Exhaust, one more time. 6" duct work?

[Just Nancy]

It's about time to put the hole in the ceiling. I'm considering a 36" range hood with 700 cfm, or a 42" with 900 cfm.

The duct work on both would only be 6" coming out the top of the hood. It would be about 30" over my bench. Standard range hoods. The duct would either go up about 4 feet, take a right turn and go another foot out, or will go straight up about 10 feet.

Seems like I should be able to make one of them work. I just don't know if the duct work is too small. I'm not sure going up in cfm can compensate for that, but I don't want to buy too small of a fan. Nor do I want to wish I'd bought one over the other. The extra $150 would help towards an oxy con or to fill in my glass supply.

Suggestions?

[MikeAurelius]

Go with 8", you will put less strain on the fan motor.

[Just Nancy]

Thanks. The hole in the top of the hood is only 6". That's why I was concerned.

[MikeAurelius]

Get a good pair of metal shears and open it up.

[Just Nancy]

OK. Thanks. I'll tell him an 8" hole. Then any opinion on the 36" hood with a 700 cfm fan or the 42" with the 900 cfm?

The main reason I considered the larger was if the vent straight up about 10 feet was too long to have the 700 cfms work well. Currently he thinks he's running it up to the 'attic' of the pole barn. That way the exhaust would have a screen to keep critters out, but no hole in the exterior steel to rust. (And need a screen or something to keep the elements out.)

The barn ceiling is like 18 feet high. The roof has a ridge cap so air is exchanged. (And I think vented sofits). There should be no reason for anyone to go up there unless maybe there is a problem with the door openers.

The open barn space is either loft space over my studio (fully enclosed, drywalled, etc) for storage, or machinery storage/repair.

[MikeAurelius]

Uh huh...it *ABSOLUTELY POSITIVELY HAS TO* be vented outside.

DO NOT vent it into the attic space. Regardless of the size of the building or the number of cubic feet, combustion byproducts must be vented outside.

10 feet is definately not too long a run for 700 CFM, especially with an 8" diameter duct. My first studio set up pushed 20 feet with 365 CFM in an 8" duct no problem at all - it just depends on the fan you are using to do the air movement.

[Just Nancy]

I have a feeling it is going to go up. There is no way we'd consider putting a hole in the roof. Too high to try to cut it out for one thing.

This isn't a typical building attic. It's not like venting a bathroom fan into my house attic. Many fans vent into pole barn 'attics'. It is a typically constructed 80x48' ag building. That's part of why the open ridge cap and vents and sofits. There is zero insulation and function to this space. The roof is mostly to keep the elements out.

The building inspector had no problem with it either.