An improved configuration for an in-line direct contact diffuser-type steam injection heater provides for a diffuser whereby steam flow is controlled using an angled sonic diffuser and plug arrangement. The steam flow is controlled by means of a steam plug which selectively exposes holes through which the steam passes. In the preferred embodiment, the configuration provides for a diffuser that had been angled approximately 30° from vertical to facilitate better mixing of the steam with the fluid. The precise angle may be altered depending on the fluid characteristics. A more durable end seal is also used. The end seal is a piston ring which prevents problems associated with the use of elastomeric rings of existing art. A more uniform diameter plug with linear travel is also an improvement over the prior art.