28 within its interior the upper ends of tubes 18 are disposed in common communication with manifold space 26 while their lower ends are in common communication with manifold space 28 a cold water inlet pipe 30 comprising an elbow enters manifold space 26 via the face of manifold 14 that is opposite the face through which tubes 18 enter a hot water outlet pipe 32 also comprising an elbow enters manifold space 28 via the face of manifold 16 that is opposite the face through which tubes 18 enter when the water heater is in use cold water enters manifold space 26 via inlet pipe 30 and is distributed around the manifold it then passes in parallel paths through the individual tubes 18 where it is heated in a manner to be subsequently described to manifold space 28 from whence it leaves the water heater via outlet pipe 32 as hot water below pipe 32 at the very bottom of the water heater is a flow control valve 34 for regulating the flow of combustion gas to a burner 36 that is disposed on the top face of manifold 16 concentric with axis 20 a combustion gas inlet conduit member 38 which is disposed between valve 34 and manifold 16 comprises a gas inlet passageway 39 that serves to convey combustion gas from a supply not shown to the inlet of flow control valve 34 a combustion gas outlet conduit member 40 comprises a gas outlet passageway 41 that extends from the flow.