It’s perfectly acceptable to have a high-efficiency furnace installed with one pipe. Just flip through the pages of any high-efficiency furnace installation manual and you’ll find the answer. I’m blogging about this because I’ve had many homeowners ask me about this. And now the rest of the story. Why Two Pipes High-efficiency furnaces come with two pipes; one pipe brings combustion air directly into the furnace and mixes it with fuel. The other pipe exhausts the combustion g