I have a natural gas forced air furnace (Ward's) from 1981 in my house. I replaced the blower unit in 1994, and have had recent problems with the control module. After a repairman came and worked on it, I was told I should probably replace the furnace. Is this furnace to old to consider just replacing the control module, or is it better to get a newer and more efficient furnace? How long can I expect a furnace to last anyway? Thanks for your help.