Find the temperature and pressure gauges of the boiler. The pressure should be between 12 and 15 pounds per square inch (psi) when the temperature is between 70 to 100 degrees F. The water pressure should be between 15 and 25 psi when the temperature is up to 240 degrees. The high limit switch should turn the boiler off when the steam pressure is above 10 psi.
Turn the thermostat of the thermometer up all the way. Listen as the water temperate reaches the preset high point, around 180 degrees. The pressure gauge will go over 10 psi and the boiler should shut off. If it does not, press the red emergency shutoff switch to turn the boiler off.
Move the limit control indicator below the water temperature in the boiler while burners are running. The burners should turn off. Raise the temperature setting on the indicator and the burners should reignite.
Set the thermostat's temperature up until the high limit switch opens and the main gas valve closes. Some boilers will have LED indicator lights that will turn off when this occurs. Others have a thermal bulb that will expand as the water is heated in the boiler to give you a visual indicator that the boiler is working.
Turn off the power to the boiler. Disconnect all wires that lead to the high limit switch. With a piece of clear tape and a felt-tip pen, label each wire according to the terminal of the old high limit switch from which you took it. Remove the screws that hold the high limit switch in place and pull it out of the boiler. Place the new high limit switch into the boiler, attaching the labeled wires to their corresponding terminals on the new device. Insert the mounting screws to secure it to the boiler.