I was told that the furnace we have has a possible hair line crack starting and should be replaced soon. So i have no idea how except for some gadget that determined this. I thought it might be wise to find out what kind of new oil furnaces are out there on the market and how much of course . I addeded on the house and now have close to 3000 square feet. the furnace i have now is older and was in this house when we bought it some 12 years ago. (its a Rheem model.John