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How much should boiler pressure rise as it heats?
How do you compare a boiler’s pressure when the system is cold with the reading after it’s been heating for a while? I’m curious what kind of change is generally expected, and whether a repeated, unusually large rise is a useful clue that the expansion vessel needs attention, rather than proof of a fault by itself.
If water is coming out of the pressure-relief discharge pipe as the pressure rises, does that point to the same issue, or could there be other causes? Which readings or symptoms are useful for a homeowner to note, and where should troubleshooting stop and a heating professional take over?
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Heating makes the system water expand, so pressure normally rises. Compare the gauge with the system fully cool to its reading after it has been heating, ideally noting both readings under similar conditions; the size and repeatability of the change matter more than a single universal “normal” figure, since systems differ. A repeated sharp rise, especially toward the boiler’s pressure limit, is worth recording and discussing with a heating professional.
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