JP3170448B2 - Gas water heater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an instantaneous gas water heater for use in kitchens, baths, floor heaters, etc., which starts heating by a gas burner simultaneously with the start of hot water supply.

[0002]

2. Description of the Related Art Generally, there are known gas water heaters of a hot water storage type in which hot water heated to a predetermined temperature is stored and an instantaneous type in which heating by a gas burner is started simultaneously with the start of hot water supply. Since the hot water storage type gas water heater stores hot water at a predetermined temperature in a tank, it has the advantage of being able to use heated hot water without delay.On the other hand, the equipment is large and the installation conditions are limited. There is a problem that there are many. On the other hand, instantaneous gas water heaters are widely used because they do not need to store hot water, are small in size, and are easy to install.

[0003]

On the other hand, since an instantaneous gas water heater starts heating by a gas burner at the same time as starting hot water supply, water staying in the gas water heater (hereinafter referred to as possessed water). There is a time delay before the water reaches the desired temperature, and there is a time delay before the hot water of the desired temperature reaches the caran, not only for the length of the pipe from the gas water heater to the caran. That is, there is a problem that it takes a relatively long time until the temperature of the hot water discharged from the curan rises.

In particular, when the time from when the hot water is stopped to when the hot water is again discharged is relatively short, the temperature of the retained water is relatively low, but the hot water is stopped and then restarted. If the time is long, the temperature of the retained water drops significantly, and as a result, the time delay until the temperature of the hot water sent out from the water heater rises greatly increases. To solve this kind of problem, for example, as shown in FIG.
In addition to a hot water supply pipe 6 to which a curran 5 for discharging hot water from the hot water supply 1 is connected, a return pipe 7 from a terminal of the hot water supply pipe 6 to the hot water supply 1 is provided to supply hot water supplied from the hot water supply 1 to the hot water supply pipe. It is considered that the water is circulated to the hot water supply 1 through the path from 6 to the return pipe 7. In other words, by providing a circulation path by the hot water supply pipe 6 and the return pipe 7, the hot water continues to flow even while the curran 5 is closed, the temperature of the circulating hot water is monitored by the water heater 1, and the circulation path The hot water is appropriately heated in the water heater 1 so that the temperature of the hot water passing through the hot water is maintained at a predetermined temperature.

If this configuration is adopted, the temperature of the hot water passing through the circulation path is maintained at a predetermined temperature, so that the hot water can be discharged while the curran 5 is opened.
Further, when the curran 5 is opened, the gas burner of the water heater 1 is ignited, so that the hot water can be continuously discharged. In this way, if the configuration shown in FIG. 7 is adopted, there is almost no time delay from the opening of the curran 5 to the rise of the temperature of the hot water to be discharged, but since the return pipe 7 is provided, the construction is It is troublesome and difficult to install in an existing building. In addition, in order to circulate hot water in a circulation path formed by the hot water supply pipe 6 and the return pipe 7, it is necessary to provide a circulating device such as a pump in the water heater 1, and the configuration of the water heater 1 becomes complicated. And increase costs. Moreover, even if the configuration shown in FIG. 7 is adopted,
If the gas water heater is restarted after stopping the operation of the gas water heater and leaving it for a long time, a large time delay occurs in the rise of the temperature of the hot water.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a start-up time from the start of hot water discharge until the hot water reaches a desired temperature even when started after being left for a long time. It is to provide a short gas water heater.

[0007]

According to the first aspect of the present invention, there is provided a heat exchanger for heating introduced water by a gas burner, a feedwater temperature detector for detecting a temperature of water introduced to the heat exchanger, and an external device. A temperature setting unit for setting a target temperature of hot water to be sent to the heat exchanger; a flow sensor for detecting a flow rate of water passing through the heat exchanger; and a fuel gas to a gas burner when a flow rate detected by the flow rate sensor becomes a predetermined value or more. Control means for starting the supply and igniting the gas burner, the control means performing a pre-heat treatment immediately after the start, and in the pre-heat treatment, the target temperature set by the temperature setting unit and the temperature detected by the feed water temperature detector Based on the temperature difference, the heating amount required to heat the water introduced into the heat exchanger to the target temperature and send it out of the heat exchanger is determined, and the heating amount is determined based on the heating amount and the heating capacity of the gas burner. After igniting the gas burner by the heating time is one that enables the delivery of water to the outside.

