A boiler gives you a few quiet signals before anything more noticeable happens. This walk-through covers the signs that tend to show up first, and what to check when you see them. None of them mean the boiler is failing. They tell you where to look and when to call.
The pressure gauge
Read the gauge when the system is cold and again when it is warm. A needle that sinks between visits is a common early sign. Look for a weeping fitting, a damp spot under a radiator, or a drip at the pressure relief valve. A fill valve that refills the system on its own may be doing its job, but it also means water is leaving the system somewhere.
Radiators that run cool at the top
Bleed each radiator with the system warm and the circulator running. Air trapped at the top leaves a cool band and often a gurgle. If the cool band returns after bleeding, the water may be carrying debris that settles in the low spots. Note which radiators are affected. A pattern across the house points at the system, not at a single emitter.
Noise from the circulator
A circulator pump that hums steadily is normal. A pump that grinds, knocks, or rattles deserves a listen with the power off. If the instructions allow it, check that the pump turns freely by hand. A noise that grows over a few heating cycles is worth a call.
Ignition and the start
Note how the burner lights. A slow start, a delayed ignition, or a repeated attempt before the flame holds are early signs worth recording. Write down what you saw and when. The technician will want the pattern, not only the final shutdown.
Who to call
Gas and combustion work belongs to a qualified technician. Describe the gauge reading, the radiator pattern, and the sounds you heard. Ask how the technician will read any shutdown code the boiler shows.
The planning date for a boiler starts with its install year. The boiler page in the index sets out the typical service life and when to plan: Boiler: typical service life and when to plan.