Ah, Min 4, that will do it. I never gone below 6, and up a little when warmer.
I wish I could get a duct up to the second floor, but I don't want to cut into anything. it does good now I have one duct in the front stairway, you can really tell a difference upstairs.
I run 1 burner when it's 40+ (Spring and fall). But when it starts staying in the 30's and below, both come on.