You should replace your furnace if it is more that 15 years old, needs frequent repairs or doesn’t start.  Before you purchase a furnace be an educated shopper by reading on line at Energy Star and other consumer publications then call your heating contractor.  He can help you purchase a new energy efficient furnace that will heat your home evenly and save you money on your heating bills.  There are energy rebates available for high efficiency furnaces and programmable thermostats.