There are many indicators that should trigger the phrase: "Check the flue!" I always measure the draft in the flue of a natural draft, 80% appliance. Too high or too low of a draft both can be an issue. HVAC Contractors tend to focus on the 'too little draft' by installing chimney liners, completing Combustion Appliance Zone and CO testing. Of course this condition could be the most dangerous, so I can't blame them. Who ever expects to pull the flue off the boiler, look into the chimney and see what is pictured? There must have been 40 years of garbage piled up! Really is a spectacular thing that after all the years of maintenance I have yet to become complacent with the Flue.
Did you know the draft of the chimney is very dependent on the temperature difference between the flue gas and outside. The greater the temperature difference, the better the draft (generally speaking). This means the draft may be just fine on a cold WInter night, yet spill gases on a warm Fall afternoon. Conversion warning: 25 Pascals = .10" w.c. I know: metric, really?...Trust me, the rest of the world has the real gripe when they see standard measurements. I bet your electronic manometers or even magnahelics (whatever you are using for draft measurements) have this scale. Take a look at this chart supplied by Building Performance Institute (T_Out is your Outdoor Ambient Temp), start testing, and be sure that boiler will be safe all year long. Oh, and don't forget about the water heater too!