JP7285140B2 - Backflow preventer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backflow prevention device provided in a supply passage for supplying hot water to a bathtub to prevent backflow of hot water from the bathtub.

A supply passage for supplying hot water to a bathtub is generally provided with a backflow prevention device for preventing hot water from flowing back from the bathtub. Such a backflow prevention device includes an air release valve connected to the downstream side (downstream passage) of the on-off valve that opens and closes the supply passage, and an air release valve that is connected to the upstream side (upstream passage) of the on-off valve of the supply passage. It is composed of a pressure detection passage that communicates with each other. The atmosphere release valve is biased in the direction of opening by a valve-opening spring, and receives the supply pressure of hot water from the upstream passage via the pressure detection passage. The valve is closed against When the supply pressure of hot water in the upstream passage falls below a predetermined pressure due to a water outage or the like, the urging force of the valve-opening spring opens the atmospheric relief valve to discharge the hot water in the downstream passage. It is possible to prevent backflow of hot water to the

In such a backflow prevention device, if foreign matter mixed in hot water supplied to the upstream passage clogs in the pressure detection passage, the supply pressure of hot water in the upstream passage will not be transmitted to the atmosphere relief valve, and the atmosphere relief valve will be closed. An open failure occurs while the valve remains open. Therefore, a structure has been proposed in which the entrance of the pressure detection passage that opens to the upstream passage is narrowed and a strainer is arranged to prevent foreign matter from entering the pressure detection passage (Patent Document 1).

JP 2018-91396 A

However, in the structure proposed in Patent Document 1, not only does the number of parts for the strainer increase, but the flow passage area at the inlet of the pressure detection passage is small, so hot water is supplied from the upstream passage to the atmosphere relief valve. There is a problem that the pressure transmission tends to be insufficient, and the air release valve may open even though the supply pressure of hot water in the upstream passage is not lowered.

SUMMARY OF THE INVENTION The present invention has been made in response to the above-mentioned problems in the prior art, and aims to sufficiently transmit hot water supply pressure in an upstream passage to an atmosphere release valve while suppressing entry of foreign matter into the pressure detection passage. To provide a backflow prevention device capable of

In order to solve the above problems, the backflow prevention device of the present invention employs the following configuration. i.e.
A backflow prevention device provided in a supply passage for supplying hot water to a bathtub to prevent backflow of hot water from the bathtub,
connected to a downstream passage on the downstream side of an on-off valve that opens and closes the supply passage, receives hot water supply pressure in an upstream passage on the upstream side of the on-off valve, and closes when the pressure is equal to or higher than a predetermined pressure; an atmosphere release valve that opens to discharge hot water from the downstream passage when the supply pressure of hot water in the upstream passage drops below the predetermined pressure;
a pressure detection passage that communicates the upstream passage with the atmosphere release valve and guides the supply pressure of hot water in the upstream passage to the atmosphere release valve;
a cylindrical member inserted inside the upstream passage,
Between the inner wall surface of the upstream passage and the tubular member, an inflow region having a gap through which hot water can flow from the upstream end side of the tubular member is provided,
The pressure detection passage is characterized in that an inlet on the upstream passage side is surrounded by the inflow region and opens to an inner wall surface of the upstream passage.

In such a backflow prevention device of the present invention, since the pressure detection passage communicates with the upstream passage through the inflow region between the inner wall surface of the upstream passage and the cylindrical member, the inlet of the pressure detection passage is the upstream passage. Intrusion of foreign matter into the pressure detection passage can be suppressed as compared with the case where the pressure detection passage is exposed. Moreover, although the space between the inner wall surface of the upstream passage and the cylindrical member is narrow, the entire periphery of the inlet of the pressure detection passage is surrounded by the inflow region and is in communication with the water flowing into the pressure detection passage. Since a large total flow passage area can be ensured, it is possible to sufficiently transmit the supply pressure of hot water in the upstream passage to the atmosphere release valve.

In the above-described backflow prevention device of the present invention, the inflow region may be provided over the entire circumference of the inner wall surface of the upstream passage.

In this way, compared to the case where the inflow area surrounding the entrance of the pressure detection passage is provided only in a part of the entire circumference of the inner wall surface of the upstream passage on the side where the entrance of the pressure detection passage opens. can flow into the gap from the upstream end side of the cylindrical member and easily secure a route leading to the inlet of the pressure detection passage, which is advantageous in transmitting the supply pressure of hot water in the upstream passage to the atmosphere relief valve.