According to this configuration, immediately after the start, the pre-heat treatment for heating the retained water retained inside is performed, and in the pre-heat treatment, the temperature of the retained water reaches around the target temperature set in the temperature setting section. Heat the retained water for about the heating time,
After that, since the hot water can be supplied to the outside, the temperature of the retained water rises when the hot water is started to be used, and the usability is improved by raising the temperature of the hot water to be sent to the outside in a short time. In addition, the configuration of the conventional water heater is less changed than the conventional water heater, and the water heater can be provided at a relatively low cost.
Furthermore, in the pre-heat treatment, the temperature of the retained water is detected by the feedwater temperature detector that detects the temperature of the water introduced into the heat exchanger. It is possible to determine the appropriate heating time according to the season. That is, hot water having a temperature near the target temperature set by the temperature setting unit can be sent out after the pre-heat treatment regardless of the season.

According to a second aspect of the present invention, in the first aspect of the present invention, a heating temperature detector for detecting a temperature of water at an outlet side of the heat exchanger is provided, and the heating temperature is detected before reaching a heating time obtained in the pre-heat treatment. When the temperature detected by the detector reaches the target temperature set by the temperature setting section, the control means stops heating by the gas burner. According to this configuration, when the temperature of the water heated by the heat exchanger reaches the target temperature, heating by the gas burner is stopped even before the heating time has elapsed, so that overheating in the heat exchanger can be prevented. it can. That is, the temperature of the retained water is not increased more than necessary, energy can be saved, and the configuration for safety protection against overheating can be reduced, leading to a reduction in cost.

According to a third aspect of the present invention, in the first aspect of the present invention, a plurality of gas burners are provided in association with each part of the heat exchanger, and a flow rate of the fuel gas is provided on a flow path of the fuel gas to the gas burner. A gas variable flow valve for adjustment is provided, and the control means adjusts the gas variable flow valve such that all gas burners are simultaneously ignited in the pre-heat treatment and the flow rate of the fuel gas becomes an allowable minimum flow rate.

In this configuration, since the number of gas burners to be ignited can be selected, the variable range of the amount of heating by the gas burners is wide, and the restriction on the relationship between the temperature of the introduced water and the temperature of the supplied hot water is reduced. In addition, since all gas burners are ignited during the pre-heat treatment, the water in the heat exchanger can be heated uniformly, and the temperature distribution in the heat exchanger does not become uneven. Since the flow rate of the fuel gas is set so as to be the minimum flow rate allowed in the state in which the fuel gas is ignited, it is difficult to overheat even if there is some error in the heating time.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a basic configuration. The housing 2 of the water heater 1 is provided with a water supply port 11 for introducing water supplied by city water or the like, a water supply port 12 for sending out hot water heated by the water heater 1, and a fuel gas supply port. A gas inlet 13 for introduction is provided. The housing 2 houses a heat exchanger 22 for heating water introduced through the water supply port 11 by a gas burner 21.
On a flow path from the water supply port 11 to the heat exchanger 22, a water supply temperature detector 23 composed of a thermistor and a flow sensor 24 that measures the flow rate of water introduced into the heat exchanger 22 through the water supply path are provided. Be placed. A heating temperature detector 25 composed of a thermistor and a variable flow valve 2 for adjusting the flow rate of hot water in accordance with a control signal from a microcomputer 20 described later are provided on a flow path from the heat exchanger 22 to the tap hole 12.
6 and a tapping temperature detector 27 composed of a thermistor for detecting the temperature of tapping water sent from tapping port 12 are provided. Further, the connection between the feedwater temperature detector 23 and the variable flow valve 26 is also established by the bypass pipe 14,
4 is provided with a bypass valve 28. The variable flow valve 26 is an electromagnetic valve or a motor-operated valve capable of adjusting a flow rate in a plurality of stages, and an electromagnetic valve is used as the bypass valve 28. Although not shown, a pressure relief valve for releasing the pressure when the pressure of the heat exchanger 22 rises is provided.

