A heater unit (10) for a respiratory system is designed for natural convection cooling air to pass first through a transformer (60) in the heater unit and then through a heat sink (90) in the heater unit to which is mounted a power switch (52, 56 or 58) to conduct heat from the power switch to the cooling air passing through the heat sink (90). A thermal break may be provided between the heat sink and the transformer. The transformer may be a toroidal transformer with the air passing through an aperture (84) thereof such as from a hollow projection (140) of the heater unit extending into the aperture. One side of the transformer may be situated along a surface (94) of the heater unit and the heat sink secured to the heater unit against the opposite side of the transformer to hold the transformer in place. A venting chamber (184) may be associated with an air exhaust (168) of the heater unit to provide a circuitous exit path for air exiting the heater unit. The venting chamber (184) may extend in generally surrounding relationship with the heater (32) of the heater unit.