Further, in the backflow prevention device of the present invention, the cross-sectional shape of the upstream passage taken along a plane perpendicular to the flow direction of hot water is circular, and an impeller that rotates in response to the flow of hot water is incorporated as a cylindrical member. A cylindrical outer wall forming the outer frame of the flow rate sensor may be used.

In this way, since the inflow region is provided between the inner wall surface of the upstream passage and the outer wall of the flow rate sensor installed in the upstream passage, there is no need to add a separate cylindrical member, which is convenient. can be realized.

Further, in the backflow prevention device of the present invention, the tubular member is inserted into the upstream passage from upstream to downstream, and the upstream side of the tubular member is formed into an annular stop along the inner wall surface of the upstream passage. It may be retained by a ring.

With this arrangement, the upstream side of the gap between the inner wall surface of the upstream passage and the cylindrical member is covered with the retaining ring, so that the path through which hot water flows into the gap from the upstream end side of the cylindrical member can be bent. , it is possible to further suppress the intrusion of foreign matter into the pressure detection passage.

1 is an explanatory diagram showing the overall configuration of a hot water filling system 1 that fills a bathtub 2 with hot water. FIG. FIG. 2 is an explanatory diagram showing the configuration of the hot water filling control unit 20 of the embodiment; FIG. 3 is a cross-sectional view illustrating the structure of an air release valve 26; 4 is an enlarged perspective view showing a connecting portion between the water supply passage 4 and the pressure detection passage 7; FIG. 4 is a cross-sectional view of a connecting portion cut along a plane including the center line of the water supply passage 4 and the center line of the pressure detection passage 7 with the flow rate sensor 6 inserted inside the water supply passage 4. FIG. 4 is a cross-sectional view of a connecting portion cut along a plane perpendicular to the center line of the water supply passage 4 and including the center line of the pressure detection passage 7, with the flow rate sensor 6 inserted inside the water supply passage 4. FIG.

FIG. 1 is an explanatory diagram showing the overall configuration of a hot water filling system 1 that fills a bathtub 2 with hot water. As shown, the hot water filling system 1 includes a hot water supply device 10 for generating hot water, a hot water supply passage 5 for supplying hot water from the hot water supply device 10 to the bathtub 2, and a hot water supply passage provided in the middle of the hot water supply passage 5 to fill the hot water. and a hot water filling control unit 20 for controlling.

The water heater 10 includes a burner 11 for combusting the fuel gas supplied through the gas passage 3, a combustion fan 12 for sending combustion air to the burner 11, and a combustion exhaust gas generated by combustion in the burner 11 for heat exchange. A heat exchanger 13 is provided. Clean water is supplied to the heat exchanger 13 through the water supply passage 4 , and the supplied clean water is heated by heat exchange with combustion exhaust gas in the heat exchanger 13 , and then turns into hot water and flows into the hot water supply passage 5 . and flow out. A water supply passage 4 connected to the heat exchanger 13 is provided with a flow rate sensor 6 for detecting the flow rate of clean water supplied to the heat exchanger 13 . The water supply passage 4 and the hot water supply passage 5 of this embodiment correspond to the "supply passage" of the present invention.

A hot water filling control unit 20 is provided in the hot water supply passage 5 connected to the downstream side of the heat exchanger 13 , and hot water is supplied to the bathtub 2 via the hot water filling control unit 20 . Further, although the details will be described later, the hot water filling control unit 20 is connected to a pressure detection passage 7 branched from the water supply passage 4, and the supply pressure of the clean water in the water supply passage 4 is detected through the pressure detection passage 7 to the hot water. It is designed to be led to the tension control unit 20 .

FIG. 2 is an explanatory diagram showing the configuration of the hot water filling control unit 20 of this embodiment. As shown, the hot water filling control unit 20 includes a hot water filling electromagnetic valve 21, two check valves (a first check valve 22 and a second check valve 23), and an air release valve 26. . The hot water filling solenoid valve 21 can open and close the hot water supply passage 5, starts filling hot water when the valve is open, and stops filling hot water when the valve is closed. A well-known pilot type solenoid valve is used for the hot water filling solenoid valve 21 of this embodiment, but a direct acting type solenoid valve may be used. A pilot-type solenoid valve that utilizes differential pressure generally opens and closes with a smaller force than a direct-acting solenoid valve, so it is possible to reduce the size of the solenoid and reduce power consumption. The hot water filling electromagnetic valve 21 of this embodiment corresponds to the "on-off valve" of the present invention. Further, the hot water supply passage 5 and the water supply passage 4 upstream of the hot water filling electromagnetic valve 21 of the present embodiment correspond to the "upstream passage" of the present invention, and the hot water supply passage 5 downstream of the hot water filling electromagnetic valve 21. corresponds to the "downstream passage" of the present invention.