A supply path of the fuel gas to the gas burner 21 is provided with a gas on-off valve 15 comprising an electromagnetic valve for selecting supply and cutoff of the fuel gas according to a control signal from a microcomputer 20 described later, and a microcomputer 20 described later. And a gas variable flow valve 16 comprising an electromagnetic valve or an electric valve capable of adjusting the flow rate of the fuel gas in a plurality of stages in accordance with a control signal from the controller. Further, two or three gas burners 21 are provided so that the number of the burners can be controlled in accordance with the hot water supply amount, and an opening / closing valve 17 is inserted in a supply path of the fuel gas to the gas burners 21. Therefore, the amount of heating can be increased by igniting all the gas burners 21, and the amount of heating can be reduced by closing the on-off valve 17 and supplying the fuel gas to only one gas burner 21. That is, by using the gas variable flow valve 16 and the on-off valve 17 in combination, it is possible to adjust the heating amount in a wide range. Although not shown, when the gas on-off valve 15 is opened, an ignition device for igniting the gas burner 21 is started.

As shown in FIG. 2, the variable flow valve 26, the bypass valve 28, the gas on-off valve 15, the gas variable flow valve 16 and the on-off valve 17 include a microcomputer 20 as a control means housed in the housing 2. Is controlled by A remote control device 30 is connected to the microcomputer 20 via a connection line 29. The remote control device 30 includes an operation switch 31 for instructing the operation of the water heater 1 and a temperature setting unit 32 for setting the temperature of the hot water to be sent out. The water supply temperature detector 23, the heating temperature detector 25, and the tap water temperature detector 27
The gas on-off valve 15, the gas variable flow valve 16, the on-off valve 17, and the variable flow valve 2 are set so that the temperature of the hot water to be sent approaches the hot water supply temperature set by the temperature setting unit 32 based on the detected temperature at
6. Control the bypass valve 28.

Next, the normal operation will be described with reference to FIG. In the normal operation, as shown in FIG. 3, the hot water supply temperature set by the remote control device 30 is read and (S
1) The flow rate detected by the flow rate sensor 24 is read (S2). The hot water supply temperature is set as the temperature of the hot water sent from the hot water outlet 12, and when the hot water supply temperature is set, the required target temperature (the hot water sent from the heat exchanger 22 is passed through the bypass pipe 14) at the outlet side of the heat exchanger 22. (The temperature at which hot water supply temperature is obtained when mixed with the water that has passed through). If the passing flow rate is equal to or less than the predetermined value (S3), the gas on-off valve 15 is kept closed.

On the other hand, when the flow rate detected by the flow rate sensor 24 is equal to or higher than the predetermined value, the gas on / off valve 15 is opened and the gas burner 21 is ignited (S4). The calorie of the gas burner 21 is determined based on the difference from the set hot water supply temperature and the flow rate detected by the flow rate sensor 24, and the opening amount of the gas variable flow valve 16 is adjusted so that the calorie is obtained. The valve 17 is controlled (S5). At this time, the heat quantity of the gas burner 21 is set so that the temperature of the hot water detected by the heating temperature detector 25 becomes higher than the hot water supply temperature, and the hot water discharged from the heat exchanger 22 by opening the bypass valve 28. The desired hot water temperature is obtained when the water and the water introduced through the water supply port 11 are mixed. Further, since the temperature of the hot water to be sent out may not match the hot water supply temperature only by adjusting the gas variable flow valve 16, the gas variable flow valve may be adjusted so that the temperature detected by the tapping water temperature detector 27 and the hot water supply temperature match. The opening amount of the valve 16 is corrected (S6).
Further, when the gas variable flow valve 16 alone cannot cope with the situation, the tapping amount is controlled by the variable flow valve 26. Thereafter, the flow rate is detected by the flow rate sensor 24 (S7). If the flow rate decreases (S8), the gas burner 21 is extinguished (S9).

By the way, in this type of water heater 1, the tap 1
When the discharge of hot water was stopped after discharging 10 liters of 40 ° C. water per minute from 2 to 5 m ahead, 5 minutes from the stop
It is desired that the temperature is kept within ± 3 ° C. with respect to the hot water supply temperature set by the remote control device 30 within minutes.
In other words, when washing dishes, the use of hot water is often stopped once and then reused after a short time. In such a usage form, the temperature of the discharged hot water fluctuates. If the temperature change at the time of reuse is not reduced, a so-called sandwich phenomenon in which hot water and cold water are alternately discharged will occur, giving the user discomfort.

