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A fan unit which in use forms part of a gases supply unit, the gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, the fan unit having a casing that has an inlet aperture and an outlet passage, the outlet passage including an exit aperture, the fan

A fan unit which in use forms part of a gases supply unit, the gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, the fan unit having a casing that has an inlet aperture and an outlet passage, the outlet passage including an exit aperture, the fan unit also including a fan which is located inside the casing and which is adapted for connection to a motor to drive rotation of the fan in use, the fan drawing gases into the casing via the inlet aperture, and forcing these gases out of the casing via the outlet passage as a gases stream, the outlet passage further including at least one bypass vent hole independent of the exit aperture and arranged at an angle to the path of the gases stream through the outlet passage.

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1. A fan unit that in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said outlet passage having an exit aperture, and at least one vent hole

1. A fan unit that in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said fan unit comprising: a casing having an inlet aperture and an outlet passage, said outlet passage having an exit aperture, and at least one vent hole independent of said exit aperture and arranged at an angle to a path of a gases stream through said outlet passage; and a fan located inside said casing and adapted for connection to a motor to drive rotation of said fan in use so that in use said fan draws gases into said casing via said inlet aperture and forces said gases out of said casing via said outlet passage as said gases stream, said outlet passage and said at least one vent hole configured so that, if said outlet of said outlet passage is substantially blocked, a flow in use through said at least one vent hole matches a flow through said outlet passage at which an onset of surge would occur in a substantially blocked outlet passage without a vent hole or holes. 2. A fan unit as claimed in claim 1 wherein said exit aperture is larger than a cross-sectional area of said at least one vent hole by approximately three orders of magnitude. 3. A fan unit as claimed in claim 1 wherein said at least one vent hole comprises two or more vent holes and said exit aperture is larger than a total cross-sectional area of said two or more vent holes by approximately one to two orders of magnitude. 4. A fan unit as claimed in claim 1 wherein said angle of said at least one vent hole to said path of said gases stream is substantially perpendicular. 5. A fan unit as claimed in claim 1 wherein said outlet passage has a plurality of said at least one vent holes. 6. A fan unit as claimed in claim 5 wherein said outlet passage comprises two opposed sides and said vent holes are located on said two opposed sides of said outlet passage. 7. A fan unit as claimed in claim 6 wherein substantially the same number of vent holes are formed on each of said two opposed sides. 8. A fan unit as claimed in claim 5 wherein said at least one vent holes number between 65 and 75. 9. A fan unit as claimed in claim 5 wherein each of said vent holes is substantially the same size as the others of said vent holes. 10. A fan unit as claimed in claim 5 wherein said exit aperture has a cross-sectional area of substantially between 210 and 270 mm2. 11. A fan unit as claimed in claim 9 wherein a cross-sectional area of each of said vent holes on an inner surface of said outlet passage is substantially between 0.22 mm2 and 0.26 mm2. 12. A fan unit as claimed in claim 9 wherein a total cross-sectional area of said vent holes on an inner surface of said outlet passage is substantially between 15.00 mm2 and 17.00 mm2. 13. A fan unit as claimed in claim 5 wherein said vent holes are formed with a draft of substantially between 5° and 15°. 14. A fan unit as claimed in claim 13 wherein said vent holes are formed with a draft of 10°. 15. A fan unit as claimed in claim 5 wherein said casing has a generally circular form and said outlet passage is tangential to said casing. 16. A fan unit as claimed in claim 15 wherein said exit aperture has a generally rectangular cross-section. 17. A fan unit as claimed in claim 16 wherein said inlet aperture is located on a lower face of said casing in use. 18. A fan unit as claimed in claim 17 wherein said inlet aperture is located generally centrally on said casing. 19. A fan unit as claimed in any one of claims 15 to 18 wherein said fan is an impeller unit. 20. A fan unit as claimed in claim 19 wherein said impeller unit has a spindle, adapted for connection to a motor in use to drive said impeller unit, said spindle passing out of a bottom of said casing. 21. A housing for a fan unit that in use forms part of a gases supply unit suitable for use as part of a system for providing heated humidified gases to a user, said housing comprising: an outer casing having an inlet aperture and an outlet passage, said outlet passage having an exit aperture and at least one vent hole, said at least one vent hole independent of said exit aperture and arranged at an angle to an overall path of a gases stream passing along said outlet passage in use, said outer casing adapted to enclose a fan in use, and wherein said outlet passage and said at least one vent hole are configured so that, if in use an outlet of said outlet passage is substantially blocked, a flow through said at least one vent hole matches a flow through said outlet passage at which an onset of surge would occur in a substantially blocked outlet passage without a vent hole or holes. 