Duct size for 15' x 40' room

[Brianc500]

I have a 15' x 40' fully insulated room attached to my barn that I'm looking to heat. The room has a sloped ceiling of 11' on the one side down to 8' on the other side. It used to be a lean to that I closed in and framed all the walls and insulated with R19 in the walls and R64 in the ceiling. I'd like to just run one duct down the entire length of the room in the highest point of the sloped ceiling and pop some vents in every 10' or so. I'm curious as to what diameter duct I could use to effectively heat the space. The attached photo is a similar example as to what I'm looking at doing with the space.

 

[PoorUB]

12" would do it.

What is the heat source and BTU output?

 

[Brianc500]

It will likely be a 40k btu 80% efficient furnace

 

[Shiftless]

You might need air retur

It will likely be a 40k btu 80% efficient furnace

If the furnace only has to heat that one big room, I bet it will be completely adequate.
You might need two air intakes, one on each end to make the heat even.

 

[PoorUB]

It will likely be a 40k btu 80% efficient furnace

12" is plenty. Are you adding air conditioning?

 

[Brianc500]

12" is plenty. Are you adding air conditioning?

I'll have a separate Mini Split for AC.