The overheat thermostat requires continal re-setting.
The boiler fires for about five minutes then the overheat thermostat activates and shuts the boiler down. Could it be inaffectual fan? excessive air in the system? Faulty programmer? or something else?
Need to know temp on flow pipe
Use boiler stat on 3 if still plays up lower temp
If temp to system not hot enough boiler stat ok
Could be over heat stat U/S or pump over run stat not cooling down boiler when it shuts down
or pump not man enough