The first check valve 22 and the second check valve 23 are provided in series on the downstream side (bathtub 2 side) of the hot water filling solenoid valve 21, and are biased in the valve closing direction to close the hot water supply passage 5. . When the pressure of hot water supplied from hot water supply device 10 exceeds a predetermined valve opening pressure due to the opening of hot water filling solenoid valve 21, first check valve 22 and second check valve 23 open to allow hot water to pass through. Therefore, the bathtub 2 is filled with hot water. When the pressure of the hot water supplied from the hot water supply device 10 drops due to a water outage or the like during hot water filling (while the hot water filling solenoid valve 21 is open), the first check valve 22 and the second check valve 23 are closed. The valve is closed to prevent hot water from flowing back from the bathtub 2 side to the hot water supply device 10 side. In addition, since the two check valves (the first check valve 22 and the second check valve 23) are arranged in series, the backflow of hot water can be prevented more reliably than when there is only one check valve.

The atmosphere release valve 26 is connected between the first check valve 22 and the second check valve 23 of the hot water supply passage 5 via a lead-in passage 36 . The pressure detection passage 7 and the open passage 37 open to the atmosphere are connected to the atmosphere release valve 26, thereby forming the backflow prevention device 25 of this embodiment. If the supply pressure of clean water drops during hot water filling, the atmospheric release valve 26 opens, and if for some reason the first check valve 22 and the second check valve 23 are not completely closed, Even so, it is possible to prevent hot water from flowing back from the bathtub 2 side to the hot water supply device 10 side.

FIG. 3 is a cross-sectional view illustrating the structure of the air release valve 26. As shown in FIG. First, FIG. 3(a) shows a state in which the air release valve 26 is closed. As shown in the figure, the atmosphere release valve 26 has a structure in which a diaphragm 30 partitions the primary chamber 31 and the secondary chamber 32 . A pressure detection passage 7 is connected to the primary chamber 31 , and clean water is introduced from the water supply passage 4 . On the other hand, the secondary chamber 32 is connected to the hot water supply passage 5 via the lead-in passage 36, and hot water between the first check valve 22 and the second check valve 23 is guided. Further, the secondary chamber 32 is provided with a valve body 33 supported by the diaphragm 30 and a valve opening spring 35 that biases the valve body 33 toward the primary chamber 31, and is open to the atmosphere. A passageway 37 is connected.

If clean water is normally supplied to the water supply passage 4, the supply pressure of clean water is introduced into the primary chamber 31 through the pressure detection passage 7, and the diaphragm 30 is pushed into the secondary chamber 32 side, so the valve is opened. The valve body 33 is pressed against the valve seat 34 against the urging force of the spring 35, and the atmosphere release valve 26 is closed. During hot water filling, the pressure of hot water supplied from the hot water supply device 10 by opening the hot water filling solenoid valve 21 causes the first check valve 22 and the second check valve 23 to open, and the lead-in passage 36 is closed. Although it extends to the secondary chamber 32 of the air release valve 26 via the air release valve 26, the supply pressure of tap water is generally lower than the pressure of hot water supplied via the hot water supply device 10, the hot water filling solenoid valve 21, and the first check valve 22. Since the water level is higher, the air release valve 26 remains closed even during hot water filling.

On the other hand, when the supply pressure of clean water drops due to a water outage or the like, as shown in FIG. By moving away from the valve seat 34, the atmosphere release valve 26 is opened. As a result, the hot water between the first check valve 22 and the second check valve 23 can pass through the air release valve 26 and is discharged through the open passage 37, so that the water from the bathtub 2 to the hot water supply device 10 is discharged. It can prevent backflow of hot water.

In the backflow prevention device 25 having such an air release valve 26, if a foreign object enters the pressure detection passage 7 and clogs it, the supply pressure of clean water is not sufficiently transmitted to the primary chamber 31 of the air release valve 26, and the pressure is released to the atmosphere. An open failure occurs in which the valve 26 remains open. Therefore, in the backflow prevention device 25 of this embodiment, the following configuration is adopted in order to prevent foreign matter from entering the pressure detection passage 7 .