22. A housing for a gases supply unit that in use forms part of a system for providing heated humidified gases to a user and that is adapted to in use provide a pressurised gases stream, said housing comprising: an enclosing external shell that encloses and forms part of a gases supply unit, said enclosing external shell having an atmospheric inlet adapted to allow gases from atmosphere to enter said enclosing external shell in use, and a humidifier inlet aperture adapted to allow said pressurised gases stream to exit said enclosing external shell, said enclosing external shell further comprising an internal recess adapted to hold a fan casing in use, an outlet duct, having an outlet end adjacent to and fluidically connected to said humidifier inlet aperture, and an inlet end, a passage that in use fluidically connects an outlet of said fan casing and said inlet end of said outlet duct, said passage having at least one vent hole, arranged at an angle to an overall path of said pressurized gases stream through said passage, said passage and said at least one vent hole configured so that, if said pressurized gases stream cannot pass freely along said passage, a flow in use through said at least one vent hole matches a flow through said passage at which an onset of surge would occur in a substantially blocked outlet passage not having a vent hole or holes. 23. A housing for a gases supply unit as claimed in claim 22 wherein said passage is formed as an integral part of said housing. 24. A housing for a gases supply unit as claimed in claim 22 wherein said passage is formed as a separate item to said enclosing external shell, and is connected to said enclosing external shell in use. 25. A housing for a gases supply unit as claimed in claim 22 wherein said passage is located and arranged in said enclosing external shell so that in use gases from said fan enter said passage substantially at a tangent to said fan. 26. A gases supply unit that in use forms part of a system for providing heated humidified gases to a user, said gases supply unit comprising: a housing having an external shell that encloses and forms part of said gases supply unit, said external shell having an atmospheric inlet adapted to allow gases from atmosphere to enter said external shell in use, and a humidifier inlet aperture adapted to allow a pressurised gases stream to exit said external shell, and an internal recess adapted to hold a fan unit, said fan unit located in said internal recess, said fan unit having an external fan casing with an inlet aperture and an outlet aperture, and a fan located inside said external fan casing, said fan unit adapted to draw air into said gases supply unit through said atmospheric inlet and provide a pressurised gases stream through said outlet aperture in use, a motor located within said external shell and adapted for connection to said fan unit to drive said fan in use, an outlet duct having an outlet end adjacent to and fluidically connected to said humidifier inlet aperture and an inlet end, a passage that in use fluidically connects said outlet of said external fan casing and said inlet end of said outlet duct, said passage having at least one vent hole arranged at an angle to an overall path of said gases stream through said passage, wherein said passage and said at least one vent hole are configured so that, if said gases pressurized stream cannot pass freely along said passage, a flow in use through said at least one vent hole matches a flow through said passage at which an onset of surge would occur in a substantially blocked outlet passage not having a vent hole or holes. 27. A housing for a gases supply unit as claimed in claim 26 wherein said passage is formed as an integral part of said housing. 28. A housing fir a gases supply unit as claimed in claim 26 wherein said passage is formed as a separate item to said external shell, and is connected to said external shell in use. 29. A housing for a gases supply unit as claimed in claim 26 wherein said passage is located and arranged in said external shell so that in use gases from said fan enter said passage substantially at a tangent to said fan. 30. A medical breathing system suitable for supplying heated humidified gases to a user, the system comprising: a gases supply unit adapted to provide a pressurised gases stream, a humidifier chamber fluidically connected to said gases supply unit in use so that said gases stream can pass into and through said humidifier chamber, said humidifier chamber containing a volume of water that in use is heated to heat and humidify said gases stream as said gases stream passes through said humidifier chamber, a patient interface adapted to provide said heated humidified gases stream to a user, said patient interface including a conduit to transport said heated humidified gases from said humidifier chamber to said user, said gases supply unit comprising: a housing having an external shell that encloses and forms part of said gases supply unit, said external shell having an atmospheric inlet adapted to allow gases from atmosphere to enter said external shell in use, a humidifier inlet aperture adapted to allow said pressurised gases stream to exit said external shell, and an internal recess adapted to hold a fan unit, a fan unit located in said internal recess, said fan unit having an external fan casing with an inlet aperture and an outlet aperture, a fan located inside said external fan casing, said fan unit adapted to draw air into said external shell through said atmospheric inlet and provide said pressurised gases stream through said outlet aperture in use, a motor located within said external shell and adapted for connection to said fan unit to drive said fan in use, an outlet duct having an outlet end adjacent to and fluidically connected to said humidifier inlet aperture and an inlet end, a passage that in use fluidically connects an outlet of said external fan casing and said inlet end of said outlet duct, said passage having at least one vent hole arranged at an angle to an overall path of said pressurized gases stream through said passage, wherein said passage and said at least one vent hole are configured so that, if said pressurized gases stream cannot pass freely along said passage, a flow in use through said at least one vent hole matches a flow through said passage at which an onset of surge would occur in a substantially blocked outlet passage not having a vent hole or holes. 31. A medical breathing system as claimed in claim 30 wherein said external shell is further configured to at least partially enclose said humidifier chamber in use. 32. A medical breathing system as claimed in claim 30 or claim 31 wherein said external shell and said humidifier chamber are mutually configured so that said outlet aperture can mate directly with an inlet port to said humidifier chamber in use.