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Sudden Overpressure in Wood Boiler

Hi all, I have a Tarm Solo Plus 30 with storage (820 gallon STSS tank). I am now on my 10th season with this system which has met all performance expectations so far. Suddenly, this week, I have noticed that as the boiler temp rises on startup, as it approaches the point that the main circulator kicks on (170 degF), the pressure rises enough to activate the pressure relief valve. I have not seen this spike in pressure before. Normally, the pressure rises up to operating pressure as the temp in the boiler increases. I do NOT have makeup water on auto-fill. The system normally operates about 15 PSI. The only thought I can come up with is that somehow suddenly the main circulator is preventing any expansion flow through it as the temp rises. Can anyone comment on whether this is a reasonable guess (I'll just replace it) or if there is some other explanation? The main circulator is a Taco OOR-MSF1-ifc

thanks in advance!