If the Pilot Light does not Stay Lit on Boiler

On my Weil-Mclain Boiler, you remove the front cover by sliding it up and the instructions for lighting the pilot are on the back side of the front cover. One of the first things you need to do is turn off the pilot button, which is behind the wires about in center. The settings are written around the button (off, pilot) but they face the side of the boiler unit. Sal used a mirror so he could read them. The cover to the boiler mentioned in the instructions is at the bottom back of this part of the boiler. It is a rectangular piece of metal. The pilot is behind here is the middle of the heating units. There is a hole in the cover so you can see if the pilot is lit.

When relighting the pilot, if you let go of the pilot button too quickly, you may find that the pilot sputters and goes out. This is because the thermocouple has not had enough time to heat up. Unless it reaches a certain temperature threshold, it will not send the signal to keep the valve gate open. Hold the button down for about 1 minute.

If the pilot flame ignites but goes out after you release the gas control knob, the cause may be a dirty or defective thermocouple. If the gas is on but the flame will not ignite at all, a pilot tube obstruction is the most likely issue. Remove the pilot tube from the gas valve and spray compressed air to clear it.

But if the pilot does not stay lit, the thermocouple may need replacing due to corrosion or different metals in the thermocouple getting welded together. A thermocouple is a safety mechanism that turns off electricity and thus shuts off gas to the pilot and the burners to prevent an explosion. Replacing a thermocouple becomes necessary when the pilot light won’t stay lit even after the old thermocouple has been cleaned.

Pro Tip: Test the thermocouple with a multimeter with a DC volt scale. If the thermocouple reads more than 25 millivolts, it’s not the source of the problem.

When working properly in most gas appliances, the pilot flame should hit the upper 1/2 to 3/8 of the thermocouple. If the pilot has a weak, orange-yellow flame or barely reaches the thermocouple, it’s too low. If the flame extends well above the thermocouple, it’s too high.

Choose the new thermocouple based on the size and mounting style of the old one. Commercial thermocouples range in size from 18-48 inches and are interchangeable. Universal thermocouples have millivolt rating of 25-35 mV. Type K, J and N thermocouples are commonly used with residential appliances, while other thermocouple types have industrial applications.