Therefore, in this embodiment, in order to suppress such a sandwich phenomenon, the time from ignition of the gas burner 21 to opening of the bypass valve 28 is adjusted according to the temperature detected by the heating temperature detector 25. I have to.
That is, as shown by a broken line in FIG. 4, the water temperature inside the heat exchanger 22 decreases in a relatively short time,
In order to suppress a decrease in the temperature of the hot water sent out from the furnace, the amount of cold water mixed through the bypass pipe 14 may be reduced. Therefore, by adjusting the timing from the ignition of the gas burner 21 to the opening of the bypass valve 28, the change in the temperature of the hot water sent from the tap 12 as shown by the solid line in FIG. 4 is reduced. At the same time, the variable flow valve 26 is also controlled, and the temperature of the hot water is suppressed by the gas variable flow valve 16 while reducing the amount of hot water to be sent out from the tap hole 12. With such control,
Even if the tapping and stopping are repeated, it is possible to reduce fluctuations in the temperature of the tap water sent out from tap hole 12.

Even if the control is performed as described above, if the hot water supply is restarted after the hot water supply is stopped for a relatively long time, the temperature of the water retained in the water heater 1 drops, and It takes a long time before the hot water at the desired temperature is sent out from the gate 12. this is,
This is because the heating power of the gas burner 21 is set so that the water introduced from the inlet of the heat exchanger 22 reaches the outlet when the water reaches the outlet. Hot water of a desired temperature is not discharged until the temperature reaches the vicinity. Here, the amount of water retained in the heat exchanger 22 is, for example, about 1.5 liters, so that it takes about 10 seconds for the water introduced into the heat exchanger 22 to pass through the tap hole 12. Will be. That is, there is a time delay of about 10 seconds from the start of the supply of water from the tap hole 12 until the temperature of the hot water reaches the hot water supply temperature set by the temperature setting unit 32.

Therefore, in the present invention, when the microcomputer 20 is started, it does not immediately shift to the steady operation, but first performs the pre-heat treatment, and then shifts to the steady operation. In the pre-heat treatment, as shown in FIG. 5, first, the variable flow valve 26 is kept in a squeezed state (preferably, the passage of water is stopped) (S11), and the target temperature set by the temperature setting unit 32 (the above-described temperature) As well as the temperature set based on the hot water supply temperature) as well as (S
12) The temperature of the retained water detected by the feedwater temperature detector 23 is read (S13). The heat exchanger 22 is arranged so that the inlet side is lower than the outlet side, and by using the temperature detected by the feedwater temperature detector 23, the heat exchanger 2
The minimum temperature of the holding water of No. 2 can be obtained. Next, based on the target temperature Td set by the temperature setting unit 32 and the detected temperature Ts detected by the feedwater temperature detector 23, a heating amount necessary for discharging hot water at the target temperature Td is obtained. That is, since the volume V of the retained water inside the water heater 1 is known, if the specific heat of the water is 1, the heating amount Q is Q = (T
d−Ts) × V (S14).

In consideration of the heating capacity of the gas burner 21, if the amount of heat per unit time at which the water in the heat exchanger 22 is heated by the gas burner 21 is Qu, the heating required for heating to the target temperature Td is obtained. Time t is expressed as t = Q /
It can be obtained in the form of Qu (S15). When the heating time t is determined, the gas variable flow valve 16 and the on-off valve 17 are controlled so that the amount of heat per unit time given to water becomes Qu, and the supply amount of the fuel gas to the gas burner 21 is adjusted. 15 is opened and the gas burner 21 is ignited (S16). Thereafter, the gas burner 21 is continuously ignited for the heating time t to heat the heat exchanger 22 (S17, S18). After the elapse of the heating time t, the gas burner 21 is extinguished (S19), and the variable flow valve 26 is opened to prepare for hot water supply (S20).

In winter, the detected temperature Ts is, for example, 1
It will be about 0 ° C. The target temperature is 70 ° C., the volume V of the water retained inside the heat exchanger 22 is 1.5 liters, and the amount of heat per unit time at the time of ignition of the gas burner 21 is 18,000.
Assuming that kcal / h and the ratio (efficiency) of the heat generated from the gas burner 21 used for heating water is 0.7, the heat quantity Qu per second is 18000 kcal × 0.7 ÷ 36.
Since 00 = 3.5 kcal / sec, the heating time t is
t = (70−10) × 1.5 ÷ 3.5 ≒ 26 seconds. As described above, under the above conditions, the target temperature is reached by heating for about 26 seconds.