FIG. 4 is a perspective view showing an enlarged connection portion between the water supply passage 4 and the pressure detection passage 7. As shown in FIG. The water supply passage 4 of this embodiment can be divided by a joint 4a provided on the upstream side (lower side in the figure) of the connection portion of the pressure detection passage 7, and the diagram shows the divided state. ing. A flow rate sensor 6 is inserted into the water supply passage 4 from the joint 4a.

The water supply passage 4 has a circular cross-sectional shape along a plane perpendicular to the direction of water supply (vertical direction in the drawing), and is formed with an accommodating portion 40 that accommodates the flow rate sensor 6 by expanding the inner diameter. Further, the storage portion 40 is provided with an enlarged diameter portion 40a by further expanding the inner diameter on the upstream side of the center in the flow direction, and the inlet 7a of the pressure detection passage 7 opens to the inner wall surface of the enlarged diameter portion 40a. ing. Further, on the upstream side of the housing portion 40, a large-diameter portion 41 having an inner diameter that is larger than that of the enlarged-diameter portion 40a is provided.

The flow rate sensor 6 has an impeller, which will be described later, inside a cylindrical outer peripheral wall 42 that forms an outer frame. After the flow rate sensor 6 is accommodated in the accommodation portion 40 of the water supply passage 4 , it is retained by an annular retaining ring 46 . The retaining ring 46 has a plurality of (three in this embodiment) protrusions 46a protruding radially outward on the outer periphery. is pressed in contact with Incidentally, the outer peripheral wall 42 of this embodiment corresponds to the "cylindrical member" of the present invention.

FIG. 5 is a cross-sectional view of a connecting portion cut along a plane including the center line of the water supply passage 4 and the center line of the pressure detection passage 7 with the flow rate sensor 6 inserted inside the water supply passage 4 . As shown in the figure, the flow sensor 6 has an impeller 43 inside the outer peripheral wall 42 , and both ends of a rotating shaft 43 a of the impeller 43 are rotatably shafted by an upstream side bearing portion 44 and a downstream side bearing portion 45 . supported. These bearings 44 and 45 are supported inside the outer peripheral wall 42 with radial spokes so that clean water can pass therethrough. In the flow rate sensor 6 , since the impeller 43 is rotated by the flow of clean water in the outer peripheral wall 42 , the flow rate of clean water is detected based on the rotation speed of the impeller 43 .

An outer peripheral wall 42 of the flow rate sensor 6 is housed in the housing portion 40 inside the water supply passage 4 . Further, the inner wall surface and the outer peripheral wall 42 are in close contact with each other on the downstream side (upper side in the figure) of the housing portion 40 . As a result, the clean water supplied to the heat exchanger 13 passes through the inside of the outer peripheral wall 42, so that the flow rate sensor 6 can accurately detect the flow rate of clean water supplied to the heat exchanger 13. can.

On the other hand, on the upstream side (lower side in the drawing) of the housing portion 40, the enlarged diameter portion 40a is provided as described above. A gap A is formed in Since the inlet 7a of the pressure detection passage 7 is open to the inner wall surface of the expanded diameter portion 40a, the entire circumference of the inlet 7a is surrounded by the gap A and communicated with the outer wall 42 while being covered.

Further, the outer peripheral wall 42 is retained by an annular retaining ring 46 press-fitted into the large diameter portion 41 on the upstream side. As a result, although the upstream side (lower side in the figure) of the gap A between the inner wall surface of the enlarged diameter portion 40a and the outer peripheral wall 42 is covered with the retaining ring 46, The length of the outer peripheral wall 42 is slightly shorter than the length of the housing portion 40, and the upstream end of the outer peripheral wall 42, which is pushed downstream by the flow of clean water, does not contact the retaining ring 46, so that the outer peripheral wall Clean water can flow into the gap A from the upstream end side of 42 .

FIG. 6 is a cross-sectional view of the connecting portion cut along a plane perpendicular to the center line of the water supply passage 4 and including the center line of the pressure detection passage 7, with the flow rate sensor 6 inserted inside the water supply passage 4. As shown in the figure, a gap A between the inner wall surface of the enlarged diameter portion 40a of the water supply passage 4 and the outer peripheral wall 42 of the flow rate sensor 6 is provided over the entire circumference of the inner wall surface of the water supply passage 4 (enlarged diameter portion 40a). , and the inlet 7a of the pressure detection passage 7 opened in the inner wall surface of the enlarged diameter portion 40a. In this embodiment, the area of the housing portion 40 of the water supply passage 4 housing the outer peripheral wall 42 of the flow rate sensor 6, in which the enlarged diameter portion 40a is provided, corresponds to the "inflow area" of the present invention.

