The disclosure is of an all weather gas supply unit for delivering generally inert gas to a sealed storage structure for atmosphere control. In the system, a gas generator combustion chamber exhausts generally oxygen-free gas to a condenser and thence to an icing arrester, the condenser being cooled by ambient air in a cooling shroud system enveloping the condenser and combustion chamber, and the icing arrester serving to collect ice from the gas exhaust when the arrester is in a freezing ambient. The condenser and arrester have water collecting and draining assemblies which are kept from freezing by a heating system such as scavenger air shrouds extending from the cooling shroud. A heating system in the cooling air shroud intake keeps the exhaust in the condenser to temperatures above freezing.