Shop forge question

[SteelSlaver]

I am going to start the addition on my shop soon. It will be 24'x28' and connected to a portion of the current shop that is 10'x24' so it will give me an area 38'x24' with 11' ceiling. I want to be able to forge inside this area, close to one wall. I want to install a vent pipe over the forge area up through the ceiling and roof with a inline fan inside the pipe. What size pipe should I use for this. I don't want overkill that sucks a bunch of heat out of the building in the winter which can get quite chilly here in Montana. Is 6" enough.

 

[Overmountain]

It will depend on the size of you'd forge as much as the area to be cleared out. If you have a large forge and it is rather open with no hood over it then it might be too little, but it might not either
AFAIK The roof geometry plays into getting the heat out of there as well. I helped put a vent hood in an insulated shop in Colorado (also quite cold... lol) that I shared for some of my work. I wasn't making knives yet at the time, but was there with the forge running when I was doing my stuff. We set it up with an inline fan, can't remember how many cfm but think around 300, will have to ask, and also a flue damper door like is used for wood stoves to knock down the chimney effect in winter. The fan was one of the simple low pressure fans. It made a 90 above the hood, then through the shop wall near the eaves of the apex of the roof. The pipe used was black stove pipe that was 8" in diameter. We put the door before the fan FYI to control the flow with the fan on, and I know they have speed controllers for those fans, but we didn't have one. It will kill one of those fans to push against an obstruction but not to pull against one. The hood was the type they use in pizza kitchens above the conveyor type ovens (well at least that's where it came from, don't know if it was a fabbed up bespoke piece for that kitchen) but it worked really well. That was a 2 burner forge with doors front and rear with an internal dimension of about 8x18x5" and the shop was about 24'x20'. I know with it closed it could run you out of the shop in winter, but it also struggled to keep up in summer. I don't know if a better fan would have helped.
I elected to mount my stuff on carts with my shop and forge outside, but would eventually like to set up something similar. Hopefully someone who has done a few of these Will chime in and better advise you.