As described above, in the backflow prevention device 25 of the present embodiment, the flow rate sensor 6 is inserted inside the water supply passage 4, and the enlarged diameter portion 40a is formed upstream of the center of the accommodation portion 40 that accommodates the outer peripheral wall 42. A gap A is formed between the inner wall surface of the enlarged diameter portion 40a and the outer peripheral wall 42 by providing the gap A, and clean water can flow into the gap A from the upstream end side of the outer peripheral wall 42. As shown in FIG. An inlet 7a of the pressure detection passage 7 opens to the inner wall surface of the enlarged diameter portion 40a and communicates with the gap A. As shown in FIG. As a result, the pressure detection passage 7 communicates with the water supply passage 4 through the gap A between the inner wall surface of the enlarged diameter portion 40a and the outer peripheral wall 42, so that the inlet 7a of the pressure detection passage 7 is exposed to the water supply passage 4. Intrusion of foreign matter into the pressure detection passage 7 can be suppressed as compared with the case. Moreover, although the gap A between the inner wall surface of the enlarged diameter portion 40a and the outer peripheral wall 42 is narrow, the entire periphery of the inlet 7a of the pressure detection passage 7 is surrounded by the gap A and is in communication with the gap A. Since a large total flow area for the clean water flowing into the passage 7 can be ensured, the supply pressure of the clean water in the water supply passage 4 can be sufficiently transmitted to the atmosphere release valve 26 .

In addition, since the gap A is provided between the outer peripheral wall 42 and the entire inner wall surface of the water supply passage 4 (enlarged diameter portion 40a) where the inlet 7a of the pressure detection passage 7 opens, the inner wall surface can be Compared to the case where the gap A is provided surrounding the inlet 7a only in a part of the area on the side where the inlet 7a opens, clean water flows into the gap A from the upstream end side of the outer peripheral wall 42 and is detected. Since the route leading to the inlet 7 a of the pressure passage 7 can be easily secured, it is advantageous in transmitting the supply pressure of the clean water from the water supply passage 4 to the atmosphere release valve 26 .

In addition, since the outer wall 42 of the flow rate sensor 6 installed in the water supply passage 4 is diverted to provide a gap A between the inner wall surface of the water supply passage 4 (expanded diameter portion 40a), a separate cylindrical member is required. This is not necessary and can be easily implemented.

Furthermore, since the upstream side of the gap A between the inner wall surface of the enlarged diameter portion 40a and the outer peripheral wall 42 is covered with the retaining ring 46 that prevents the outer peripheral wall 42 from coming off, clean water flows from the upstream end side of the outer peripheral wall 42 into the gap. Since the path for flowing into A can be bent, it is possible to further suppress foreign matter from entering the pressure detection passage 7 .

Although the backflow prevention device 25 of this embodiment has been described above, the present invention is not limited to the above embodiment, and can be implemented in various forms without departing from the scope of the invention.

For example, in the above-described embodiment, the gap A was provided between the outer wall 42 and the inner wall surface of the water supply passage 4 (enlarged diameter portion 40a) over the entire circumference. It is sufficient that a gap A is provided surrounding the entrance 7a on the side where the entrance 7a of the passage 7 opens. In addition, clean water cannot flow in from the inlet 7a side of the pressure detection passage 7 in the outer periphery of the upstream end of the outer peripheral wall 42, and the clean water that has flowed in from the side opposite to the inlet 7a wraps around the outside of the outer peripheral wall 42. A gap A may be provided so as to reach the periphery of the inlet 7a. However, if the gap A is provided along the entire circumference of the inner wall surface of the water supply passage 4 as in the embodiment, as described above, clean water flows into the gap A from the upstream end side of the outer peripheral wall 42 and the pressure detection passage. It is easy to secure the route leading to the inlet 7 a of the water supply passage 4 , which is advantageous in transmitting the supply pressure of clean water from the water supply passage 4 to the atmosphere release valve 26 .

In the above-described embodiment, the inlet 7a of the pressure detection passage 7 is connected to the water supply passage 4. 4 , and may be connected to the hot water supply passage 5 between the heat exchanger 13 and the hot water filling electromagnetic valve 21 . In this case, the flow sensor 6 may be inserted inside the hot water supply passage 5 to which the pressure detection passage 7 is connected so that the flow rate of hot water flowing out of the heat exchanger 13 can be detected by the flow sensor 6 .

