My repairman told us we needed to replace our 35 year old furnace. He also said we probably need to line our flue since they used to be sized for much bigger furnaces. Does this make sense? Is the flue liner something that can be done later since the chimney also needs repair(spalling). We were thinking about doing some major repairs next year (new roof and addition) so I'd like to do all the work then and replace the furnace this year. Thanks.