Hydrogen concentration inside the battery room
 

my case: A battery room with batteries producing hydrogen. there is an air inlet duct and there is a mixture out let duct. As shown in figure attached. I have to findout the hydrogen concentration inside the room. I m using Species transport model without reaction.
i have problem with B.C. When i take "mass flow inlet" BC with "out flow" BC., ANSYS-FLUENT give me error
Primitive error: Gauss Elimination error: Singular Matrix

Anybody can guid me to right path...about model and BC