I've put 2 new sets of thermostats in and it's still going out on the high limit stat.
I solved mine by reducing the settings knob on the front of the boiler from full (setting 6) to setting 4. A higher setting reduces the effects of any sulphur in the oil condensing out to form a corrosive acid. This is fine if it does not cause the high limit stat to operate but if you can't use the boiler then it will corrode anyway.
If your boiler is going out on High Limit, this means it is heating faster than your system can absorb the heat. In our house the high-limit is 192F, and if the weather is very cold out, the system will heat to that point but the stat may be still calling for heat. Then, circulator will continue to operate but the flame will stop. This may be what is happening with your system. It is actually rather more common with oil systems than with gas as an oil system burns hotter than a gas if properly maintained. But if the system is actually in runaway, it may well be that the thermostat wire is shorted somewhere between the stat and the boiler. What happens if you disconnect the thermostat wires at the boiler itself? Does that shut it down? If not and the boiler is still in a runaway condition, you need to have it serviced.
