heated by two burner groups 11, and a third combustion is set in the first combustion.
As shown by the solid line A in the figure, the combustion area that provides the hot water supply amount from No. 2 to No. 8 is set, and in the second combustion, as shown by the solid line B in Fig.
The combustion range that provides the amount of hot water supplied is set.

The blower control section 34 drives the blower 12 according to the heating amount Q of the temperature control section 32 based on the control state of the switching control section 36 .

Here, in the case of the first combustion, the temperature control section 3
The blower 12 is driven as it is according to the heating amount Q determined in step 2, and in the second combustion, the blower 12 is driven according to the heating amount Q of the temperature adjustment control section 32 in the control state switched to the second combustion by the switching control section 36. to drive the blower 12.

In addition, when switching the number of burners, the switching control section 36
The switching control is performed, 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 changed accordingly, and when the switching is completed, the operating state of the blower 12 is changed again to the operating state for switching. The blower 12 is driven according to the amount of heating Q.

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 heating amount Q is supplied respectively. 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 section 3
6 performs switching control, and at this time, the current value to the governor proportional valve 23 is changed so as to offset 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 section 32. Correct.

The switching control unit 36 controls the solenoid valve 24 by determining the number of burners to be used according to the heating amount (number) Q determined by the temperature control control unit 32, and also controls the heating amount Q for each number of burners. The voltage applied to the blower 12 is determined according to the voltage.

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.

Air-fuel ratio change control is performed to ensure that the flame transfer occurs safely and reliably to avoid sudden changes in the combustion state when the flame transfers from the first combustion to the second combustion. This is to reduce fluctuations in the amount of heating during heating and stabilize the temperature of the hot water.

Here, air correction information Φα for reducing the supply amount of combustion air is set to the blower with respect to the air blowing amount information Φ ₁ determined according to the heating amount Q ₁ at the time of fire transfer determined by the temperature control unit 32. It is sent to the control unit 34 and determines air blowing amount information Φch at the time of burning so that Φch=Φ ₁ −Φα . . . .

At this time, when the blower 12 is controlled accordingly, the fuel supply amount F adjusted by the governor proportional valve 23 decreases by Fα corresponding to the air correction information Φα, and the heating amount Q ₁ at the time of fire transfer decreases. will no longer be compatible. 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 ₁ = By setting Fch+Fα..., we obtain the corrected fuel supply amount _F1 ,
The fuel supply amount F by the governor proportional valve 23 controlled according to the operation of the blower 12 is corrected.

As a result, a corrected fuel supply amount F ₁ corresponding to the heating amount Q ₁ at the time of fire transfer is supplied.

Therefore, for the heating amount Q ₁ of the temperature control section 32,
By reducing only the amount of combustion air, the air-fuel ratio is changed and becomes gas-rich, so that reliable fire transfer is obtained, and an appropriate amount of corrected fuel is supplied according to the heating amount Q ₁ of the temperature control section 32. Since the amount F ₁ is supplied, for the water passing through the heat exchanger 13,
Excessive heating is not performed, and fluctuations in the hot water temperature can be reduced.

Further, 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 includes a timer section 36.
a is provided, and in the fire shifting control, the air-fuel ratio is changed to the flame shifting air-fuel ratio at once by sending air correction information Φα to the blower control unit 34 and sending fuel correction information Fα to the proportional valve control unit 35. Instead, each correction information is gradually increased according to 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 information in the elapsed unit time tn after the start of the switching control are Fn is obtained by Φn=Φα×tn/ta...Fn=Fα×tn/ta...

Therefore, the airflow amount information Φch in the elapsed unit time tn after the start of the switching control at the time of fire transfer is Φch=Φ ₁ −Φα×tn/ta...

In addition, the corrected fuel supply amount F ₁ in the elapsed unit time tn after the start of the switching control at the time of fire shifting is F ₁ =Fch×Fα×tn/ta... As shown by, the elapsed time after the start of the switching control at the time of fire shifting. Regardless of the unit time tn, the corrected fuel supply amount F ₁ corresponds to the heating amount Q ₁ at the time of flame transfer.

In addition, in the air-fuel ratio change control, from the first combustion to the second
After the transition to combustion is complete, the air-fuel ratio is returned to the one before switching. In this case, as before switching,
Gradually return the air-fuel ratio to normal.

In this case, in the switching control, the solenoid valve 24
If tb is the setting time from when the number of burners is opened and the number of burners is changed until the air-fuel ratio is returned to the original value, then the air correction information Φn and fuel correction information Fn in the elapsed unit time tn after switching are as follows: Φn = Φα × (tn −tb)/tb...Fn=Fα×(tn−tb)/tb...The air flow rate information Φch in the elapsed unit time tn after switching at the time of fire transfer is Φch=Φ ₁ +Φα×(tn −tb)/tb...

In addition, the corrected fuel supply amount F ₁ in the elapsed unit time tn after the start of the switching control at the time of fire shifting is F ₁ = Fch - Fα × (tn - tb) / tb ......, the start of the switching control at the time of flame shifting. Regardless of the subsequent elapsed unit time tn, the corrected fuel supply amount F ₁ corresponds to the heating amount Q ₁ at the time of flame transfer.

In this way, in the switching control, the air-fuel ratio is gradually changed to a non-flammable air-fuel ratio over time, and when the air-fuel ratio reaches a non-flammable air-fuel ratio, the solenoid valve 24 is opened, 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 does not change suddenly. Therefore, the flame can be transferred smoothly and no resonance noise or the like is generated.

