This is just a guess but how about a rain catcher that drains the rain or snow from going into the exhaust manifold if the unit is shut down. I am thinkingnthe top would be louvered so that the exhaust pressure would open the louvers and allow normal operation while closing and sealing if the engine is off and not running... Sort of like a flapper on a tractor or construction equipment perhaps. Just my educated guess though...
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