The amount of heat per unit time at the time of ignition of the gas burners 21 can be adjusted by the gas flow valve 16 and the on-off valve 17, but in the present embodiment, the minimum allowable value when all the gas burners 21 are ignited. (The minimum supply amount of fuel gas). As described above, by igniting all the gas burners 21 at the same time, the water held in the heat exchanger 22 can be uniformly heated so that the temperature distribution does not become uneven. In addition, the larger the heat quantity of the gas burner 21, the shorter the heating time can be. However, since the heating time t is determined as described above, the heat quantity per unit time is large. Overheating can occur. On the other hand, the amount of heat per unit time is
By setting the minimum value at which the holding water of No. 2 can be heated uniformly, the possibility of overheating can be greatly reduced.

As described above, the calorific value Qu of the gas burner 21 in the preheat treatment is fixed, and the volume V of the water retained in the heat exchanger 22 is known, so that the temperature difference between the target temperature Td and the detected temperature Ts is determined. In this case, the heating time t can be determined. Therefore, in the calculation for actually calculating the heating time t, a table in which the temperature difference between the target temperature Td and the detected temperature Ts is associated with the heating time t is created, and the heating temperature t is easily obtained by obtaining the temperature difference. To be required. Specifically, a temperature difference is obtained by an analog subtraction circuit (differential amplifier), and then an analog-digital conversion is performed to obtain a digital value corresponding to the temperature difference. The temperature difference is used as an address and the heating time t is used as data. By giving the digital value of the temperature difference as an address to the memory in which the table is created, the heating time t corresponding to the temperature difference can be read.

Further, in this embodiment, the temperature detected by the heating temperature detector 25 during the pre-heat treatment is taken in. When this temperature reaches the target temperature Td (S17), even if the heating time t is not exceeded. Extinguish the gas burner 21 (S
19). By this processing, overheating can be reliably prevented. After the gas burner 21 is extinguished, the variable flow valve 26 is opened to prepare for the delivery of hot water (S20). That is, in the specific example described above, the microcomputer 2
About 26 seconds elapse after the start-up of 0, the hot water can be used.

In the present embodiment, it is necessary to start the microcomputer 20 and perform the pre-heat treatment before using the hot water. However, after the pre-heat treatment, the hot water at the desired temperature can be obtained immediately. Instantaneous water heater 1 with little time lag from hot water to discharge of hot water at desired temperature
This can solve the problem of delay in rising of the temperature of hot water, which was a drawback of the above.

FIG. 6 shows the temperature change of hot water when the conventional configuration and the configuration of the present embodiment are used for a shower.
6A and 6B show the temperature of hot water at the hot water supply port 12 of the hot water supply 1, and C and D show when 10 liters of hot water is discharged per minute from the discharge port of the shower 5 m away from the hot water supply port 12. Shows the temperature of hot water. A broken line indicates a conventional configuration, and a solid line indicates the present embodiment. As is clear from FIG. 6, in the conventional configuration, heating by the gas burner is started at the same time as the start of use of the shower at time t ₀ , whereas in the present embodiment, the microcomputer 20 is turned on prior to the start of use of the shower. Since the gas burner 21 is ignited for the heating time t (12 seconds in this case) from the time of power-on to perform the pre-heat treatment, the rise time from the start of use of the shower at time t ₀ is significantly reduced. I understand. That is, a difference of about 10 seconds occurs at the outlet of the shower until the desired temperature (40 ° C.) is reached.

In the above-described embodiment, the method of controlling the opening timing of the bypass valve 28 has been described as an example of the temperature control method when the hot water supply is repeatedly stopped and restarted in the steady operation. However, the present invention is not limited to this. Not a thing,
Other similar techniques can be used. It is also possible to select whether or not to perform a pre-heat treatment by adding a pre-heating switch to the remote control device 30. A pre-heating switch may be provided separately from the operation switch 31 and the temperature setting unit 32, or the operation switch 31 and the temperature When the setting unit 32 is operated at the same time, it can function as a preheating switch.

[0029]

According to the first aspect of the present invention, the pre-heat treatment is performed immediately after the control means is started. In the pre-heat treatment, the temperature difference between the target temperature set by the temperature setting unit and the temperature detected by the feed water temperature detector is detected. The heating time required to heat the water introduced into the heat exchanger to the target temperature and send it out of the heat exchanger is determined based on the heating amount and the heating time determined based on the heating amount and the heating capacity of the gas burner. Only after the gas burner is ignited, hot water can be sent to the outside, and immediately after startup, a pre-heat treatment for heating the retained water inside is performed. The heated water is heated only for the heating time to reach the vicinity of the target temperature set in, and the hot water can be supplied to the outside after that. Temperature of hot water to send But there is an advantage of improving usability by rises in a short period of time. In addition, there is an advantage that the configuration of the conventional water heater can be provided at a relatively low cost with few configuration changes. further,
In the pre-heat treatment, the temperature of the retained water is detected by a feedwater temperature detector that detects the temperature of the water introduced into the heat exchanger. Since an appropriate heating time according to the season can be determined, there is an advantage that regardless of the season, hot water having a temperature near the target temperature set by the temperature setting unit can be sent out after the pre-heat treatment.

