Abstract: The apparatus is a non-condensible gas venting device for vapor sources which can include a sonic orifice for vapor pressure reduction in high vapor pressure systems. A typical vapor source has a evaporator chamber with an evaporating wick containing liquid which is heated and produces vapor. The invention is a venting chamber connected to the evaporator chamber so that the vapor has access to the venting chamber. The venting chamber also includes a condensing wick interconnected to the evaporating wick in the evaporator chamber by a capillary capillary device through which the condensed liquid is returned to the evaporating wick. The condenser chamber is vented to a lower pressure region or vacuum so that non-condensible gas present moves out of the condenser chamber into the lower pressure while the vapor is trapped by the condensing wick and is retained in the system.