What is this chimney vent(?) for?

[very_rachel]

I'm new to this woodstove and bought this house in June. It's a Pacific Energy D Series insert stove installed in the late '90s. We had an energy assessment done on our home, which included a blower door test that said the silver rectangular vent (?) on the chimney is a major source of air leakage. Is this a vent? If not, what is it and what purpose does it serve? And yes, this stove should have a blower. It's on order but in the meantime we're getting by with a fan.

 

[Gearhead660]

I am guessing that used to be an open fireplace? That looks like the vent for the blower that is used for open fireplaces.

 

[EbS-P]

I think question is where/how is outside air coming out. The surround does not match the insert. So I’m questioning what kind of fireplace is it?

Is the grill removable?

 

[john26]

I think question is where/how is outside air coming out. The surround does not match the insert. So I’m questioning what kind of fireplace is it?

Is the grill removable?

Looks like the correct surround, just looks odd with the covers off for the blower assembly. That surround looks very much Like my PE design D surround. A lot of Heatilator and copies of them had a grill there, my guess is it's that type of fireplace.