Oil Boiler Overheating

My boiler is set to the aquastat all the way down to 140 and the burner will fire until 200 is reached then shut off after the relief valve shoots out steam and water. I am not sure if it does this because there is a limit, but seems like the connection from my thermostat to the boiler isn't working. Thermostat is wired properly, but when I set the thermostat to 15C, it goes up to 20C at times, can anyone help? I have a technician coming in during the week but is cold and would like to see if i can fix this. I have heat just have to manual shut down the power to the boiler because it reaches 200 which throws the relief valve open. With the high and low points on the relay set to the lowest, why is it going to 200? wiring problem? I can manually turn on the power and run the burner until 160 and shut it down but the curc pump will not run as well. The boil was to heat a small apartment building maybe 8 units, would i have to replace the nozzle to a lower btu nozzle?

The boiler i am using is a Clare HMT with water heater coil which is removed, would the wiring to the aquasat need to be modified without the coil possibly?

If your high limit is set at 140 and the burner continues to fire past that setting then the aquastat is bad or the thermocouple is not fully seated in the aquastat well

As Tom said or an accumulation of mud or crud on the outside of the well restrcting heat transfer.

Many boilers rely on a well into the water where a sensor is installed. The sensor stays dry, but it must be fully inserted. It might be somethings (as already noted) as the sensor (aquastat) is not seated all the way in, and is thus sensing air temp, rather than the water temp. It could be bad as well. Testing the sensor can usually be done with a multimeter and the spec sheet. The resistance across it will vary based on the temperature.