The biggest problem I find with boilers on old Minneapolis and Saint Paul homes is leaking exhaust gas. While every heating contractor knows to look for backdrafting at the vent while performing maintenance checkups on boilers, I've found that many contractors don't bother to check for exhaust gas leaking around the jacket of the boiler, which can be just as serious of an issue.
To identify these exhaust gas leaks, I start by just looking at the jacket, or 'cover' of the boiler. If I find black or brown scorch marks around openings in the jacket, it's usually a dead giveaway that the boiler is leaking exhaust gas.
Another obvious sign of exhaust gas leakage at a boiler is heavy rusting in one particular area on the jacket; if you see rusting like the kind pictured below, it's bad news for the boiler.
To confirm these exhaust gas leaks, I use a combustible gas detector. While the most common use for a combustible gas detector is to detect natural gas leaks, they can also be used to detect exhaust gas leakage. When I inspect boilers, I check around the entire boiler for exhaust gas leaks while it's running. A combustion gas analyzer can also be used to confirm exhaust gas leaks, but this method takes much more time, because there is no instant audible feedback. The video below shows me finding just such a leak with my combustible gas detector - a TIF8800A.
If you don't own a combustible gas detector or a combustion gas analyzer, we can't hang... but you can still check for exhaust gas leakage on your boiler. Simply hold a room-temperature mirror above the suspected areas. If there is exhaust gas leaking out, it will fog up the mirror. It probably won't be as dramatic as the fogged mirror shown below, but it's the same principal.
Exhaust gas leakage is a problem because it could contain carbon monoxide, which can be deadly in high enough concentrations. Even if the exhaust gas leaking out of a boiler has a very low level of carbon monoxide, this is still a potential safety hazard that needs repair. There is no acceptable amount of exhaust gas leakage.
The repair for leaking exhaust gas at a boiler is often to have the boiler replaced, which is an expensive repair. In some cases, the exhaust gas leakage can be repaired by a qualified heating contractor who specializes in boiler repairs, but this is usually an expensive repair.
Reuben Saltzman, Structure Tech Home Inspections