(continued ...) Are you really in the second group? If your furnace or boiler is more than 20 to 25 years old, you should consider replacing it with a new high energy efficient model. This is about the time that things start to malfunction due to normal wear-and-tear. This also puts your equipment firmly outside of the high energy efficient equipment's technological maturity that makes it a reliable choice. If you have not had any trouble to date then count yourself as fortunate. This is now an opportunity to plan a replacement rather than to scramble for a replacement in the dead of winter.
I have been in the scramble mode before. I know a fair amount about my equipment but repair was just not feasible. It was the middle of winter and in the mid teens F outside. I thought, "No problem, I'll just call up my local heating contractor and we'll have heat by afternoon"; NOT! This is the same time of year that everyone else's equipment is failing. I was on a waiting list. I had to run out and buy six electric heaters. I dug out an old kerosene heater from the shed. It was all bad. Instead of living through that, I recommend you start making payments to your own special high energy efficient boiler/furnace replacement account. Get some quotes for replacement to a high energy efficient system so you know what your budget needs to be. Start your self-payments so as to target mid-spring in one or two years. This is the time of year that heating contractors are looking for work and are low–balling quotes just to keep busy. Look for their advertising screaming about a high energy efficient deal. This is when you will make your move to a high energy efficient system.