Further, in the above-described embodiment, the gap A is formed between the inner wall surface of the water supply passage 4 and the outer peripheral wall 42 by providing the enlarged diameter portion 40a. However, the gap A may be formed between the outer wall 42 and the inner wall surface of the water supply passage 4 by reducing the outer diameter of the outer wall 42 and providing a reduced diameter portion. Of course, the gap A may be formed by both the enlarged diameter portion 40 a of the water supply passage 4 and the reduced diameter portion of the outer peripheral wall 42 .

Further, in the above-described embodiment, the outer wall 42 of the flow sensor 6 was used to provide the gap A between the inner wall surface of the water supply passage 4 (enlarged diameter portion 40a), but instead of the flow sensor 6, The clearance A may be provided by inserting a simple cylindrical member inside the water supply passage 4 . In this case, since the downstream side of the center of the cylindrical member does not need to be in close contact with the inner wall surface of the water supply passage 4 (accommodating portion 40), clean water can pass through the gap A not only inside the cylindrical member but also outside. may be In addition, by providing a plurality of protrusions protruding radially outward on the outer periphery of the cylindrical member and fixing the cylindrical member by press-fitting these protrusions into the water supply passage 4, The retaining ring 46 may be omitted.

Furthermore, in the above-described embodiment, the cross-sectional shape of the inner wall surface of the water supply passage 4 and the cylindrical member (outer peripheral wall 42) cut along a plane perpendicular to the direction of water supply is circular. , may have a rectangular cross-section.

1... hot water filling system, 2... bathtub, 3... gas passage,
4... Water supply passage, 4a... Joint, 5... Hot water supply passage,
6... Flow sensor, 7... Pressure detection passage, 7a... Inlet,
DESCRIPTION OF SYMBOLS 10... Water heater, 11... Burner, 12... Combustion fan,
13... heat exchanger, 20... hot water filling control unit, 21... hot water filling solenoid valve,
22... First check valve, 23... Second check valve, 25... Backflow prevention device,
26... Air release valve, 30... Diaphragm, 31... Primary chamber,
32...Secondary chamber, 33...Valve element, 34...Valve seat,
35... Valve opening spring, 36... Retracting passage, 37... Opening passage,
40...Accommodating portion 40a...Expanded diameter part 41...Large diameter part
42... Peripheral wall, 43... Impeller, 43a... Rotating shaft,
44...Upstream side bearing portion, 45...Downstream side bearing portion, 46...Retaining ring,
46a... Protrusion, A... Gap.

Claims (4)

A backflow prevention device provided in a supply passage for supplying hot water to a bathtub to prevent backflow of hot water from the bathtub,
connected to a downstream passage downstream of an on-off valve that opens and closes the supply passage, and receives hot water supply pressure in an upstream passage on the upstream side of the on-off valve and closes when the pressure is equal to or higher than a predetermined pressure; an atmosphere release valve that opens to discharge hot water from the downstream passage when the supply pressure of hot water in the upstream passage drops below the predetermined pressure;
a pressure detection passage that communicates the upstream passage with the atmosphere release valve and guides the supply pressure of hot water in the upstream passage to the atmosphere release valve;
a cylindrical member inserted inside the upstream passage,
Between the inner wall surface of the upstream passage and the tubular member, there is provided an inflow region having a gap through which hot water can flow from the upstream end side of the tubular member,
A backflow prevention device, wherein an inlet of the pressure detection passage on the side of the upstream passage is surrounded by the inflow region and opens to an inner wall surface of the upstream passage. In the backflow prevention device according to claim 1,
The backflow prevention device, wherein the inflow region is provided along the entire circumference of the inner wall surface of the upstream passage. In the backflow prevention device according to claim 1 or claim 2,
The upstream passage has a circular cross-sectional shape taken along a plane perpendicular to the direction of hot water flow,
The backflow prevention device, wherein the tubular member is a cylindrical outer peripheral wall that constitutes an outer frame of a flow rate sensor that incorporates an impeller that rotates in response to the flow of hot water. In the backflow prevention device according to any one of claims 1 to 3,
the cylindrical member is inserted into the upstream passage from upstream to downstream,
A backflow prevention device, wherein the upstream side of the cylindrical member is retained by an annular retaining ring along the inner wall surface of the upstream passage.

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