It sounds like your chimney is sub standard and that it might not have had flue tiles installed, but just brick. If your chimney has failed in the manner you describe then you definitely need to fix the situation ASAP. It makes me wonder what the overall condition of the chimney is. Are there any other problems with the chimney that is noticeable from the outside ie: loose or missing mortar, cracks, etc.? If the flue situation is the only problem with the chimney then I would fix it with new flue liner(s). This probably wouldn't be that much different in price from a whole new exterior chimney (metal) running outside your house for the furnace and you would end up maintaining the value of your home by having a servicable chimney and fireplace. Resale value of house with broken chimney is thousands less than one with working, well maintained chimney. Not only that, but it will look better and be safer because sooner or later someone will want to make a fire in the fireplace.