This is the third time I am installing one of these jotul DV stoves. Installing a 600DV running on propane. Book says you need to chop a 10x10 hole through the wall for wall termination and use a wall thimble which I did on the last two. This one however is going through a concrete block wall. I don’t see a need for a wall thimble and would prefer to keep the hole through the wall as small as possible. I am wondering if the wall thimble is for going through a combustible material wall ?