CT, a "spike" is something that happens short term. if it climbed steadily up to 650 over a few hours, your vent settings had to be off somehow. if it spiked in just a few minutes, then (as CW said), the daisy must have been bumped open or something.just doesn't make sense...leaky gaskets let smoke out, not air in. that would be some amazing airflow to come in the gaskets, nosedive down to the fire, then flow up and out of the chimney while also being fed from below.
