A stainless steel liner is frequently an important part of extending the usable life of your chimney.  Over time, the terra cotta flue liner of your chimney breaks down, potentially allowing deadly Carbon Monoxide gases to enter your home.  A stainless steel liner will be connected directly to your boiler flue pipe, and will safely contain all poisonous gases from your boiler, allowing the boiler to vent all these dangerous gases outside your home.  Surprisingly, some old chimneys were built without terra cotta liners at all!  This is extremely dangerous building practice which is not permitted by current building codes.  If you have a very old chimney, there is a high likelihood that your chimney is leaking gases into your home.  If you have unexplained headaches when you are home, particularly if you are in basement, please consider this as a possible cause.