Think I may have narrowed it down. So it only overshoots when the upstairs zone is on. (Tri level). Which is located directly above boiler. My max return temp is set to the max of 158f. My return water temps from upstairs zone is 149f. Boiler set at 145f.
What I think is happening is the return water is just as hot as boiler water so when it’s lit in heating mode the return water is “helping” heat the boiler water as it recirculates. Causing the set temp to overshoot. I set burner off temp to 12 degrees over but it’s still doing so.
When 2 zones are calling for heat it doesn’t do this near at all if any time. I think the longer runs from multiple zones helps cool the water some before it gets back to boiler so it doesn’t overshoot set temp.
Does my theory sound plausible?