On the other hand, here, the blower 12 is controlled according to the determined combustion amount, and the governor proportional valve 23 is controlled according to the rotation speed of the blower 12, so when switching between the first combustion and the second combustion, the blower 12, the change in fuel supply amount by the governor proportional valve 23 is delayed.

As a result, if the solenoid valve 24 is switched at the start of switching control, the third
As shown by broken lines C and D in the figure, since the switching is performed after the heating amount is significantly different from the heating amount before switching, the fluctuation in the heating amount becomes large.

For this reason, the blower 12 and the governor proportional valve 23
In order to perform the actual switching of the solenoid valve 24 and the switching of the solenoid valve 24 at the same timing, the timer section 36a performs timing after the switching of the blower 12 and the governor proportional valve 23 is started, and the timer section 36a measures the time after the above-mentioned set time. ta
The solenoid valve 24 is switched after the period has elapsed. As a result, since the burners are switched as shown by broken lines E and F, the influence of response delay of the blower 12 can be reduced.
The switching timing of this solenoid valve 24 is as follows:
This is done gradually not only 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 accordingly, fluctuations in the outlet temperature Tout 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 issued when the heating amount Q decreases and falls below the switching number to the first combustion. When the duration t of this switching command is measured by the timer section 36a, if the duration t exceeds a certain time _t1 , switching control is performed and the duration t is changed to a certain time t. If the number is less than ₁ , switching is not performed.

The water amount control unit 37 adjusts the opening degree of the electric water amount control device 18 based on the incoming water temperature, and limits the amount of water that exceeds the heating capacity from flowing into the heat exchanger 13.

The operation of the gas water heater of this embodiment having the above configuration will be explained based on FIG. 4.

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.
Combustion is performed under temperature control (step 2).

If the heating amount Q increases or decreases during combustion due to temperature control and a switching command is issued (in step 3)
YES), the elapsed time t of the switching command is measured (step 4).

If the switching command is continuously issued and the predetermined time period _t1 or more is exceeded (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.

In response to the switching command from the switching control section 36, the heating amount Q 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 12 is changed. Started (step
11).

Further, the proportional valve control section 35 detects the rotation speed of the blower 12 whose rotation speed has been changed, and the switching control section 35 detects the rotation speed of the blower 12 whose rotation speed has been changed.
In response to the switching command No. 6, a change in the current value applied to the governor proportional valve 23 is started in synchronization with the operation of the blower 12 (step 12).

When the change of the governor proportional valve 23 is started, the timer 36a starts measuring time, 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 when the change of the rotation speed of the blower 12 is completed (step
15), then the change in the opening degree of the governor proportional valve 23 is completed (step 16).

In this switching control, in the case of fire transfer, the air flow information Φch is gradually decreased with the elapsed unit time tn according to the above equation, Φch = Φ ₁ − Φα × tn / ta, so that the combustion The amount of air used gradually decreases.

Conversely, the corrected fuel supply amount F ₁ is gradually corrected according to the elapsed unit time tn after the start of switching control from the above equation, with the relationship F ₁ = Fch + Fα × th/ta, so the heating amount Q A fixed amount is supplied according to the amount.

Therefore, the air-fuel ratio gradually becomes gas-rich,
At the time of switching, smooth fire transfer is ensured. Furthermore, since the air-fuel ratio is gradually changed, the combustion state does 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, since 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, the rotational speed of the blower 12 is changed to start the switching operation of the number of burners. 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 broken line C, so an excessive amount of heating is given as a result of switching. Never.

Conversely, when the number of burners decreases, the amount of heating changes as shown by broken line F in FIG. 3, and therefore becomes smaller compared to the conventional case shown by 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 predetermined 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, in a water heater that is equipped with a plurality of burners and in which the number of burners used can be switched, the amount of heating does not change excessively when the number of burners is switched, and the amount of heat is not changed continuously compared to the conventional method. This reduces fluctuations in the hot water temperature at the time of switching.

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 water 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 embodiment, a gas combustion type water heater has been described, but stable hot water supply can be similarly performed using other fuels such as petroleum.

Moreover, the liquid to be heated is not limited to water.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the functional configuration 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 of this embodiment, and FIG.
The figure is 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 this embodiment, Figure 4 is a flowchart for explaining the operation of this embodiment, and Figure 5 is the main It is a flowchart for explaining the operation of switching control in the embodiment. 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 device (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 fluid passing through a heat exchanger, and a fuel pipe is branched 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 in use, and the opening degree of the control valve is changed in relation to the opening/closing of the solenoid valve, wherein combustion air is supplied to the plurality of burners. an air-fuel ratio that controls the control valve according to the operating state of the blower and changes 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 is characterized in that the air-fuel ratio changing means changes the operating state of the blower and also 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 fluid passing through the heat exchanger, and a fuel pipe is branched downstream of a control valve for adjusting the fuel supply amount to supply fuel to each of the plurality of burners, and 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, so that the plurality of burners can be connected to each other. A combustion control device for a liquid heater that switches the number of burners used 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; and an air-fuel ratio changing means for changing the ratio between the amount of air supplied to the burner and the amount of fuel supplied when switching the number of burners used, and the air-fuel ratio is gradually changed. Combustion control device.