Have a propane furnace venting thru roof of manufactured home. The intake freezes up and shuts down furnace and all pipes etc frozen. We have used poles to knock ice off chimney each year. But only takes one mistake to freeze up house. Twice now in 12 years. There are no other sewer or dryer or kit/bath fans venting near the chimney. Near International Peace Gardens in ND. Put in new furnace two years ago to fix the problem. It did not. Contractors said some federal regulation requires newer furnaces to have exhaust vent pipe inside the combustion air intake pipe on the roof. They won't remodel with two separate pipes to roof. Any ideas on how to "fix" this?