100-270 Degree high limit aquastat (manual reset) keeps
opening the circuit. It is set at 210 degrees. I understand that even after the unit is done firing that it is still heating and therefore the temp can still increase. But the manual reset part means that the heat won't come back on until it is manually pushed. Can I just change this out with an automatic reset one?
What type of boiler?
What is the operating temp?
This problem can normally be solved by installing a post purge relay on the boiler circulator. This helps dissipate the extra heat after the cycle.
A manual reset high limit should not be used as an operating control. You should have another aquastat set at a lower temperature as the operating control. It has to be manual reset to get some one there to realize there is a problem and take some kind of action.
If you have a closed system with proper expansion tank and a pressure relief I would turn the(reset) high limit setting up a few degrees and then try to find out why the operating control is overshooting the set point.