A planar light source unit has a transparent light leading plate and a light source provided opposite to the incidence surface of the lead plate. The piston includes an upper wall, base wall, outer rib, and inner rib defining a cooling chamber therebetween, and a plurality of ring grooves formed in the outer rib. The sealed combustion chamber is supported over the base under an inner casing of the hot water heater which is adapted to contain water to be heated by a primary burner in the combustion chamber. A wafer having a hardness greater than the base wall is positioned beneath the base wall to enhance dimensional stability of the receptacle in a vertical or other direction perpendicular to the substrate on which the receptacle is mounted.