You are considering replacing your aging propane furnace for a natural gas model. The propane model originally cost $2,200, will last 6 more years, and will have no salvage value. The gas model costs $2,200 and offers a $400 trade-in on the old furnace. It lasts 13 years and can be salvaged for $500 at the end of year 13. Annual fuel costs are $800 for the propane furnace and $600 for the gas furnace. The real interest rate is 9% per year. Using cash-flow replacement and annual worth analysis, should the propane furnace be replaced with the gas model?