I’m trying to wrap my mind around this. You went from an 80k btu unit down to 40k? How does this work? Why did you have an 80k unit if a 40k is sufficient?
I still don’t understand how the math would work. If your 80k unit was a mid efficiency one, ie 80%, then your max output would be 64k btu. Assuming you went with a high efficiency 97% 40k unit, your output would be 38.8k btus. That’s a gap of 25k btu’s!