According to a second aspect of the present invention, there is provided a heating temperature detector for detecting the temperature of water at the outlet side of the heat exchanger, and the temperature detected by the heating temperature detector before reaching the heating time obtained in the pre-heat treatment. When the temperature reaches the target temperature set by the temperature setting unit, the control means stops heating by the gas burner,
There is an advantage that overheating in the heat exchanger can be prevented. That is, there is an advantage that the temperature of the retained water is not increased more than necessary, energy is saved, and a configuration for safety protection against overheating can be reduced, leading to a reduction in cost.

According to a third aspect of the present invention, a plurality of gas burners are provided in association with each part of the heat exchanger, and a gas variable flow valve for adjusting a flow rate of the fuel gas on a flow path of the fuel gas to the gas burner is provided. The control means ignites all gas burners simultaneously in the pre-heat treatment and adjusts the gas variable flow valve so that the flow rate of the fuel gas becomes the allowable minimum flow rate. At the same time, the variable gas flow rate valve is adjusted so that the fuel gas flow rate is the minimum flow rate allowed, and the number of gas burners to be ignited can be selected. This has the advantage that the restriction on the relationship between the temperature of the introduced water and the temperature of the hot water to be delivered is reduced. In addition, since all gas burners are ignited during pre-heating,
The water in the heat exchanger can be heated uniformly, the temperature distribution in the heat exchanger does not become uneven, and the minimum flow rate allowed with all gas burners ignited Because the fuel gas flow rate is set as follows
There is an effect that even if there is some error in the heating time, it is difficult to overheat.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a main part block diagram of the same.

FIG. 3 is an operation explanatory diagram in a normal operation of the above.

FIG. 4 is an explanatory diagram of an operation in a normal operation of the embodiment.

FIG. 5 is an operation explanatory view showing a preheating treatment according to the first embodiment;

FIG. 6 is a diagram showing an operation example of the above.

FIG. 7 is a schematic configuration diagram showing a conventional example.

[Explanation of symbols]

 Reference Signs List 15 gas on-off valve 16 gas variable flow valve 17 on-off valve 20 microcomputer 21 gas burner 22 heat exchanger 23 feedwater temperature detector 24 flow sensor 25 heating temperature detector 32 temperature setting unit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F24H 1/10 302

Claims (3)

(57) [Claims] 1. A heat exchanger for heating introduced water by a gas burner, a feedwater temperature detector for detecting a temperature of water introduced to the heat exchanger, and a temperature for setting a target temperature of hot water to be sent out. A setting unit, a flow sensor for detecting a flow rate of water passing through the heat exchanger, and control means for starting the supply of fuel gas to the gas burner and igniting the gas burner when the flow rate detected by the flow sensor becomes a predetermined value or more. The control means performs a pre-heat treatment immediately after starting, and in the pre-heat treatment, based on a temperature difference between the target temperature set by the temperature setting unit and the temperature detected by the feedwater temperature detector, The heating amount required to heat the introduced water to the target temperature and send it out of the heat exchanger was determined, and the gas burner was ignited for the heating time determined based on the heating amount and the heating capacity of the gas burner. A gas water heater characterized in that hot water can be sent to the outside later. 2. A heating temperature detector for detecting a temperature of water at an outlet side of the heat exchanger, wherein the control means detects a temperature detected by the heating temperature detector before reaching a heating time obtained by the preheat treatment. The gas water heater according to claim 1, wherein the heating by the gas burner is stopped when the target temperature set by the setting unit is reached. 3. A plurality of gas burners are provided in association with each part of the heat exchanger, and a gas variable flow valve for adjusting a flow rate of the fuel gas is disposed on a flow path of the fuel gas to the gas burner. 2. The gas water heater according to claim 1, wherein all gas burners are ignited simultaneously in the pre-heat treatment, and the gas variable flow valve is adjusted so that the flow rate of the fuel gas becomes an allowable minimum flow rate.